Slide Rule Terms, Glossary and Encyclopedia

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This is a work in progress of words and terms used by slide rule collectors and the relevant history associated of the inventors and manufacturers. When I run across something that I haven't seen before or do not understand, I research it, then add it to this page, as much for my own edification and reference as for yours. Corrections and additions are readily accepted and appreciated. Many topics will have pictures or Links associated with them that you can click.

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1° - A gauge mark on Chinese Shanghai Flying Fish slide rules for finding the sinΘ and cosΘ of small angles from 0.573°-5.73°. Located on the C and D scales, rather than Sin and T scales, between 1.74 and 1.75. When the angle is very small, SinΘ is about equal to tanΘ and about equal to the radian of Θ. Set the index of C over the 1° mark on D. For any angle(Θ) on C, read the corresponding sinΘ or tanΘ on D.

4 Inch Slide Rule - These small (literally) group of pocket slide rules are the most compact of all useful slide rules, other than possibly a 5cm (2 inch) diameter circular slide rules. Most have magnifiers on the cursor, like the Hemmi 30, 32 and Aristo 810. The actual length of the scales is 10 cm.

5 Inch Slide Rule - A short, pocket sized,  version of a slide rule with the scales compressed to 12.5 cm (4.92 inch) on a body typically about 15.2cm (6 inches) long in length. Typical precision of a pocket slide rule varies from two to three significant digits, as it becomes difficult to extrapolate between the lines at the right end of the scales. High end examples of pocket slider rules are the K&E 68-1130 (Deci-lon 5), Hemmi 149, and the Faber-Castell 62/83N.

8 Inch Slide Rule - Relatively rare, a shorter version of a standard 10 inch slide rule with the scales compressed to 8 inches, but typically having the same number of tick marks and precision of the longer 10 inch scale. For examples, see Post 1445, K&E 4053-2 and Unique J180.

10 cm Slide Rule - Same as a 4 inch slide rule. 10 cm is the actual starting length upon which the divisions are calculated for a 4 inch slide rule.

10 Inch Slide Rule - a standard length slide rule having the basic scales divided on 25 cm (9.84 inches) with an overall length from 10.5 to 13 inches long. Depending on the construction method, closed or open, simplex or duplex, the body may extend beyond the length of the scales. Standard length slide rules may vary from 25.4 cm (10 inches) to over 30 cm (11.8 inches). The general precision of a standard length slide rule is three (3) significant digits.

12 - A gauge mark on the C or D scale of a commerce or financial slide rule used to denote a dozen, or months.

12.5 cm Slide Rule - Same as a 5 inch or most common standard length slide rule. 12.5 cm  is the actual starting length upon which the divisions are calculated for an 5 inch slide rule.

20 cm Slide Rule - Same as a 8 inch slide rule. 20 cm is the actual starting length upon which the divisions are calculated for an 8 inch slide rule.

20 Inch Slide Rule - An extended length slide rule having the basic scales stretched to twice the standard length of 25 cm (9.84 inches), to 50 cm (19.685 inches) which allows the interpolation of the readings of three (3) significant digits to be more precise. Over time, temperature and humidity, a 20 inch long laminated wood scale is more susceptible to variations in shrinkage and become less accurate than a shorter slide rule as the errors may be magnified because of the length. A slide rule made from an engineering material like aluminum or magnesium, and some plastics, would be proportionally more accurate than one having a wood stock.

25 cm Slide Rule - Same as a 10 inch or most common standard length slide rule. 25 cm  is the actual starting length upon which the divisions are calculated for an 10 inch slide rule.

50 cm Slide Rule - Same as a 20 inch or most common standard length slide rule. 50 cm  is the actual starting length upon which the divisions are calculated for an 20 inch slide rule.

144 - A gauge mark on the C or D scale of a commerce or financial slide rule used to denote 12 dozen, or a gross.

314 gauge point - A mark on the A and B scales and on the C and D scales. It is actually π= 3.14 and is used to find the circumference or perimeter of a circle. Set the index of B under the gauge mark of A, and for any diameter on B, read the circumference on A. See π.

360 hairline - Found on some European multi-lined cursors. Used in conjunction with the central hairline in the center of the cursor. Used as a conversion factor that makes use of the fact that the upper CF scale on the slide and the adjacent DF scale on the stock are displaced relative to the C and D scales by a factor of π or 3.14159. The distance from the "360" hairline to the central line is, in effect 3.6/π or 1.14 (rounded). Positioning the center line over the value of 2 m/s on the D scale read 7.2, the conversion to km/h, on the DF scale.

785 gauge point - A mark on the A and B scales and on some circular slide rules on the C and D scales. It is actually π/4= 0.7854 and is used to find the area of a circle or the volume of a cylinder. Set the index of B under the gauge mark of A, and for any diameter on B, read the area on A. See π/4.

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A

A scale - x² - The scale of squares on the body, The square of the D scale.  To determine a square root, find the number on A and read the root on D. The process is reversed to find a square.

Accuracy (of a slide rule) - The accuracy of a slide rule is dependant upon the design and construction methodology of the slide rule which determine how well the engraving or printing of all of the scale graduations correspond with the true mathematically-computed positions of every graduation on the rule at the time of manufacture. The measure of the accuracy of the scale is how close a reading or setting is to the true value of the number. The accuracy of a slide rule may diminish over time as the stock warps or shrinks at uneven rates, and as the fit and alignment of the slide and cursor become misaligned. High-end slide rules may offer features that allow the user to adjust the slide rule to enhance the alignment to improve the accuracy. See Precision (of a slide rule).

Acrylic - A transparent plastic used in the construction of post-war slide rule cursors. It is one-half the weight of glass and is easily injection molded in production. Cleaning: Wash with mild soap or detergent, with plenty of lukewarm water, dry with soft cloth or chamois. Grease, oil or tar can be removed with hexane or kerosene. Solvent residue should be removed by washing immediately. Do Not Use window cleaning sprays, scouring compounds, acetone, gasoline, benzene, carbon tetrachloride or lacquer thinner or you may ruin your cursor.

Acu-Math - American brand of slide rule made by Acu-Rule Mfg, first used in 1950. Sterling Plastics continued the use of the brand after purchasing Acu-Rule in 1968, but the name was dropped before 1972 after Borden Chemical bought Sterling around 1970.

Acu-Rule - An American manufacturer of slide rules originally known as Festus Mfg. Co. (1938-39) located in Festus, Missouri. Festus was founded by Paul Jones, jr. became the Acu-Rule Mfg Co. An August 14, 1939 business letter from Acu-Rule Mfg. Co. shows an address of 15 East 26th Street, New York. The company relocated to St. Louis, MO using the brand name of Acu-math. After 1946 the company moved to Mt. Olive, Illinois. "Acu-Design" was tried as a brand name for a brief period, probably to supply slide rules to OEM's like Charvoz-Roos. The Acu-Math Division was later purchased by Sterling Plastics in Mountainside, New Jersey. Early versions began as wood, progressed to magnesium, Like Pickett slide rules, then ended up all plastic. At different times, they supplied OEM slide rules or slide rule components to Dietzgen, Post, Charvoz-Roos, Henschel and possibly K&E. See Dietzgen 1757 Primatrig, Post 1457, Henschel 2020P and Charvoz-Roos SR105D/SR109/SR113. Many of the OEM models had laminated wood (basswood) stocks, while the main Acu-Math line was laminated or painted metal, until all-plastic slide rules became popular. More Acu-Rule History

Addiator - A flat mechanical hand-held 'Troncet Type' mechanical calculator that is used principally for addition and subtraction, sometimes found on the back side of a closed body slide rule. The invention of the addiator is credited to J. Louis Troncet of France in 1889, the first version called the Arithmographe. One of the most popular versions of the Troncet-type calculator was the Addiator, which was created in Germany by Addiator Gesellschaft. Introduced in 1920, over 100,000 units were sold in the first year, and versions of this device remained in production as late as 1982. Featuring a novel carry mechanism, the Addiator could be used to add or subtract, but addition and subtraction required two different control panels. In early versions of these devices, the two panels were provided by flipping an overlay on the front or by having the addition function on the front and the subtraction function on the back (later models had two separate panels on the front). The addiator is comprised of three components, the body containing multiple slides used for calculation, a stylus, and a handle to reset the addiator. The calculations are performed by inserting the tip of the stylus in notches in the metal plates in the addiator. See Pascaline Mechanical Calculator

Ahrend - Dutch office supply house that sold slide rules made by Hemmi under the Ahrend name.

AI scale - 1/x², Reciprocal of square of D.

Al¹ - Gauge mark equivalent to √(4ρ/π) = 0.1922. Note:ρ=0.029Ωmm²/m at 20°C, ρ is the small Greek letter rho. Used for electrical calculation for aluminum conductors.

Al² - Gauge mark equivalent to √(4/πγ) = 0..680. Note:γ=2.69g/cm³, γ is the small Greek letter gamma. g is the gravitational constant 9.81 meters/sec². Used for electrical calculation for aluminum conductors.

Allen, Elias - (1580-1653) Considered by some to be the father of the slide rule. He was a mathematical instrument maker who worked near St. Clements Church in the Strand in London. He fabricated William Oughtred's first slide rule, advising that it would be easier to make a circular slide rule with opening cursors, rather than the two disk model that Oughtred first designed.

Allied - A U.S. radio and electronics catalog company based that sold U.S. made Pickett and Dietzgen slide rules. It is unknown if any were re-branded.

Alpha or α - Greek letter α

Aluminum - A non-ferrous light weight metal used by slide rule manufacturers for the body and slide. Alloying of aluminum rapidly developed during World War II which improved the metal to the point that it replaced slide rules previously constructed of magnesium. Reiss and Pickett were large manufacturer's of aluminum slide rules. A thin film of petroleum jelly (Vaseline) is recommended by Pickett to aid in preventing galling ( a problem of aluminum on aluminum contact).

ALRO - A Dutch slide rule company, in the Hague, Holland, that designed and manufactured circular slide rules with characteristic metal boxes.

American Slide Rule Brands - Slide rules manufactured or re-branded by American companies were Keuffel & Esser, Richardson (Geo. Richardson), Gilson, Thacher, Fuller, Pickett or Pickett & Eckel, Dietzgen, Frederick Post, Roos, Charvoz-Roos, Acu-Rule, Acumath, Acu-design, Festus, Lawrence, Engineering Instruments, Charles Bruning, C-thru, Union Instruments, Sterling, Scientific Instruments Co (SIC)(SINCO), Lafayette, Compass, Dieterich-Post, Engineering Sales, Gerber, Guedon, Henschel, Indeco, Jason, Kel-co, Lietz, Novotni, Master Electric, Sans & Streiffe, Micronta, Olson, Omega, Lutz, Slide Rule and Scale Engineering, U.S. Blueprint, Weber, Chirchlow, Mascot, and many others that were promotional rules (list incomplete).

Analon - A revolutionary slide rule by K&E who's introduction was ill-timed with the advent of electronic hand held calculators. The Analon was for dimensional analysis to verify units when doing calculations in physics. Approximately 1000 rules were built in 1967. A large portion of the lot was rejected for quality issues and 597 were known to actually have been delivered to retailers. There were so many returns accompanied with complaints about the difficulty in understanding the manual that the rule was pulled from distribution. The plan was to rewrite the manual was thwarted with the sudden arrival of electronic calculators, makings the Analon semi rare. If you find this white plastic slide rule with gold colored braces in a flea market, take it and run.

Antilogarithm - The number corresponding to a given logarithm.

Apollo 11 - NASA's Lunar Lander space vehicle, with the crew Neil Armstrong, Buzz Aldrin and Michael Collins, carried slide rules, for the first time in history, into space and onto the moon, July, 1969. The slide rule of choice was the aluminum pocket Pickett 600-T Dual base Log Log. “We went to the moon with slide rules,” retiree Norman Chaffee, who worked on the spacecraft propulsion system, “I didn’t even have my first full-function calculator until 1972." Note: When you watch the movie Apollo 13 look for the engineers using slide rules.

Approximate Calculation - A technique, used by experienced engineers and physicists, of rounding your numbers to single significant digits to get a quick approximation of a calculation to verify that your actual result falls in a range that makes sense.

Archimedes - A Brazilian manufacturer of slide rules located in Rio de Janeiro, Brazil (RJ - Brasil).

Aristo - A slide rule manufacturer since 1872, Dennert & Pape was founded by Johann Dennert & Martin Pape, in Hamburg, Germany, and continued under that name until 1936. D&P were the first to laminate and engrave celluloid on boxwood wood slide rules in 1886. Mahogany was used after 1888 and in 1902 started producing slide rules using the System Rietz scales designed by Max Rietz. In 1924 D&P introduced the System Darmstadt slide rule, based on the scales set designed by Prof. Dr. Alwin Walther and began using 'DUPA' as a trademark from 1924 to 1936. All plastic slide rules, using Astrolon for the stock and Plexiglas for the cursor, were introduced in 1936 using the trademark 'Aristo'. D&P officially became Aristo as a company in 1948 when they introduced all-plastic Duplex slide rules and discontinued mahogany wood models. Aristo stayed in the slide rule business until the end of 1978, a run of 106 years. Aristo was one of the most important slide rule manufacturers world wide. Expanded Dennert&Pape/Aristo timeline

Arithmographe - French for 'Computing Machine'. Several people experimented with versions of simple mechanical calculators that involved strips of metal with numbers marked on them mounted in a frame, where a stylus was used to slide these strips up and down. A few names that usually rise to the top of the pile in this regard are Claude Perrot (1613 - 1688), who invented an early form of this class of device sometime around the mid 1660s, and the Frenchman Caze, who created his version around 1720. One problem with Caze’s machine was that it wasn’t possible to perform carries from one column to another. This issue was solved around 1845 by the Prussian-German mathematician Ernst Eduard Kummer (1810-1893). In 1889, a Frenchman named Louis Troncet created his version of this type of calculator, which he called the Arithmographe. Troncet's invention became so popular that the term “Troncet-type” is often used to refer to this class of device, what is now popularly called the Addiator.

Arithmometer - Term used to refer early calculating machines. to an Addiator, which is a mechanical means of addition and subtraction, in actuality, the Arithmometer(s) also were capable of mutliplication and division. Charles Xavier Thomas (1785 -1870) of Colmar, France, or Thomas de Colmar, as he is more commonly known was the first to establish, in 1820, the manufacture of calculating machines as an industry goes to Like Hahn, Thomas used the stepped cylinder invented by Leibniz as his digital-value actuator." Ref:George C. Chase, "History of Mechanical Computing Machinery." Annals of the History of Computing, Vol. 2, No. 3, July 1980. Article: Making the Arithmometer Count

Ash - A type of wood that was commonly used in the construction of Soviet Era slide rules. Fraxinus excelsior (Yasen obiknovenni) and Fraxinus mandschurica (Yasen manjurski - Mongolian Ash) grows in Russia's eastern Siberia and the Ukraine. The wood is very tough and resilient, and is used for farm implements, tool handles, vehicle bodies, baseball bats, tennis rackets, oars, spring boards, piano frames, and to some extent for furniture and interior trim.

Associated Spring Corporation - A U.S. spring manufacturer, based in Bristol, Connecticut, that commissioned specialized slide rules for aiding mechanical engineers in the design of springs. Pickett made some of the earliest versions (copyright 1952) used for designing extension and compression springs and K&E made a slide rule (computer) for designing Belleville spring washers (copyright 1967). There were also cardboard versions copyrighted 1947 through 1960. Today Associated Spring uses computerized software for designing springs.

Astralon - A proprietary PVC plastic used by the British manufacturer Blundell Rules Limited (B.R.L.). It replaced Bakelite as the preferred material for slide rules.

Asymptotic - Mathematical definition: A straight line associated with a curve such that the point moves along an infinite branch of the curve; as in: Vertical Asymptote. Mike's definition: Engineer 'speak' for going vertical or through the roof, what the wife does when she sees more slide rules show up in the mail, or "Becky went asymptotic when she saw the VISA bill."

A.W. Faber - German manufacturer, in Stein near Nurnberg, Germany, named for Anton Wilhelm Faber (1784-1810) who carried on the the work of Kasper Faber's pencil factory (est 1761), followed by son Georg Leonard Faber (1810-1839). Baron Lothar von Faber (4th generation) establishes a factory to build slates, drafting equipment and rulers in Geroldsgrun, Germany and starts making boxwood slide rules in 1882. Celluloid facings were added in 1897. American distributor c1904 in Newark, New Jersey. Name changed to Faber-Castell in 1905 after marriage of Lothar's daughter Ottilie to Count Alexander zu Castell-Rudenhausen. See expanded Faber-Castell timeline.

AZON Corp. - A U.S. manufacturer of drafting media and CAD products, founded in 1949. Mentioned here because in 1987 they acquired Keuffel & Esser Co. in New York, in 1988 acquired Hughes Owens Company in Montreal, Canada, and in 1994 purchased Post & Company in Detroit, MI. These companies were major slide rule manufacturers or distributors that discontinued slide rule sales and distribution in the late 70's. Company is now named InteliCoat Technologies Azon L.L.C.

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B

Bakelite - A non-flammable phenolic resin, invented by Dr. Leo Baekeland, and founder of Bakelite Corp. that was used from 1910 through WWII. Used for early electrical appliances for its insulating and decorative properties. Used in some slide rules like the Novotni Weight Slide Rule for blacksmiths.

Bamboo - The wooden-like material from the Bamboo grass, used by Asian manufacturers of slide rules for the stock, especially out of Japan. See: Sun Hemmi and Relay. The variety of bamboo used in Japanese slide rules (at least by Hemmi) is mosochiku (scientific name: phyllostachys pubescens). Hemmi shipped it in from Kagoshima Prefecture on Kyushu, the southernmost island of the main four islands of Japan. With a fairly large diameter, it was easy to obtain pieces of this variety of bamboo that were large enough to use in slide rules.^{William Lise}. Bamboo is a light color with light yellow throughout. Easy to precision mill into tongue & groove planks. Over time the colors mellow to a more uniform yellow.

Basic Scales - See Primary Scales.

Basswood - A type of wood used in the construction of U.S. made slide rules with painted or silk-screened scales, from companies such as Lawrence, Engineering Instruments and Acu-Rule. OEM slide rules with Post and Charvoz-Roos brands also may have been constructed with basswood. Basswood grows in the eastern U.S., Great Lakes states, Appalachians and Canada. Related species grow in Mexico. Description: Light, soft, with very fine, even texture, easily worked, good strength for its weight. Experiences high shrinkage when seasoned, but is stable once dry. Color light tan to nearly white. Wood is odorless and tasteless. One of the best carving woods in the world.

Battery - What? Why? History texts will state that the demise of the slide rule was caused by the invention of the electronic calculator, but in some circles it is believed there was a world wide conspiracy of battery manufacturers that sought the end of the non-powered slide rule industry. ...or at least it seems that way (where's my charger?).

Bavaria, Made in - Bavaria is a southern state in modern Germany, Munich being it's most prominent city. In 1806, Nuremberg, the former Free City of the Empire, and large parts of Franconia were incorporated into the newly formed Kingdom of Bavaria. Some A.W.Faber-Castell, Nestler and D&P slide rules are marked with Germany, Made in Germany or Made in Bavaria. The back-stamp was to fulfill the requirements of the McKinley Tariff that was enacted by the U.S. Congress in October 1890 and went into effect in 1891. This law required that all products exported to the United States had to be marked with the country of origin. At first, countries marked their wares with words such as "England," "France," "Germany" and so forth. Sometime after the turn of the 20th century, the U.S. Treasury Department began requiring countries to be more specific in their markings and phrases like "Made in England," and "Made in France" began to appear. The phrase "Made in Bavaria" appeared to be used in the early first quarter of the 20th century, 1900 through WWI (1914-1918), during the realignment of Eastern Europe and the Balkan States, and possibly up to WWII. There was a lot of anti-German sentiment after WWI, so marking products as "Made in Bavaria", helped to continue imports into the US and England.

Beechwood - A wood used to make slide rules in eastern Europe by Löbker & Company and later Meissner K.G.

Beghin, Auguste - Produced his Règle des Escoles in France in 1907. This had a displaced scale by √10 and, with the Mannheim rule, became another standard used by a number of manufacturers. He also produced his Règle à Calculs in 1898.

Bevel - An angular surface that runs along the length of one or both sides of many closed body slide rules. The bevel may contain a linear scale used for measuring metric (centimeters) and/or English (inches), but in some cases may be provide an additional Log scale which read in conjunction, using a line marked on the indicator frame, with the other scales on the face of the slide rule.

Bissaker, Robert - In 1654, Robert Bissaker made the first real slide rule in which the slide worked between parts of a fixed stock. The term, slide rule, applies to all instruments designed so as to allow relative motion between the indices and the scales.

Blundell Rules Limited (B.R.L.) - British manufacturer of slide rules in Luton, England. BRL originated as an effort to create ongoing post-war work for a machine shop created during WW2. The Blundell G.I., was their first slide rule (c.1925) which was made of laminated black Bakelite. This highly brittle material was used for two years before switching to Astralon (PVC) as the prior material had a 40% reject rate. BRL later moved to Weymouth in the UK in January of 1956 and refined their rule making techniques, with the British government as a prime customer. The B.R.L. logo was used at this time. In January of 1964, BRL purchased the assets of W.H. Harling Limited of Clapton, London, and with it, a line of precision drawing instruments, T-squares and related items. Harling had previously made wooden slide rules, but dropped them in favor of its other instruments. The resulting company became Blundell-Harling, and stayed in Weymouth which still manufactures specialist drawing equipment.

Body - The non-movable portion , Stock or Stator of the slide rule made of one or more pieces. Bodies were made of wood (Bamboo, Mahogany, Rosewood, Basswood, Magnolia, Pearwood, Boxwood), celluloid laminated wood, sheet metal (steel), aluminum extrusions, machined from stock and molded plastic. Cheap models may have a body made of folded sheet plastic and cardboard.

Boos, Wilfred J. - (1934-1989) Became president of Acu-Rule Mfg Co. in 1946. Lived in Clayton, Missouri circa 1949. Inventor of record of U.S. Patent 2,594,857 which was assigned to Acu-Rule in St. Louis, MO. The patent was filed in 1949 and issued in 1952. An implementation of this patent is the Charvoz-Roos 105D. It is believed that Boos operated Acu-Design for a short time, independant of the Acu-Rule Company, to provide scale artwork to other slide rule manufacturers.

Boxwood - A wood used to make early slide rules, such as by A.W. Faber. After 1897, Faber began to put a celluloid facing on top of a boxwood body.

Braces - Also called straps. The end pieces on an Open-Body slide rule that connect the two parallel Stator pieces together. Finer slide rules have braces that are assembled with mechanical fasteners that allow adjustment and calibration of the scales. Braces are typically made of non-ferrous metal or plastic.

Braun, Werner von - German rocket scientist responsible for the design of the Nazi V-2 Rocket, and later one of the foremost drivers of NASA space projects. Wernher von Braun brought two 1930s vintage Nestler slide rules with him when he moved to the U.S. after WWII to work on the American space program. Throughout his life he never used any other pocket calculating devices; slide rules obviously served him perfectly well for making quick estimates of rocket design parameters and other figures. (ref: Wikipedia.org) Portrait Slide rule

British Currency - The former British currency system, found on many European slide rule scales, was based on 12 pennies ("pence") to a shilling (sh) and 20 shillings to a Pound. The entire system was: 2 farthings = 1 halfpenny;  2 halfpennies = 1 penny; 12 pennies = 1 shilling;  5 shillings = 1 crown;  20 shillings = 1 pound;  21 shillings = 1 guinea.  Guineas were used in auctions, specially livestock.  Crowns were not in circulation in the 1900's (collectors items, but valid tender). A 6d piece was a 'tanner', a shilling was a 'bob'. The symbol for the penny (d) was based on the abbreviation of the Latin "denarius", a Roman silver coin (which, confusingly, was initially valued at 10 asses and later at 18). The Pound sign "£" was a handwritten "L", based on the Latin for pound,"libra". When Britain decimalized its money in 1971 it changed to 100 New Pence (p) to the Pound, and retained the same Pound sign. Australia, in 1966, and New Zealand, in 1967, decimalized their former British-based currency systems differently, like several other former British colonies, they completely abandoned the Pound, and adopted a dollar of 100 cents. The Australian and New Zealand dollars were equivalent to half the value of their former Pounds. (reference: Cyril Catt)

British pound (£) scale - Reference to the old British pre-decimal currency. There used to be 20 shillings (sh) to a pound (£), and 12 pence (d) to a shilling. Therefore sh.d scale reads 0 to 20 shilling, and 12 divisions denote pence. These designations, sh.d along with the £ symbol were quite common on European business rules, often in combination with percent, interest or compound interest scales. Because it was not a decimal system, calculations involving parts of a monetary Pound had to be converted to a decimal of a Pound before they could be calculated on a slide rule. Hence the conversions on the log 10 scale.

British Slide Rule Brands - Slide rules manufactured or re-branded by British companies were British Thornton, P.I.C., Unique, Classic, Otis King, Mechanical engineer, Blundell Harling, Boots Ringplan, Jakar, Norton & Gregory, Simplon, Stanley, Technical Standards, W.H. Smith & Son, Fowler, (list incomplete).

Burgi, Jost - Jost Burgi (1552-1632) is also attributed to the invention of logarithms along with John Napier (1550-1617) who gets most of the credit.

Business and Finance Rule See also Financial Rule. A special rule (ex: Pickett 510) that has scales that permit one familiar with finance formulas to solve such problems directly, the basic formula being (1 + i)ⁿ.

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C

C' - Gauge mark found on the C scale denoting √(40/π) = 3.568

C scale - x, Principle scale used with D scale to solve multiplication and division problems. To multiply two numbers on a typical slide rule, the user set the left index (start of the scale) on the C scale to line up with one factor on the D scale Division is performed by reversing the multiplication steps by setting the divisor on the C scale opposite the dividend on the D scale and reading the result of the D scale under the C scale index.

C scale (electrical) - Capacitive value in Farads [Pickett N16-ES Electronic slide rule].

C1 scale - Found on some pocket slide rules to give the same precision as a 10" long slide rule. Also D1. Not to be confused with the CI scale.

C2 scale - fx, Shifted scale, functions vary from manufacturers.

C% scale - On a Pickett financial slide rule denoting percent Profit.

Cajori, Florian - (1859-1930) Emigrated to the United States from Switzerland in 1875. Educated at Wisconsin University and John Hopkins and later was Deamn of mathematics at Colorado College. Received his doctorate from Tulane University. He is best known amongst slide rule historians for his book History of the Logarithmic Slide Rule (5.6MB PDF) published in 1909, in which he described 256 designs of slide rules since 1800, 90 of which were produced in the first nine years of the 20th century. Florian Cajori Biography by J J O'Connor and E F Robertson (2000)

Calculigraphe - A watch style circular slide rule manufactured by Henri Chatelaine of Paris, with double-sided bevelled glass. The case was nickel plated. The instrument has two faces, one for numbers, the other (spiral) for sines and tangents. The main key turns the logarithmic scale and the smaller key large pointers on each face. Using the two pointers (one fixed) on the face shown, any interval on the scales (the outer being uniform, the inner logarithmic) may be recorded, and by the subsequent movement of the scales added to any other part of the scales. This was of the Boucher's style design based on the French Patent Breveté, no. 114,520, 13th September 1876. Boucher's patent specification (no. 4310) was taken out in 1876.

Capitol Radio Engineering Institute - CREI - A correspondence school in Washington, D.C. that taught vacuum tube theory from 1927-1954. After WWII, CREI commissioned Pickett to make a slide rule specifically for their students. CREI became CIT in 1955, then in 1984 became Capitol College, still located in Laurel, Maryland. The Pickett N531-ES had the following scales: L, Ln, A [B, CI, C] 2p ,K and LL2, LL1 [S, ST, T, C] D, LL3.

Celluloid - The generic name for Cellulose Nitrate, an early plastic made from plant products, that was used to coat or laminate wooden slide rules, first innovated by Dennert & Pape (D&P, later Aristo). Research on Celluloid was begun in the mid 1800's as a replacement for a dwindling supply ivory, and John Wesley Hyatt obtained several patents around 1872, specifically dealing with the manufacturing processes and its use in dental products. In 1886 D&P first introduced white celluloid scales on slide rules and rulers. No maker of slide rules after 1890 ever used ivory, but it is possible prior to 1890. Celluloid compounds were given different names by various manufacturers: Pyralin/Dietzgen, Xlonite/K&E (later Ivorite) and Ivorine/Dennert&Pape (Aristo). From an 1897 Pickworth manual, Instructions for the use of A.W. Faber's Calculating Rule: "The white surface of the celluloid throws the division lines into more distinct relief than is the case with the all-boxwood construction, and in general it is less fatiguing to the eyes. The only objection to the celluloid-faced rule is, that this facing material is somewhat unreliable under variations of atmospheric conditions. However, as the graduations of the Calculating Rule are in every case comparative and not absolute, the expansion and contraction cannot affect the accuracy of the instrument" .Today, highly-flammable celluloid is only used to make billiard balls, having been replaced by other engineered plastics.

CF scale - Folded principle scale where every number on the C scale is multiplied by π. CF= C x π. Note: Scale begins at π (3.14) with 1 in the middle, rather than starting at 1.

CF/π scale - Pickett labeling of a normal CF scale folded at π. Labeled this way to differentiate from their CF/M scale on a model N4.

CF/M scale - Scale unique to Pickett N4 Dual Base Log Log slide rules folded at 1/M=2.30, where the modulus M=log₁₀e. See DF/M. With these scales, logarithms to base 10 may be read from the ordinary C or D scales.

Chained Calculations - Repeated multiplications, where the cursor is used to hold the result of one calculation on D, while the index on C is brought up for the next.

Chan Street - Chan Street of Street Laboratory and Industries of El Segundo, California, designed the Pickett Electronic N-16-ES slide rule, circa 1960, which is labeled as having been "Designed by Chan Street, Los Angeles" (California). This is a specialty slide rule having scales on one side for electronic frequency and impedance calculations to determine tuned circuits. Θ Θ, db, D or Q, XL , Zs or Xc [ C or L, F, λ, ω, T, TR' or X'c, Cr ] Lr, db, CosΘ. The opposite side had a standard Pickett hyperbolic scale set. It is unknown if Chan Street's layout was used on other slide rules. More would like to be known about this individual.

Characteristic - The integral part of the logarithm of a number. See Logarithm and Mantissa.

Charvoz, Andre and Angele R. - Andre Charvoz (1892-July, 1969 – died Clifton, New Jersey), Andre Charvoz, born in France, and became president of Manufacture Francais de Compas of Ligney-en-Barre, France. Later immigrated to the U.S.A. and was a founder of Charvoz, Inc in 1939 and with his wife Angele, were partners with another family, the Gottheimer's, in the Roos Company which manufactured slide rules in New York. Andre's wife, and business partner was Angele Charvoz (1900-February 1982 – died Clifton, New Jersey).

Charvoz-Roos - A U.S. distributor of drafting and engineering supplies based at one time in 50 Colfax Ave, Clifton, New Jersey, that sold Charvoz-Roos branded slide rules. In November, 1939, Charvoz, Inc., formed at 185 Hackensack Street in East Rutherford, NJ. as a joint venture between Andre Charvoz, president of Manufacture Francais de Compas of Ligney-en-Barre, France, and Samuel Sanders of Teaneck, N.J., president of L. Oppleman, Inc. The Roos Company of 280 Madison Ave., New York, was a partnership between Andre and Angele Charvoz, and Abel and Ethel R. Gottheimer, that was created (c1941) to manufacture painted wood and laminated wood slide rules in the United States presumably to replace foreign imports of slide rules during WWII. It is possible that parts of the Charvoz-Roos early slide rules were supplied by the U.S. slide rule manufacturer Acu-Rule as many parts are identical with other brands of the time. Roos began distributing Charvoz drafting supplies by 1945. After the war, Roos Company merged with Charvoz Inc. into Charvoz-Roos Corporation on June 14, 1946 and began importing re-branded Relay/Ricoh slide rules from "Occupied Japan". The manufacturing plant in East Rutherford, New Jersey was sold in 1953 and the Charvoz-Roos Manufacturing Company was in receivership. Around 1954 the company became just Charvoz, and slide rules were imported from the German company Aristo. Early Roos and Charvoz-Roos slide rules were identical to Post 1442, 1447-D, 1452-D, 1462 and Henschel 2020. A Charvoz-Roos PSR-200 is identical to a Relay 403 and an Aristo 909 is branded with just 'Charvoz'.

Chemical Milling - See Photoetching

Chemist's Rule - Most major manufacturers made a version of a slide rule with specific scales to aid in the mixing of chemical compounds, to solve reacting-weight equations (K&E 4160, Post 1490, Hemmi 157).

Chinese Slide Rule Brands - Slide rules manufactured or re-branded by Chinese companies were China Sha, China Shanghai, Ding Feng, Flying Fish, Haiou Pai, Hangzhou, Jang, Paper, Ruyi, Shanghai, Sida, Xuesha, and Tianjin Squirrel (list incomplete).

Chisel Cursor - (or Metallic Runner) A one piece metal, typically brass, cursor used prior to 1900 and as late as 1905, to assist in the transposition of reading the values on simple slide rules, typically with only the A[B,C]D scale set. Either the left or right side was used to align the divisions on the scales. The chisel cursor was replaced by glass cursors with central hairlines mounted on celluloid spacers or metal frames.

CI scale - 1/x, This scale is the reciprocal of C scale.

CIF scale  - Folded scale of the reciprocal of C. See CF scale.

Circular Slide Rule - A slide rule bent around in a circle. The cursor is a pair of radial arms that move around the origin. The advantage of the circular slide rule is that you can fit a longer scale, and many more tick marks, within a certain area. Circular slide rules and rules with spiral scales were made before 1733, but their inventors are unknown. One model is still being manufactured today by Concise in Japan.

Cleveland Institute of Electronics - CIE - Founded in 1934, CIE was a home-study electronics school and after WWII, commissioned Pickett to make a slide rule specifically for their students. CIE still operates out of Cleveland, Ohio. The Pickett N515-ES had the following scales: (L_G), (fx)2p, A [(Cr)B, S, T, (Lxor Cx)CI, Xlor Fr-C] D(XL), L, L_n.

Closed-Body Slide Rule - A slide rule having a body fashioned out of one or more pieces that has a bridge between the upper and lower halves, common to earlier designs. The channel formed by the bridge sometimes contains additional scales or tables. See Open-Body Slide Rule.

Code Maker - A specialty type of slide rule, popular with kids, that was made by Lawrence Engineering right after WWII for making secret messages by reassigning the values of the alphabet. The slide rule could be used for coding and decoding messages, as long as the user knew what 'key' was assigned by the original writer. The pocket version was the Dick Tracy Secret Code Maker named after the famous cartoon detective.

Coggeshall, Henry - (1623-1690) Suffolk, England, an expert in mensuration and timber gauging. In 1677, he combined the proportional circle with a slide rule, thus developing the 'polymeter', the earliest standard slide rule design. This instrument was a a standard linear design, but in 1682, he used the basis to establish the carpenter's two-fold, two-foot rule, which incorporated a 'sliding Gunter' in one leg of the rule for performing timber calculations.

Coggeshall Rule - Made by Stanley Rule and Level Co., New Britain, Connecticut, 32x4x.4 cm. Wood and Brass hinged on two sides. Had Navigational scales including meridian, chords, latitudes, and hours inscribed.

Compass - American brand of slide rule manufactured by Relay/Ricoh out of Japan. The Compass 1321 was the same as a Relay 150 and the Compass 505 is the same as a Relay 505. Known date codes span 1956 through 1961.

Computer - Up to 1960, "computer" was understood to be a person who computed usually with a slide rule close at hand.. A computer who must make many difficult calculations. Now dictionaries begin the only definition of "computer" with "An electronic machine..."

Competition, Slide Rule Competitions were and are still held to challenge mathematical problem solving using slide rules. Japanese students in Tokyo competed as early as 1952. US Schools in the 40's had student contests in "Slide Rule and Number Sense". Up until 1980, a national US competition was held in Texas by the University Interscholastic League (UIL) an extension of The University of Texas founded in 1909. The Pickett 905-ES Texas Speed Rule was designed for this competition. Cole's manuals (like Cliff Notes) were available to prepare for district, regional, and state slide rule competition in the UIL. The UIL still holds competitions but the slide rule contest was phased out for the calculator. Results from 1945-1980 Since 2001, there is an on-going World Championship Slide Rule Competition in Texas, promoted mostly by slide rule collectors. The 2006 championship was held on March 4th in Irving, TX.

Concise - A Japanese company that still makes slide rules of the circular variety. They provided re-branded, promotional and custom products to Pickett, Scientific Instruments, Lafayette, Fullerton, Sama & Etani and others. Company Link

Concrete Quantity Calculator - A slide rule with scales to calculate the volume in cubic yards or meters of concrete required to fill a 3-dimensional space. Typical scales are Thickness(A), Width or Height(B), Length(C) and Volume(D).

Condition of Slide Rule The condition of a slide rule may be described with the slide rule condition rating system of the Dutch Circle of Slide Rule Collectors, endorsed and used by The Oughtred Society :
    C0 Mint condition, with all extras such as box, case, instruction
        manual present also in mint condition, factory clean.
    C1 Mint condition with all extras such as box, case, instruction
         manual present, but these extras not all in mint condition.
    C2 Mint condition, but without one or more of the extras.
    C3 Very minimal signs of use (first impression is that it is new,
        but careful inspection reveals a slight scratch).
    C4 Minimal signs of use (say, on the back side of an end
        plate one can see that it was used; otherwise very nice).
    C5 Signs of normal use (one-third of all slide rules are C5).
    C6 Signs of heavy use (ugly, but not damaged).
    C7 Small damage, but the rest quality C4 or better.
    C8 Damaged and well worn.

Connor Spring Co. - An American spring manufacturer that commissioned Pickett to make a specialized helical spring design slide rule called the Pickett Model 1090-ES (c1962). Connor Spring Manufacturing Company had offices at the time in Los Angeles and San Francisco, California.

Connecticut Spring Corporation - An American spring manufacturer that commissioned spring design slide rules from Pickett as well as Acu-Rule. Pickett Model 1072 (c1962)show CSC in Farmington, Connecticut. The Acu-rule version shows CSC in Hartford, Connecticut. See also Associated Spring Corp for main competitor.

Cooke Radio Rule - The 4139 Duplex was manufactured by K&E for Cooke Radio to speed up calculations for electrical and radio engineers. It has an LC scales used to determine the frequency (Kilohertz (F)) of an LC (Inductor (L) Capacitor (C)) circuit as in F=1/(2π√LC).

Copyright - "A legal device that gives the author of a work the right to control how the work is used and protects the author’s expression of ideas or facts, but not the ideas or facts themselves. The life of a copyright depends on the publication date. For recent works after 1977, the life of the author plus 70 years, or if work is for hire or anonymous, then 95 to 120 years. For works during 1923 and 1977, 95 years. Works before 1923 are in the public domain." (reference: Joe Pasquale University of California). The copyright law of the United States (Title 17, U.S. Code) governs the making of photocopies or other reproductions of copyrighted material. Under certain conditions specified in the law, libraries and archives are authorized to furnish a photocopy or other reproduction. One of these specified conditions is that the photocopy or reproductions is not to be used for any purpose other than private study, scholarship, or research. If a photocopy or reproduction is used for purposes in excess of "fair use," the user may be liable for copyright infringement. Link to Cornell Law school

Cos S scale - An alternate labeling of the S scale. See S scale.

Cos Θ scale - Relative gain in coupling circuit [Pickett N16-ES Electronic slide rule].

Cox, William - A mathematical consultant to Keuffel and Esser Co.,  invented a revolutionary slide rule construction, with scales on both the front and back surfaces of body and slide and with a double-faced indicator referring to all scales simultaneously. This Duplex slide rule was patented in 1891. The first Cox rules were made by Dennert & Pape, Germany for K&E. With the manufacture of Mannheim rules and this new rule, K&E became the first commercial manufacturer of slide rules in the United States.

C_r & L_r scale - On a Pickett N16-ES Electronic slide rule, for any frequency read at arrow on F scale, the values of resonant condensor and inductance are opposite each other on C_r and L_r scale.

Cu¹ - Gauge mark equivalent to √(4/πρ) = 0.1493. Note:ρ=0.0175Ωmm²/m at 20°C, ρ is the small Greek letter rho. Used for electrical calculation for copper conductors.

Cu² - Gauge mark equivalent to √(4/πγ) = 0.1493. Note:γ=8.87g/cm³, γ is the small Greek letter gamma. g is the gravitational constant 9.81 meters/sec². Used for electrical calculation for copper conductors.

Cube Root or 3√ scale - Cube and cube roots on the number on C or D. (Pickett).

Cubes and Cube Roots - On a Mannheim rule, with only A [B, C] D scales, cubes and cube roots as well as higher powers and roots can be found by the use of tables of logarithms and antilogarithms. On a rule with a K scale, the cube on A is found on K. The cube root on K is found on A.

Cursor - the sliding window, Runner or Indicator, with one or more hairlines used to transfer readings from one scale on the stock, or body, to another scale on the slide or stock. The cursor, perhaps invented either by Robert Bissaker or Isaac Newton, became known around 1778, in a slide rule designed by John Robertson, and the use of C and D scales for multiplication, which required the aid of an indicator to transpose calculations, had been introduced even earlier, by a British excise examiner named J. Vero (or Verie). In 1851, Victor Mayer Mannheim, introduced a transparent slab movable cursor for the slide rule of his design. Other modifications and improvements were introduced by Dennert (1886), Cox (1897), Beghin (1899), Furle (1899), Schweth (1901), Rietz (1902), Walther from the Politechnic of Darmstadt (1936). Early cursors were metal frames holding glass, followed by glass mounted on celluloid guides and spacers. Later cursors used molded acrylics or polycarbonates sliding on Teflon bearings (Dietzgen).

Cursor Scale Labels - Scale labels imprinted on the lens of the cursor. Found on 1960 and later S.I.C. (Scientific Instruments Corp) slide rules as an aid to the user. Although a unique product differentiator, no other slide rule manufacturers found it practical to implement.

Curta - A sophisticated hand-held cylindrical mechanical calculating machine made in Liechtenstein after World War II to a design by Curt Herzstark, a Austrian concentration camp inmate. It has a small crank on the top used to process the calculations. More at The Curta Page

CV - A French unit for tax horsepower, short for chevaux vapeur ("steam horses") or cheval-vapeur. CV is Metric Horsepower 1 CV = 736 watts, as compared to US mechanical horsepower of 746 Watts. CV is found on French slide rules such as the Marc(Paris)(UNIS France). In Italian ("Cavalli"), Spanish ("Caballos"), and Portuguese ("Cavalos"), 'CV' is the equivalent to the German 'PS'.

C.V. Ore - There are two known examples of a S-M slide rule labeled C.V. Ore, Harvey, Illinois. It is unknown if C.V. Ore is a person's name or a mining company. The earlier one is a simple wooden slide rule, assumed to have been manufactured by Lawrence Engineering, as the cursor is identical to the standard models and the slide is also interchangeable . The other is a later all-plastic design assumed to be manufactured by Acu-rule or Acu-math. Both are labeled 'Copyright 1955' and have the same scales: DS [ CI-a, CI-b, C ] D, DM . The function of the S-M scales is unknown, but could refer to statistical calculations or they could have been used for teaching multiplication of division of integers as there are no sub-divided divisions between the main divisions (whole numbers) on the scales. The DS scale is in 1's, the DM scale is in 10's. The C scale is the conventional numbers with divisions between the whole numbers. This may have been an easier way to learn slide rule basics.

Cycle(s) - The term cycle is used in slide rules to define one logarithmic scale is numbered from 1 to 10, such as found on the C, DCI, DI, CIF, DIF scales. A two-cycle scale or double cycle logarithmic scale is numbered 1 to 10 to 100, such as found on theA and B scales. The K scale is a three-cycle or triple logarithmic scale.

Cylindrical Slide Rule - Similar in principle to a linear slide rule, but the scales are wrapped around a cylinder. Cylindrical calculators have extra long logarithmic scales, over 60" long, and were invented by George Fuller, of Belfast, Ireland, in 1878 and Edwin Thacher, of New York, in 1881.These are the most accurate of slide rules as the scale have more divisions.

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D

D scale - x, Principle scale used with C scale to solve multiplication and division problems. Consists of one logarithmic cycle 1-10. See C scale.

D or Q - Quality factor of capacitance or inductive circuits (capacitance will normally have some resistive factor). A coil always has some losses so Q will always equal something less than infinity [Pickett N16-ES Electronic slide rule].

D1 scale - Found on some pocket slide rules, like the Roos-108 and Post 1442, to give the same precision as a 10" long slide rule. Also C1. Not to be confused with the DI scale.

D2 scale - fx, Shifted scale, functions vary from manufacturers.

D* scale - Used in conjunction with the D scale to keep track of the decimal place in a result. Found on some Pickett slide rules. The scale ranges from 10^-10 to 0 to 10⁺¹⁰. See Decimal keeper and Decimal Tracking

Darmstadt - Also called System Darmstadt. An arrangement of scales originally found on German made slide rules by Aristo (D&P). Professor Alwin Walther of Darmstadt, in 1935, shifted the scales for the trigonometric functions of the slide, or tongue, onto the body of the slide rule. On the slide an exponential scale was placed along with other scales. Trigonometric scales that are dual-labeled, in black and red, with complementary angles, are considered "Darmstadt" style. In order to be called a Darmstadt, the slide rule must have a P scale and three LL scales. Scales: L, K, A, [ B, CI, C ] D, P, S, T and on back of slide: [ LL1, LL2, LL3].

Data Center - A slide rule designed by Sama & Etani and manufactured by Concise, Japan that incorporated a circular slide rule mounted on a rectangular sleeve that held several pull-out data cards with mathematical tables and information like the Periodic Table of the Elements. The manufacture of the Data Center that had a periodic table was discontinued, purportedly, due to complaints from teachers, who said that students were using them to cheat on exams.

Data Slide Rule - The data slide rule has only one special purpose with scales arranged in such an order to facilitate rapid sequential calculations in the particular technical field for which it is designed. Additional information in the form of tables, diagrams, pictographs, marks, arrows and formula may be supplied in order to assist the user in its operation. In the case of a regular slide rule, the user must assign the decimal point, units of measurement and the context to the scales and apply his knowledge to those tasks. Pickett, K&E, Perrygraf, Concise, IWA and others produced such slide rules and slide charts.

Date Code - A method of marking a slide rule that denotes its date of manufacture. Generally only found on upper end slide rules. This could take the form of a small, almost indiscernible, two letter year/month code pressed into the stock (Hemmi, Post) or by the use of a serial number (Hemmi, Post, K&E, Dietzgen). Later models of Aristo rules all have a manufacturing code on one edge of the rule. This is usually a combination of alpha and numeric codes which defines the rule's factory of manufacture, year of manufacture and batch number.

Dating - The method of determining the age or date of manufacture of a slide rule. Dating a slide rule becomes difficult if a manufacturer did not imprinting a date code or serial number on the stock. Even then, some manufacturers recycled their serial numbers over time, and the stock could have been left on a shelf for a period of time before the slide rule was assembled and shipped. Sometimes the copyright date in a manual is used to determine the approximate age, but one printing could span a decade. Dates that a patent was issued only suggests the first year a model may have been produced, but the patent may only refer to a certain combination of scales used over a thirty year time on different models. Having contact with the original owner of a slide rule that remembers the year it was purchased helps on determining the age. See Slide Rule Dates, Determine the Age

db scale - Decibels, Power or voltage loss in coupling circuit (ratio of voltages or power) same as extended range scale on lower stator [Pickett N16-ES Electronic slide rule].

DDR - Deutschen Demokratischen Republik (DDR) or German Democratic Republic (GDR). Marks slide rules as being made in East Germany, after 1949, such as the VEB Mantissa and Reiss.

Decimal Keeper Slide Rule - A slide rule model made by Pickett with special scales to assist the user or student in keeping track of the order of magnitude of the result of a calculation. Picket models 115 Basic math and 904-ES/904-T Decimal Keeper Speed Rule use scales Marked with an asterisk(*) to perform this function. The one-sided simplex Pickett 115 has: D*, X* [ Y*, CI, C ] D, A, L scales. The 904 duplex has one side dedicated to decimal tracking: K*, A* [ B*, T*, S*, CI*, C*] D*, L* which are used with the standard scales on the front: K, A [ B, T, S, CI, C] D, L.

Decimal Tracking Cursor - A cursor that appeared generally pre-WWII on many brands including Hemmi, Post Faber-Castell, K&E and Nestler. It consisted of a small arrow that would be positioned by the user during a series of calculations to keep track of the order of magnitude of the calculated results. Typically: -10, 15, 0, +5, +10 in various increments, depending on the manufacturer.

Decimal Tracking Scale - Special scales which have range, such as -10, 15, 0, +5, +10 in various increments, that cover several log cycles. The zero is generally positioned in the center of the scale on the slide rule. Calculations are performed once with the special scales, to get the order of magnitude correct, and then with with C and D scales to get the answer to a higher degree of precision. These scales appear on a Pickett (Model 1) Deci-point and later on the more modern Pickett 115 and 904. See Decimal Keeper

Decimal Point, Determining the (Slider position method) - Where only two factors are involved, a practice is used by which the position of the decimal point in the answer can always be determined directly. Consider: 348.4 has 3 places, 0.456 has 0 places and .00678 has -2 places. Depending on whether the slider projects to the left or projects to the right, you add (multiplication) or subtract (division) the number of places for your final answer according to:
  Slider Projects to Left - Multiplication: add number of places of each factor.
  Slider Projects to Right - Multiplication: add number of places of each factor - 1.
  Slider Projects to Left - Division: subtract the difference of the number of places of each factor.
  Slider Projects to Right - Division: subtract the difference of the number of places of each factor + 1.
You will notice on some early Hemmi's in the corners a symbol like (+ | -)/(- | +). These are aids or hints for remembering how to apply the above rules: (Mult=Sum | Mult=Sum-1) / (Div=Diff | Div=Diff+1)

Decitrig  - A Decimal-Trig rule with the trigonometric scales (S, ST, T) marked in degrees and tenths of a degree rather rather than in degrees and minutes of degree.

Deci-Lon - Trademark name by K&E for their most comprehensive slide rule, with 28 scales. It was first introduced in 1962. Standard size Deci-Lon's were models model 68-1100 and the pocket version was model 68-1130. A 6 foot training version of the pocket Deci-Lon was model 68-1929.

Deeva - Brand of plastic slide rules made in India under license by Diwa, Denmark. circa 1970. Known models: Deeva 601, Polytek 707, Teknilog 710, and Polylog 720.

Delta - Small Greek letter δ or Large Greek letter Δ.

Dennert, Johann Christian - (1829-1920) co-founded of the firm Dennert & Pape (D&P) in 1862. Patented (DRP 34583) celluloid scales laminated on wood slide rules in 1886. Became the sole owner of D&P when Martin Pape died in 1884. Children Richard (1865-1924) and Jean Dennert (1869-1916) join (1869-1916) join the company as managers in 1904. J.C. Dennert worked until age 80 in 1908, died in 1920.

Dennert, George - (1900-1992) Grandson of Johann Christian Dennert, the co-founder of Dennert & Pape, and son of Richard Dennert. Became joint owner of D&P, along with his cousin Christian Dennert (1896-1944) son of Jean Dennert, after death of their parents, Jean (1916) and Richard (1924) in 1924. Created the brand name DUPA for the slide rules.

Dennert, Hans - (1926-2000) Born in Hamburg Germany to George Dennert, and great-grandson of Johann Christian Dennert, the co-founder of Dennert & Pape, later Aristo. Graduated from Darmstadt University with an engineering degree, and one year later he took over the company from his father in 1952. Aristo became one of the largest slide rule manufacturers in the world, but ceased production in 1978. Hans continued to provide assistance to slide rule collectors and was a highly sought after lecturer up until his death in 2000.

Dennert & Pape. See: Aristo.

DF scale - πx, Folded scale where every number on the D scale is multiplied by π.. DF= D x π. Note: Scale begins at π (3.14) with 1 in the middle, rather than starting at 1. For use where calculations involve π to reduce the number of settings.

DF/π scale - Pickett labeling of a normal DF scale folded at π. Labeled this way to differentiate from their DF/M scale on a model N4.

DF/M scale - Scale unique to Pickett N4 Dual Base Log Log slide rules folded at 1/M=2.30, where the modulas M=log₁₀e. See CF/M. With these scales, logarithms to base 10 may be read from the ordinary C or D scales.

DI scale - 1/x,  the reciprocal of D.

Dieterich-Post Co. - A California based, US company, founded in 1917, that currently sells modern printer, plotters, CAD media and related engineering drafting supplies. A 1949 catalog shows that they sold slide rules along with other technical equipment of the day. The slide rules looked like they were made by Hemmi in Japan. An all-plastic slide rule, labeled Dieterich-Post 3060P, was made in the USA and appears to have been produced by Charles Bruning Co. It is unclear if there was a relationship with Frederick Post Co. or if the names were coincidental. Company motto: "Builders of Goodwill".

Dietzgen - An American slide rule manufacturer, founded in 1885 by Eugene Dietzgen. The first manufacturing company opened in 1893 producing T-squares, wooden triangles, drawing boards, hectograph inks, surveying instruments, blue print and Van Dyke papers, Dietzgen's historic line of engineering products. In 1908 the company acquired its own drafting instruments factory in Germany and introduced their "Lifetime Service Policy" on drafting Instruments. The company later OEM'ed slide rules from Faber-Castell and other engineering items and drafting instruments from Germany and from Ricoh in Japan for the American Market. Often mistaken for K&E slide rules, as many parts are interchangeable as the physical dimensions and construction are similar. The main factory and head office was in Chicago, Illinois, and branch offices were in New York, San Francisco, New Orleans, Pittsburgh and Los Angeles.

Dietzgen, Eugene - (1862-1929) A German inventor, born in Uckerrath, Germany on May 6, 1862, who immigrated to America in 1878 and founded his own engineering supply house, as both a dealership and manufacturers' representative in Chicago, Illinois. The company was founded on November 13, 1885 under the name of Luhring & Dietzgen. This partnership was succeeded by Eugene Dietzgen & Co. in 1891. Successor corporation under the name of Eugene Dietzgen Co. of Illinois occurred on February 3, 1893 and the present corporate company, Eugene Dietzgen Co. of Delaware was incorporated in Delaware on October 29, 1917. He died in Chicago on December 1, 1929. His son, Joseph E. Dietzgen, continued to work in the company and had a U.S. Patent #2,369,819 issued in 1945.

Dietzgen, Joseph E. - Son of Eugene Dietzgen, founder of Eugene Dietzgen Company. Joseph E. Dietzgen, continued to work in the company after his father's death, residing in Wanatka, Illinois, and had a U.S. Patent #2,369,819 issued in 1945 for a Log Log Duplex slide rule and later U.S. Patent #2,543,313 in 1951 for a belt clip for a slide rule sleeve.

DIF scale - 1/πx, folded scale of the reciprocal of the principle scale D. See CF scale.

Digit Registering Cursor - A counter on the cursor that keeps track of the number of decimal places  when using Scientific Notation. The rules with these cursors were only produced for about 10 years in the early 20th century, principally by Faber-Castell.

Directional symbol - < - This is a Pickett innovation that is placed next to numbers (left side) on reciprocal and Log scales to inform the user that the scale is increasing numerically from right to left. This is handy on low-end slide rules that are inked all in one color, black. High end rules have the direction noted by Red Scales and K&E and others lean the numbers in the direction of increasing magnitude.

Displaced Scale - A scale may have its index placed near the middle of the the rule, which can reduce the number of movements during a calculation. Typically the A and B scale are chosen to be displaced, the distance varying by manufacturer, the most common being √10 and π.

Dividing and Numbering of Slide Rules The subdivisions of the 5 inch rule range from 50 down to 10 between the prime numbers. the 8 inch rule is divided twice as close as the 5 inch, having subdivisions ranging from 100 down to 20 between the prime numbers. The 10 inch rule, considered the standard, is subdivided the same as the 8 inch. The 20 inch rule is divided more closely than the 8 and 10 inch rules, having subdivisions ranging from 200 down to 50 between the prime numbers (text from a 1938 Dietzgen catalog).

Dividing Engine - A machine consisting of gears, used to accurately and repeatedly engrave the divisions on a slide rule. Although dividing engines were in existence in the 1700's, most slide rules were hand scribed from a master pattern using calipers to transfer the measurements, until such time as automatic machinery became popular in the late 1800's.

Division (Calculating) - In division, the divisor set on the slide is brought to the dividend on the rule, and opposite the index on the slide will be the quotient on the rule.

Divisions - The small lines, tick marks, or graduations on the scales of a slide rules that correspond to numeric values in descending decimal places. Generally, the more divisions a slide rule has the better the resolution and precision. The longest divisions, associated with the most significant digit of a number, is called the Primary, followed by the Secondary or second significant digit, then the Tertiary or third significant digit. A fourth significant digit is possible through approximation. See also Sub-divisions.

Diwa - Regnestockefabrik, gentofte, Denmark. Established in 1924 as a small shop making of slide rules and measuring devices in Denmark, primarily to Scandinavian countries. After WWII, 16 models were produced, and exported. Some sold through a Lawes Rabjohns Catalog. Students at the university and teachers' colleges in Denmark were required to use the Rietz-Ideal slide rule. Supplier to India brand Deeva. DIWA no longer exists as a company.

Donkin and Company, Ltd - Marine engineering company (1879-1977) that started out making Steering gear registers. They had a ship's wheel as part of their logo and provided a promotional slide rule, manufacturer unknown.

DRGM - Deutches Reichs Gebrauchsmuster. Same as U.S. "registered trademark" and designs found on Aristo, Faber-Castell and other German slide rules. Equivalent to the French "marque depose". See Trademarks.

DR/DRP - German patent, Deutsches Reichs Patent. See Patents.

Dual 10"/5" Slide Rule - A slide rule which combines the convenience of the 5-inch pocket size rule with the precision of the standard 10-inch rule. One one side the C and D scales are split into the C1, C2, D1 and D2 scales which are folded at √10 (3.162). Charvoz-Roos SR-108, Post 1442 (circa 1944) and Unique Five-Ten are good examples. The Unique Ten-Twenty utilizes the same principle, turning a 10-inch rule into the equivalent 20-inch rule.

Dual Base - A rule with scales that read both common log base 10 and natural log base e of a given number.

DUPA - A trademark or brand name instituted by George Dennert after taking over the management of Dennert & Pape and used on all products between 1925 and 1936. After 1936, D&P slide rules were entirely made of plastic (Aristopal) and the slide rules were marked with the brand name of Aristo. DUPA was still used on other types of products.

Duplo - A brand name for duplex plastic slide rules made by Deeva in India. A quote from the Deeva 1720 Polylog manual: "The DUPLO Slide Rule is a delicate precision-instrument and should be treated as such. Above all protect the rule from heat over 50 C (120 F) and from direct exposure to glaring sunlight".

Duplex - An open double-faced slide rule. See Open-Body Slide Rule. Typically added three folded scales (CF, CIF, DF) to those of the Polyphase rule to make many problems easier to solve. Generally with a wrap around cursor. More advanced models having multiple cursor hairlines. Invented by a K&E mathematician in 1891. See Cox, William.

Duraplast - A brand name of plastic used in the construction of Staedtler Mars, a German brand of slide rules.

Dutch Circle of Slide Rule Collectors (Kring) Kring was founded in 1991 and has grown into an informal group of about 50 collectors, sharing the following goals of mutual support of members for the collecting and research and the promotion of public knowledge and awareness of the historical interest of the slide rule. These goals are pursued by issuing their periodical "MIR", by organizing meetings, presence at fairs, by publishing papers, books, catalogues, interviews and by communicating with Dutch and international communities of collectors and other interested parties. In 2004 the area of interest was expanded, and the name of the Circle was changed to Kring or Historische Rekeninstrumenten (Dutch Circle for Historical Calculating Instruments). The web site is translated into many languages.

Dynamo/Motor - %, Electrical efficiency, numbered 20 to 100, often in well of stock.

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E

E1 scale -  Same as LL2 scale, e^0.1x, Exponent of x.10^-1, 1.1 to 3.0, Raising to a power x>1.

E2 scale - Same as LL3 scale, e^x, Exponent of x., 2.6 to 10⁵, Raising to a power x>1.

E6B - A circular slide rule with a slide used as an aid in aviation navigation and pilotage. Developed prior to WWII, this slide rule is still manufactured today for student private pilots. Calculates distance, air and ground speed, wind direction, fuel range, pressure altitude.

Easterly, John I. - (1885-1974), born in rural Missouri became a licensed California Architect in 1926. Served two three-year hitches in the U.S. Army and fought in the Philippines. Mfg. Received his first patent in 1936 for a "Combination Square and Slide Rule" and in 1940 in Healdsburg, CA, Easterly designed the Easterly Rafter Calculator slide rule. Two wood versions were produced, along with a plastic circular model and an aluminum model by Pickett. As an architect, Easterly designed churches and schools for many years. John I. Easterly (Pop) portrait.

Easterly Rafter Calculator - Used to calculate the length of wooden rafters in building construction based on the horizontal span and rise in inches per foot. Designed and built by John I. Easterly, Mfg. of Healdsburg, CA the Easterly Rafter Calculator slide rule, around 1940. It was a wood stock model with a paper laminate with hand drawn scales and brass brackets. It was soon revised to incorporate printed scales. A circular plastic version was also made in 1955 and a later linear version, copyright 1957, with the company located in Watsonville, CA, was made by Pickett as Model 1082-ES and called the Easterly Rafter Framer.

Eckel, Arthur F. - (?-?) A.F. Eckel, cofounded R.C. Pickett & A.F. Eckel Companyin 1943 with Ross C. Pickett. He was married to Myrtle Eckel, while residing in Evanston, Illinois. He left the company in 1948, which was then renamed P&E. A U.S. 1949 patent #2.466,983 was issued to A.F. Eckel with Louise Pickett being the assignee. Eckel later formed Precision Scale Company located in Phoenix, Arizona, producing all-metal slide rules to limited markets. George Washington University in Washington D.C. shows an A.F. Eckel as having graduated there in 1922. His estate sale in Phoenix produced several unfinished slide rules.

Ecobra - Bayerische Reisszeugfabrik AG, formerly Reisszeligfabrik Eichmoller & Co. An obscure German manufacturer, in Nuremberg, who also sold OEM aluminum slide rules for to Dietzgen (US) and promotional purposes. Ecobra was bought out by Dietzgen in the 1970's.

Eff scale - Efficiency scale for generators and motors.

EI scale - A doubled CI scale found on Scientific Instruments Co. (SIC) circular slide rules, models 120 and 130. Where the CI scale is completed or joined together in 360 degrees, the EI scale is completed in 720 degrees, offering better interpolation of the divisions.

Electrical Slide Rules - These are used for calculating the resistance and voltage drop in conductors as well as determining the efficiency of motors and dynamos (generators).

Electro - A French term for an electrical slide rule.

Electronic Slide Rules - These are used for not only calculating the resistance and voltage drop in conductors, but include scales for calculating the frequency and resonance of circuits using inductors and capacitors for radio design.

Elektro - A German term for an electrical slide rule.

Embellishment Lines or E-Lines - Lines added to a slide rule scale, in parallel with the slide, so that, in combination with the divisions, a series of boxes are formed. Also called longitudal lines. Used to aid hand-engraving of the scales. This style stopped being used about 1930.

Engine-Divided Scales - Originally, the process of a machine engraving the tick marks or divisions of a scale with a knife edge, followed by filling with paint or other resin. Early premium slide rules had numbers and scales machine engraved as that puts the markings below the surface of the slide rule. Later, many top quality rules (F-C, Aristo, etc.) had scales that were impressed, ground and polished to achieve the same characteristics. Aristo and Graphoplex used a special photo-chemical process which will never wear or wash away, like the rare Aristo 940 System Göttsch. Painted or imprinted slide rules are not long lasting because the markings wear off and can be dissolved by chemical cleaners. Beware: Window cleaners with ammonia will dissolve the ink used on K&E K12's or Henschel slide rules.

Engineer - You are an engineer if you still own a slide rule and you know how to use it.

Engineering Instruments, Inc - An American manufacturer in Peru, Indiana, of low-cost painted slide rules, formed in 1947 after the marriage of Vivian C. (Lawrence) and Frank Bozarth. Vivian was given Lawrence Engineering Services as part of a divorce settlement between Vivian and George L. Lawrence, the founder of the company. Engineering Instruments ceased to exist after the building burned down in 1967.

Engineering Material - A substance that has uniform physical properties that can be calculated over temperature extremes, stress (bending) and time. Metals (aluminum, copper, magnesium, zinc, etc), Polymers (plastics, rubber, etc), Ceramics (glass, silicon nitride, silicon carbide, etc) and composites (reinforced plastics, metal matrix, fiberglass, etc.) are considered engineering materials. Some wood used for structures exhibit sufficient properties for engineering purposes.

Engineering Notation - See Scientific Notation

Erdös Number - The Hungarian Paul Erdös (1913-1996, prounounced ``Ar-dish'') not only was one of the strangest mathematicians of the 20th century, he was also one of the most famous. He kept on publishing widely circulated papers up to a very high age and every mathematician having the honor of being a co-author to Erdös is well respected. Not everybody got the chance to co-author a paper with Erdös, so many people were content if they managed to publish a paper with somebody who had published a scientific paper with Erdös. This gave rise to the so-called Erdös numbers. An author who has jointly published with Erdös had Erdös number 1. An author who had not published with Erdös but with somebody with Erdös number 1 obtained Erdös number 2, and so on. Similar to 'degrees of separation'.

Esser, Hermann - (1845-1908). Hermann Esser and Wilhelm J. D. Keuffel were partners in the firm of Keuffel & Esser which was founded in July 1867. The firm was incorporated in New Jersey in February 1889 and became Keuffel & Esser Company. They first manufactured surveying instruments in 1885. Hermann was born at Wuppertal-Elberfeld, Germany in 1845. He retired from Keuffel & Esser Co. in 1902. He was back in Bad Godesberg am Rheim, Germany in April of 1902 and died there in April 1908, the same year as Wilhelm Keuffel.

Estes Industries - An American manufacturer of model rocket motors and rocket kits. Vernon Estes founded Estes Industries in 1959 to make gunpowder-based model rocket motors. Members of the National Association of Rocketry (NAR) were the primary customers of Estes rocket motors which helped to inspire a generation of aerospace engineers and scientists. Retail model rocket kits soon followed and were shown in catalogs, along with computing equipment (slide rules and charts) necessary for calculating the altitude or apogee of the flights, which involved solving a basic trigonometric triangulation problem. The 1964 catalog carried Lawrence and Pickett slide rules. The Frederick Post 1447 was sold in the 1970 catalog. Estes Industries was sold in 1969 and is currently doing business as Estes Rockets. NAR 2005 Sport Rocketry Magazine Interview with Vern and Gleda Estes 5.1 MB PDF

Eucalyptus - The wood from the Eucalyptus tree, found in Australia and Brazil. Used by Australian slide rule manufacturer, W&G (White & Gillespie) for their stocks. It was well regarded because of its stable thermal properties, which resisted the temperature-induced expansion that resulted in inaccurate calculations. It is said that "If you slide the rule really quickly, you can smell the eucalyptus from the wood as it heats up."

Evans, J.D. - c1800's. Dr. J.D. Evans described a grid-iron slide rule in 1866. This was the earliest form of a long scale slide rule, whereby the length is increased by having multiple portions of the scale in a grid.

Evolution of Scales - Over 300 years, the orientation, position and sequence of using the scales on a slide rule became more refined which made it easier to perform calculations in less steps. (Note: Evolution in this sense properly means that intelligence was added to the equation to make things better. Consequently, the term 'Evolution' from a biological standpoint is a misnomer, since everything dies off and becomes extinct, unless intelligence is added to the mix.) See Pickett Scale Evolution:

Extended Length Slide Rule - See 20 Inch Slide Rule.

Extended scales - Tick marks that extend on either end of the normal indices which help to speed up calculations. Some models change the color of the extended part of the scale to differentiate from the standard scale length.

Eye-Saver 5600 - Eye-Saver Yellow green (Angstrom 5600), the distinct yellow that is identified with Pickett slide rules, which was promoted as a help to prevent eye-strain headaches, to improve slide rule computation speed and accuracy, to reduce error and lower costs. Chromatic aberration tests by Pickett established that Angstrom "5600" is the point on the spectrum for optimum eye-ease. Pickett Eye-Saver yellow green exactly duplicates this color value. Eye-Saver "5600" reflects long wave length rays, while white reflects all colors, including the short wave length actinic rays, which create "hot" spots and the contrast with the graduations is supposed to improve precision in reading the graduations. The -ES option was available on all models of Pickett All-Metal and plastic slide rules.

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F

Faber-Castell - German manufacturer of slide rules, starting in 1882, that began as A.W. Faber pencil factory as early as 1761. Slide rules started out as boxwood, with celluloid facings added in 1897. English manuals were translated by C.N. Pickworth for American distributor in Newark, New Jersey, USA. In 1905, the name was changed to Faber-Castell. Celluloid laminated mahogany was used from 1902 to 1912, followed by celluloid faced Pearwood. In 1952 all-plastic (Geroplast) versions were introduced. The duplex slide rules began in 1961. The most complex slide rule was the multi-colored Novo Duplex 2/83N. Faber-Castell still has factories in Germany and distribution houses in over most of the world. See expanded Faber-Castell timeline.

Fabrikoid - A vinyl coated fabric used to make protective sleeves, typically for Gilson circular slide rules, similar to Naugahyde, having the external appearance of leather.

Factor of safety (FoS) - also known as Safety Factor, is a multiplier applied to the calculated maximum load (force, torque, bending moment or a combination) to which a component or assembly will be subjected. Thus, by effectively "over engineering" the design by strengthening components or including redundant systems, a Factor of Safety accounts for imperfections in materials, flaws in assembly, material degradation, and uncertainty in load estimates. An alternative way to use the safety factor is to derate the strength of the material to get a "design" strength. (Ref: Wikipedia.org)

Fagen, Daniel W. - Purchased the Gilson Slide Rule Company, Stuart, Florida, in 1960 from Clair Gilson. Fagen was an ex-machinist and operated a moving and storage business in Stuart, Florida delivering metal stock and supplies to Gilson Slide Rule in the 1950s. Fagen befriended Clair Gilson and when Clair's health was failing, offered to buy the company. Unfortunately Gilson died before he was able to explain all of the manufacturing processes to Fagen. Fagen moved the machinery into a storage warehouse and worked out all of the manufacturing and mathematical details on his own. The 1920 vintage screw machines and punch-presses used to make the Gilson slide rules were operated off of a multiple clutched pulley and belt system which required considerable maintenance. Fagen's son, Daniel II, said his father had to make many of the parts to keep the machines running. Fagen sold the company in the early 70's. ^{Fagen history contributed by son,  Daniel Fagen II.}

Fermi, Enrico - In 1943, Nobel Laureate Enrico Fermi used a K&E pocket slide rule to control the stages of the world's first self-sustaining nuclear chain reaction, beneath the stands of Stagg Field at the University Of Chicago. As assistants stood by, Fermi used his rule to determine the amounts by which the cadmium control rods were to be withdrawn from the pile of graphite and enriched uranium.

Festus Mfg. Co. - (1938-39) located in Festus, Missouri. An American manufacturer founded by Paul Jones Jr. Festus made low-end wood slide rules, as well as window and door frames. Festus became the Acu-Rule Mfg Co. in 1940 and relocated to St. Louis, MO using the brand name of Acu-math. Paul Jones Jr. died in 1945. See A Short History of the Festus Manufacturing Company by Geo. E. Keane

Filotecnica Salmoiraghi - Italian company that made optical instruments. Of particular interest was a projection slide rule which used two back lighted glass disks to project the scales on a screen. More at: Link

Financial Slide Rule - A type of rule, used in commerce, having scales to easily calculate profit, loss, mark-up, discount, amortization, interest, inventory, etc. The allocation of the standard scales are sometimes labeled on the right hand side for convenience. On a German Faber-Castell Commerce rule the scales are labeled : E (Einkauf - Purchase), V (Verkauf - Sale), T (time), Z (interest) and K (capital).

Fisher Governor Company - A U.S. company supplying control valves for the gas, steam and high-pressure-air industry out of Marshal Town, Iowa. They commissioned Pickett, and possibly Acu-Rule, to make specialized slide rules for sizing applications of their equipment. Known copyrights are 1954 and 1967.

Flying Fish - A Chinese slide rule factory and brand of slide rules that were manufactured in Shanghia, China. An earlier name of the factory was Shanghai Ding Feng which was a joint private-state owned factory.

Folded Scale - A scale that has the '1' somewhere in the middle of the scale, with the end starting at a point convenient for making quick calculations, such as the CF and DF scales are folded at π (3.14 or pi). Also used to minimize the distance of the stroke of the slider.

Fowler's calculator - Fowler (Manchester, England) was best known for making pocket watch style calculators. These were originally named "Mechanical Engineers". Fowler was originally a publishing company, "Scientific Publishing Company", selling engineering handbooks and a weekly magazine called "The Mechanical Engineer". That became kind of a brand name for them, as they also sold the calculators and a chronograph pocket watch under it. One face has a long-scale and short-scale logarithmic scale. The short scale could be used for rapid calculations or as a check that the correct sector of the long scale had been used. The other face has a short-scale and reciprocals, logs, square roots, sin, tan and sin-tan scales.

Frederick Post Company - U.S. Manufacturer and distributor of slide rules. See POST.

Fundamental Scales - x - Also called Primary Scales. Located on the body as D and on the slide as C. Typically the first variable of a calculation and upon which all other scales are mathematically calculated.

Function - A function, in mathematics, is a "rule" that assigns a unique output to each given input.

Fuji - Fujikeiko Co. Japan, est. 1949, became Fuji Slide Rule Manufacturing Co Ltd They sold under their own brand as well an OEM supplier for Wolters-Nordhoff in Holland. Their all-plastic slide rules sport a light green accent.

Fuller Spiral Slide Rule - Designed in 1878 by Professor George Fuller, the logarithmic line is arranged spirally on the surface of a cylinder. By means of movable cylinders any length of spiral line may be at once transferred to any other part of the scale, and multiplications and divisions containing a series of factors can be worked with facility. Logarithms of numbers are given by means of a scale on the longer index arm together with a circular scale on the first cylinder, so that powers and roots are obtainable. The surface of the middle cylinder bears printed tables of decimal equivalents, natural sines, etc. The axis is provided with a handle for the hand or for mounting onto a fixed base. On the cylinder is a logarithmic scale that spirals along the cylinder, having the equivalent length of 500 inches with 7,250 divisions. In comparison, a standard 10 inch slide rule has about 160 divisions. Sold by numerous slide rule distributors, one example being Dietzgen model 1794.

F. Weber & Company - See Weber, F., & Co

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G

G_θ scale - "Gudermanian" scale gives the hyperbolic angle, sinh θ found on T scale and tanh found on P scale (Hemmi/Post).

Galileo Galilei - (1564-1642)

Gamma or γ - The small Greek letter γ.

Gamma - A Hungarian slide rule manufacturer out of Budapest.

Gauge Mark or Gauge Point - Constants or numbers which are used to frequently in calculations marked on the scales of a slide rule to save the user save time by not having to remember the constant or by minimizing repetitive calculations. The most common gauge points are found at 3.14, representing pi, and .785 representing pi/4. See π/4.

Geek - A person who has chosen concentration rather than conformity; one who pursues skill (especially technical skill) and imagination, not mainstream social acceptance. As the mainstream culture becomes more dependent on technology and technical skill, mainstream attitudes have tended to shift towards grudging respect of Geeks. Quote "Old Geeks never die, they just collect slide rules", Mike Konshak, circa 2000.

Geotec - A brand of Canadian slide rules, presumed to be a spin-off company of Hughes-Owens, as they have matching model numbers to H-O slide rules. Geotec slide rule were manufactured primarily by the Japanese company Sun Hemmi.

German Slide Rule Brands - Slide rules manufactured or re-branded by major German companies (including DDR) were A.W. Castell, Faber-Castell, Nestler, Dennert-Pape, Aristo, Eco-Bra, Friedmann, H-H Universum, Meissner, Reiss, Rista, Staedtler, Staedtler-Mars, Veb Mantissa, (list incomplete).

Geroplast - The proprietary name for the plastic used in modern era German Faber-Castell slide rules. Faber-Castell recommended the use of their Geropur cleaning solution for restoring the stock. Some OEM slide rules had a prefix of "G" (such as Dietzgen G-1734) which denoted the material.

Giken - A Japanese manufacturer of all-plastic slide rules, thought to be the predecessor to Fuji. Giken had the same location as Fiji, but had a line of slide rules in 1963 that shows no overlap with Fuji models that appeared in a Fuji price list printed in 1970. ref: William Lise.

Gilson, Claire A. - (? - 1960) Founded the Gilson Slide Rule Co. in 1915, in Niles, Michigan. His first circular slide rule was printed paper glued to a 4" diameter steel disk. He purchased the Richardson's Rule Works, approximately in 1919, from George W. Richardson, presumably to obtain the printing process for scales used on Richardson Rules. The family relocated to a small island near Stuart, Florida, after 1927. The founder's son, Richard A. Gilson, computed the scales for the Gilson Atlas while still a teenager. The company was sold in in 1960 to Daniel W. Fagen. Clair died that same year.

Gilson Slide Rule Company - Made circular metal slide rules, est. 1915 by Claire Gilson in Niles, Michigan. OEM's to many other manufacturers' brands as Atlas, Midget, Post, Lietz, Dietzgen, Bruning and Travella. Gilson bought Richardson Rule Works prior to 1920. It was a good match as Richardson's printing process for the scales, black paint on metal, appeared in the second version of the Gilson Midget Slide rule, Copyrighted 1919, patent dated 1-17-22. Manuals from both companies used Richardson's "Be Wise" Owl in the manuals. The Gilson name never appeared on any of their slide rules. Known Circular models were the Gilson Binary, The Midget (several variations), Apex, Commercial, and the Atlas. While the others used concentric scales, the Atlas had a spiral scale equivalent to 75 feet in length. The founder's son, Richard A. Gilson, computed the scales for the Atlas when he was a teenager. Other special purpose slide rules were created. Claire Gilson was unable to continue the business because of illness and sold it in 1960 to Daniel W. Fagen, a local machinist and owner of a moving and storage business. Fagen manufactured Gilson Slide Rules in Stuart, FL until the early 1970's, when the company was sold again. The last owner had neither the machining or mathematical skills to continue the business and the purchase was ill-timed with the advent of the electronic calculator. ^{Fagen history contributed by son,  Daniel Fagen II.}

Gong Si He Ying - means “joint state-private ownership” and is found on sleeves of Shanghai Ding Feng (later Flying Fish) slide rules. In China, any enterprise with the words “Gong Si He Ying” existed only for a short period in the 1950’s. The PRC (Peoples Republic of China) was founded in 1949. Before that time, there were quit a few privately owned businesses in China. In the beginning of the PRC, after suffering through the Japanese invasion of WWII and later the Chinese Civil War, the Communist State had a difficult time obtaining capital for business, but slowly every thing was restored. In rural communities, farmers received farmland that been confiscated from landlords and in cities or towns, enterprises needed funding and technical support. The communist government, rather than confiscating the enterprises that were privately held, wisely decided to jointly administer many enterprises between the state and private companies, putting the words “Gong Si He Ying”on front of the enterprise or company name. However, by the 1960’s, when prosperity had increased, the “joint state-private ownership” enterprises were gradually absorbed into only state run companies. Up to 1978, no private funding or ownership was allowed in manufacturing, commerce, or any enterprise. Since 1978, many private run enterprise are growing rapidly in China, including many “joint state-private ownership” enterprises, the words “Gong Si He Ying” are not used on the front of name. ^{Contributed by Liu Ql in Beijing, China}

Gottheimer, Abel and Ethel R. - Abel (1891-1979 – died Nassau, NY) and Ethel (?-1979) Gottheimer founded the Roos Company of 280 Madison Ave., New York, in a partnership with Andre and Angele Charvoz, circa 1941, manufacturing painted wood and laminated wood slide rules, presumably to fill the void caused by import restrictions during WWII. A reasonable assumption was that Gottheimer manufactured the slide rules and Charvoz handled marketing, as by 1945, The Roos Company was distributing “Charvos” [sic] drawing instruments manufactured by Andre Charvoz, under Charvoz, Inc. The Roos Company and Charvoz Inc. were merged into Charvoz-Roos Corporation in 1946.

Graduations - The small lines or tick marks engraved in the surface of a slide rule, the major or primary graduations labeled by numbers corresponding to the value of the divisions of the scale.

Graphoplex - A French manufacturer of slide rules that also supplied slide rules for Koh-I-Noor, a European company, still in business, with subsidiaries in the USA.

Green, Alex E.S. - (1919-) Graduate Research Professor Emeritus at the University of Florida. During WWII, as a B-29 crew member, Dr. Green designed several slide rules which were used by the U.S. Military. The first "Ship length Computer" was an idea that came from a discussion with another aviator on the difficulty of identifying ships. Green's slide rule was designed to be used with the B-29 gun sights. This was used to identify the elusive Japanese battleship Yamoto. During this mission, which located the hidden Japanese fleet, which had run out of oil, the B-29 itself ran out of gas and had to make a forced landing in Xian, China. This event led Green to create the "flight Engineer's Computer" to minimize gasoline consumption on long over-water missions. Alex Green was awarded the Medal of Freedom in 1947. Currently researching thermal waste to energy conversion at U. of F. and a fellow of the Oughtred Society. Biographic article in Physics Today, 2001

Grid-Iron - A form of construction of long scale slide rules, taking the name from the grid of scales which make up the total scale length. See Evans, J.D.

Grooved Guide - The joint between the body (stator) of a slide rule and the center slide which retains the slide during manipulation. All linear slide rules incorporate some form of grooved guide to capture the slide. Some slide rules designs provide an adjustment of the clearance of the guide to improve the feel and alignment to maintain the precision of the movement.

Guedon - US manufacturer located in Camden, New Jersey. Their slide rules were made in France.

Gunter, Edmund - (London, England, 1581 - 1626), Created a number line (the Gunter Scale) in 1620, in which the positions of numbers were proportional to their logs which graphically represented the work of John Napier. In 1623 he published a description of this scale that is composed of two scales of the logarithms from I to 10 placed end to end. Although Napier conceived of the logarithm allowing multiplication or division to be accomplished by addition or subtraction, Napier relied on look up tables. Edmund Gunter Biography by John Aubrey c1650

Gunter Rule - The calculating device created by Edmund Gunter that was to be the predecessor of the modern slide rule. A pair of dividers is used to measure a distance (the multiplicand and the multiplier) along the rule and add it to another distance, the multiplicand, forming the combined distance, the product, on the rule. The accuracy of an answer is limited by the length of the rule and the user's ability to resolve a number.

gwr - A mark found on military slide rules manufactured by German manufacturer Aristo from 1942-1945. there was no company identification on these products.

G-W Photo Slide Rule - A slide rule specifically oriented to photographers to aid in determining settings for flash lamps and focus. G-W are the last initials of both inventors, C.G. Goodchild and L.V. Whitney, both PhD's. One example was manufactured by Acu-Rule for AREL Inc, St. Louis, MO.

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H

H1 scale - √(1+x²), Hyperbolic scale, range 1.005 to 1.5

H2 scale - √(1+x²), Hyperbolic scale, range 1.4 to 10

Hairline - The sharp thin vertical lines on the inside of the cursor window or glass. These were either etched or silk screened on. Originally, before glass and plastic windows were added to cursor frames, an actual horse hair was stretched across the cursor frame in open air and tightened with screws. Some more modern cursors have multiple hairlines as an aid in conversions and calculations - see Multi-Lined Cursor

Hairy Eared Engineer - 1) An engineer who's old enough to have hair growing out of undesirable places. 2) ...has been practicing engineering so long that he or she has already made all possible mistakes at least once. 3) A desirable person to have on a project to ensure the mission succeeds. 4) Quote: "Every project needs at least one hairy-eared engineer" Marvin B. Davis - Circa1980.

Halden Calculex - A circular slide rule in the form of a 2-3/8 inch diameter pocket watch. It consists of two concentric metal discs, graduated in the same manner as a linear slide rule, with two glass covers, which act as cursors on each side. Was sold as a Dietzgen 1795A, a Frederick Post 1448 and others.

Hand Scribed - Most slide rules, prior to 1890, were hand scribed from a master pattern using calipers to transfer the measurements, until such time as automatic dividing engines became economical for manufacturing.

Harling, W.H. - London manufacturer, est. 1851, purchased by Blundell in 1966 forming Blundell-Harling.

Heinlein, Robert A. - (1907-1988)American science fiction author. A character in one of his novels states: "Dad says that anyone who can't use a slide rule is a cultural illiterate and should not be allowed to vote. Mine is a beauty - a K&E 20-inch Log-log Duplex Decitrig" Have Space Suit - Will Travel 1958.

Helical Scale - A method of extending the scale length used on tubular or cylindrical slide rules, such as Otis and Fuller. The scale is wrapped as a helix around the tube's diameter thus enabling many feet of scale over a small length.

Hemmi - Hemmi Bamboo Slide Rule Company Ltd. in Japan is the oldest and most well known Japanese manufacturing company supplying OEM slide rules to several North American companies like Post, Hughes-Owens, Lafayette, Lietz, and Geotec. Their own brand was "Sun". Early examples may have had the "Made in Japan" or "Made in Occupied Japan" sanded off due to anti-Japanese sentiments after WWII. It is estimated that Hemmi produced about 15 million slide rules altogether.

Hemmi, Jiro - (November 1878 - April 4, 1953) Founder of Sun Hemmi Slide Rule, later renamed Hemmi Keisanjaku KK (Hemmi Slide Rule). Jiro Hemmis graduated in 1890 from primary school in Chiba Prefecture. In 1893 he worked for Nakamura Surveying Instruments in as a reduction scale worker then struck out on his own in March, 1899 to manage a business making slide rules as Sun Hemmi. Retired from Hemmi Slide Rule in 1941 for health reasons.

Henschel Co. - John Henschel Co. Inc. Circa 1945-1980. In 1959 the American company was located at 105 E. 29th St. in New York City, NY. Sold slide rules, drafting supplies under its brand. Also manufactured Pelikan Fount India Ink for fountain pens and typewriter ribbons. A 1979 American Medical Assn Journal mentions John Henschel Co. of 14 Albertson Avenue, Albertson, NY as a supplier of biological inks for testing stroke victims. The construction methods of known duplex slide rule samples indicate that may have been manufactured by Charvos-Roos with scales designed by Acu-Design, which is associated with Acu-rule/Acu-Math slide rules. Its possible that the scales were applied onto the stock and slide using a heat-transfer method, which became popular in 1950 (called Thermofax) and unlike engine-divided scales, the numbers and divisions are easily wiped off with a strong cleaner like Windex Ammonia-D. Colored inks did work as well as black copied by heat-transfer. Other similar slide rules can be found under the Frederick Post and Charvoz-Roos brands.

Herwijnen, Herman van - (1929-2004) Avid Dutch slide rule collector and author of the Slide Rule Catalogue© and Calculating onSlide Rule and Disc. Married to Diny van Herwijnen. Member of the Dutch Circle of Slide Rule Collectors Graduated with Mechanical Engineering degree from Technical University Delft in 1954. Spent two years in the Dutch Air Force followed by 13 years woking in Venezuala for Dutch Royal Shell. Had hi own firm Infro Software and lectured at Shell and Technical University Delft up to his death in 2004.

Historical Developments in the Evolution of the Slide Rule. - from a 1962 A.G. Thornton slide rule instruction manual with additions. See Slide Rule History.
1614 - Invention of logarithms by John Napier, Baron of Merchiston, Scotland.
1617 - Developments of logarithms 'to base 10' by Henry Briggs, Professor of Mathematics, Oxford University.
1620 - Interpretation of logarithmic scale form by Edmund Gunter, Professor of Astronomy, London.
1630 - Invention of the slide rule by the Reverend William Oughtred, London.
1657 - Development of the moving slide/fixed stock principle by Seth Partridge, Surveyor and Mathematician, England.
1775 - Development of the slide rule cursor by John Robertson of the Royal Academy.
1815 - Invention of the log log scale principle by P.M. Roget of France.
1850 - Amédée Mannheim, France, produced the modern arrangement of scales.
1886 - Dennert & Pape, Germany, introduce white celluloid as a material for inscription of scales.
1890 - William Cox of the United States patented the duplex slide rule. 1900 - re-introduction of log log scales by Professor Perry, Royal College of Science, London.
c1900 - Engine divided scales on celluloid increases precision of slide rules.
1933 - PIC Differential trigonometric and log log scales invented by Hubert Boardman, Radcliffe, Lancashire.
1976 - The final slide rule made by K&E donated to the Smithsonian Institute, Washington, DC, USA.

Hoffman - A Danish manufacturer of UTO slide rules for the OEM promotional slide rule market.

Holophane Calculator - A specialized slide rule used for calculating illumination angles and properties of holophane lamps. Produced by Blundel, Luton, England. Circa 1950.

Hope - A Japanese (Tokyo) slide rule manufacturer that also supplied OEM products to other companies such as Jakar. Hope Slide Rule Mfg. Co., Ltd. Shigets Building, No. 26-18, Asakusabashi, 2-chome, Taito-ku, Tokyo, 111 Japan

Hp, Horsepower (Electrical) - The electrical horsepower is used by the electrical industry for electric motors and is defined to be exactly 746 W (at 100% efficiency).

Hp, Horsepower (Mechanical) - The most-common definition of horsepower for engines is the one originally proposed by James Watt in 1782. Under this system, one horsepower is defined as: 1 hp = 33,000 ft·lbf·min−1 = exactly 0.74569987158227022 kW or commonly 746 Watts.

Hp, Horsepower (Metric) - Metric horsepower, is defined as 0.73549875 kW, or roughly 98.6% of mechanical horsepower, or commonly 736 Watts. Metric horsepower began in Germany in the 19th century and became popular across Europe and Asia. The various units used to indicate this definition ("PS", "CV", "pk", and "ch") all translate to "horse power" in English, so it is common to see these values referred to as "horsepower" or "hp" in the press releases or media coverage of the German, French, Italian, and Japanese automobile companies.

Hp - Gauge mark denoting Horsepower, equivalent to 746 Watts (US) or 736 Watts (Europe, Japan) depending on the country that manufactured the slide rules.

HP35 - The electronic scientific calculator with an 8-digit  light-emitting diode display that was invented by Hewlett Packard in the mid 1970's which is credited with the sudden demise of  the slide rule industry.

Hughes-Owens - A Canadian based company distributing primarily Japanese Sun Hemmi bamboo slide rules under the H-O brand, as early as 1912. Additional models of H-O metal slide rules were supplied by US manufacturer Pickett in 1952. In the late 60's the same model numbers appeared on slide rules with a Geotec logo. A 1914 catalog shows Hughes-Owens importing Hemmi slide rules from 1912.

Hydralculator - Made by Lewis & Tylor Ltd., Gripoly Mills, Cardiff, the manufacturers of "underwriter" super fire fighting hose, for the use of their "Friends in the Fire Service." patent number 396,533 To find the quantity of water discharged for any given nozzle and a known pressure, place press on scale "b" opposite nozzle on scale "a', and read discharge through window in slide. To find height of jet for given pressure and nozzle diameter, proceed as above and read opposite arrow in center of slide, the height given on scale "d" for the appropriate nozzle.

Hyperbolic  - These Vector functions, Sh, Th are found on slide rules primarily geared toward electrical engineering, but have other uses such as defining the shape of a catenary curve.

Hyperbolog - A model of slide rule (ex: Aristo 0971)  that has Hyperbolic and Log-Log scales.

Hyperlog - An Aristo 0972 Hyperlog is considered to be the most developed Aristo slide rule.

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Indicator - Another term for Runner or Cursor.

Integer - A whole number without any fractional component. Called Prime Numbers when used for labeling the scale divisions.

Interpolation - Determining the approximate value of the location of the hairlines when positioned between two tick marks on a scale.

ISd scale - α/sin^-1α), Inverted Sine differential scale, for sine range 0° to 90°. (PIC/Thornton) 

ITd scale - (α/tan^-1α), Inverted Tangent differential scale, for tangent range 0° to 60°. (PIC/Thornton) 

Ivorine - Dennert and Pape (Aristo) brand name for the white celluloid plastic they introduced in 1886 to laminate wood slide rules as a better material than wood for the inscription of scales. This allowed a much improved precision and legibility to be achieved. Other manufacturers soon followed suit. 

IWA - IWA/F.Reihle GmbH & Co., a German family operated firm, Ingenieurerbüro für wirtschafliches Arbeiten (Engineer's Office for Efficiency). Slide rules marked IWA began appearing in 1920 and in 1926, Frank Riehle, established a permanent company in Denkendorf, Germany. Product lines focused on special purpose plastic slide rules for mechanical engineers and under contract by other firms as marketing tools. Data slide rule" are still being produced for specialized purposes.

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J

Japanese Slide Rule Brands - Slide rules manufactured or re-branded by Japanese companies were Jiro Hemmi, Sans-Sai group, Relay, Ricoh, (US brands of Relay-Ricoh not listed), Giken, Fuji, Engineers, Hope, Kutsawa, Concise, (list incomplete).

Jason - American brand of slide rule manufactured by Relay/Ricoh out of Japan.

J.C Penny Company - American catalogue company that sold slide rules manufactured by Relay/Ricoh out of Japan.

Jeppeson - American manufacturer, located in Denver, Colorado, of aviation charts and pilot supplies, including a version of the E6B flight computer slide rule, called the Slide-Graphics Computer.

Joe Miner - The official mascot of the University of Missouri-Rolla depicting hard rock miner, packing a 6-gun and carrying a pick axe and a slide rule. Rugged and individualistic, Joe Miner personifies the spirit of the old west and the determination to succeed. His character is closely tied with the school’s founding in 1870 as the University of Missouri School of Mines and Metallurgy.

Jones, Paul Jr. - (1896-1945) Born in St. Louis, MO. At age 42 was associated with the Waggener & Bros Lumber Company located at 318 West