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The slide rule today: respect for the past.

For 350 years the slide rule was an indispensible logarithmic calculating machine. Variations existed for different types of calculations in engineering, surveying, chemistry, medicine, and distilling. Now, 20 years after the introduction of the scientific handheld calculator, these one-time priceless tools of calculation have been relegated to museum pieces, attracting interest only among nostalgic collectors.

The invention of logarithms in 1614 by John Napier of Scotland laid the foundation for the slide rule. In 1620, Edmund Gunter marked a 24-inch-long straight scale with logarithms. Gunter's rules were used to perform multiplication and division by adding and subtracting scale lengths with a pair of dividers. In 1621, William Oughtred, an English minister and mathematician, placed two Gunter rules side by side to avoid the use of dividers. Oughtred first arranged the scales in a circular form and later followed with a rectilinear model. These are generally recognized as being the first slide rules.

To commemorate Oughtred's invention, two enthusiasts, Bob Otnes and Rodger Shepherd, in 1991 formed the Oughtred Society in Palo Alto, Calif., for people interested in the history and collection of slide rules. As with other collectibles, a slide rule's value today depends largely on its inclusion of original parts and its show of wear or defects.

At the Buro-Antik auction, held in Koln, Germany, October 24, 1992, there were 18 slide rule lots. The Loga cylinder slide rule (a relative of the large cylindrical slide rule invented by Edwin Thacher in 1881) garnered the highest price - $863. A pocket watch - type slide rule, believed to be French but with no listed name or origin, sold for $510.

Oughtred's initial invention was refined in 1815 by Peter Roget, an English physician. He invented the Log Log slide rule with the fixed scale divided in proportion to the logarithms of the logarithms of the numbers shown on the scales. This arrangement simplified exponential calculations.

Amedee Mannheim, a French artillery officer, in 1859 invented what may be called the modern slide rule. This had the scales on one side only. The arrangement of the scales and the general form are known as the Mannheim rule, and it became the popular basic slide rule.

Some time between 1881 and 1890, Edwin Thacher sold or assigned the patent rights to his device in the United States to the Keuffel & Esser Co. in Hoboken, N.J., along with the lithographic plates from which the scales for the bars and cylinder of his calculator were printed. In 1887, Thacher's calculating instrument was listed in Keuffel & Esser's catalog for $30. In 1949 it listed for $100; by 1952 it was discontinued due to the reduced demand caused by the "refinement of electronic digital computers."

Among others, the Thacher won particular favor with railroads for calculating rate schedules and with steel companies for calculating the weights of the charges placed in the blast and open-hearth furnaces.

The period from 1884 to 1909 saw the slide rule change from the folding rule used by carpenters to the slide rule of the engineering profession. In an article in the Transactions of the ASME, Volume VIII, 1886-1887, pages 707-710, F.A. Halsey said, "For the past five years I have made habitual daily use of the slide rule and have come to regard it as an indispensable desk adjunct. As a saver of time and mental wear in tedious calculations, its value is great and unquestionable."

German-born Wilhelm Ludwig Ekrenfried Keuffel is credited with having the single most important influence on the American slide rule. Working at his uncle Wilhelm Keuffel's company from 1884 through 1945, his tenure covered the major portion of the period during which slide rules were manufactured.

W.L.E. Keuffel based the scales of his first slide rule designs on the table of logarithms. He needed to design a machine with an accurate scale cut on the face of a flat surface. His slide rule had a bed with grooves on which a workbearing carriage was guided on rails in a straight line. Between the rails a lead screw was placed to which the carriage was engaged by a split nut. At the end of the lead screw was fastened an index wheel with 200 teeth. Since the lead screw had a pitch of 2 millimeters, the travel of the carriage was controlled to [+ or -] 0.005 millimeter.

The slide rule blank was held in position on a chuck fastened to the carriage. The point of operation was a specially shaped knife in a holder free to move in a transverse direction but rigid with respect to the motion of the carriage. The length of the travel of the knife was controlled by replaceable eccentrics. The travel of the carriage was governed by a pawl engaging the index wheel of a predetermined amount, thus turning the lead screw and advancing the carriage as required. With the carriage stationary, the knife was in position to cut or scribe a graduation. Thus, by mechanical means, the spacing and lengths of the graduation of any desired scale could be produced automatically.

The advancement of the modern digital age edged slide rules out of their once-important role. In 1972, the handheld calculator emerged and relegated the slide rule to display cases in museums.
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Title Annotation:Input-Output; history of the slide rule
Author:Scuria-Fontana, Camille
Publication:Mechanical Engineering-CIME
Article Type:Column
Date:Jul 1, 1993
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