Humboldt State University
Robert A. Paselk Scientific Instrument Museum
Demonstration Slide Rule
Keuffel & Esser Co.
Humboldt S-T-C, c. 1927
Slide rules are devices used to aid in calculation, and were extremely important tools in science and engineering prior to the development of the hand calculator. Because of their importance instruction in the use of the slide rule was routinely given in Chemistry, Physics, and Engineering classes, frequently using large demonstration rules such as the one seen here. In essence a slide rule works by, 1) adding and subtracting the logarithms of numbers on adjacent sliding scales thereby enabling multiplication and division (log a*b = log a + log b, log a/b = log a - log b), and, 2) comparing various different parallel scales (in essence acting as tables of trigonometric functions, logarithms etc.).
The idea of using a logarithmic scale for calculations began with Edmund Gunter when he engraved a scale of logarithms on a piece of wood in around 1620 and used a dividers to add and subtract them to or from each other, and thus multiply and divide. In 1632, William Oughtred divised a pair of sliding scales, which when applied to each other could be used to multiply and divide. It wasn't until 1850, however that the modern form of the slide rule, with one rule sliding between two others, and using a sliding cursor, was divised by Lt. Amédée Mannheim. The resulting "Mannheim" slide rule carried the A, B, C, D, S, T, and L scales on a wooden rule. White celluloid was used in slide rules by 1886. In 1887 Keuffel and Esser began the manufacture of slide rules in the U.S. using mahogany with celluloid scales. Before this time slide rules had to be imported. In 1891 the "Duplex" slide rule, carrying folded scales, was divised by William Cox and manufactured by K&E, and by about 1900 the Log Log Trig slide rule took its "final" form.
The catalog description is from the 1933 K&E Educational Products Catalog. The scan is from Clark McCoy's K&E Catalogs site (2005).
The slide rule is made of wood (Redwood was the preferred material due to its light weight and strength) with white enamel on the front surfaces; it is unfinished on the sides, ends, and back. The rule is 84 1/2" long x 7 5/8 inches high by 1 1/2 inch thick. There are four one inch finger holes in the slider to allow the demonstrator to slide it relative to the other scales. All divisions, numbers etc. are in black enamel with the exception of the CI scale whose numbers are in red. KEUFFEL & ESSER CO. N.Y is printed in bold 1" letters in the center of the top portion of the rule, while to the left in 1/4" letters is written: PAT. JUNE 5, 1900, and to the right, again in 1/4" letters is: MADE IN U.S.A. and MODEL 100. The original cursor window (cellulose nitrate? with an engraved line filled with black paint) fragmented and has been replaced with clear acrylic. An eyebolt is attached to the top of the rule above each end of the A scale. A Chem Dept decal is on the top, front of the rule above "24" on the A scale. An oval brass tag, attached with brass pins to the right end of the rule, is stamped: HUMBOLDT S-T-C / 2094.
The basic history of the slide rule is outlined in Michael R. Williams, A History of Computing Technology, Prentice-Hall, Inc., 1985, pp. 111-118 and John P. Ellis, The Theory and Operation of the Slide Rule, Dover Pub. Inc., New York, 1961, pp. 247-250. An extensive history of the slide rule is found in A History of the Logarithmic Slide Rule and Allied Instruments by Florian Cajori, The Engineering News Publishing Co., New York (1909) [recently reprinted]. Two highly recommended recent books, by Hopp and by von Jerierski, covering the complete history of the slide rule through its effective demise with the development of personal calculators, may be found on the reference list.