Scofield-Thacher Engineer's Slide Rule

The scales on this 22-inch, two-sided wooden slide rule are printed on paper. The front has two scales on the left end of the base labeled B and C and two scales labeled A on the left end of the slide. These scales run half the length of the instrument. The B and two A scales are identical, divided logarithmically from left to right and running from 1 to 10. The C scale is divided logarithmically from right to left and runs from 10 to 1. It is labeled RECIPROCALS.

The right end of the base has two D scales, both labeled ROOTS, with two additional A scales on the right end of the slide. The A scales are divided logarithmically from left to right and run from 1 to 10. The top D scale runs from 1 to 3.16 and shows square roots of the numbers on the A scale below it. The bottom D scale runs from 3.16 to 10 and shows square roots of the numbers on the A scale above it, between 10 and 100. Written in pencil on the back of the slide and underneath the slide is the number 1096.

The back of the rule contains two scales on the base labeled F and two scales on the slide labeled E. The top F scale runs the length of the scale and is divided logarithmically from 1 to 10. The sections between 1 and 2, between 2 and 3, between 3 and 4, and between 4 and 5 are subdivided. The lower F scale is similarly divided, but starts from the center of the rule (running from the square root of 10, or 3.16, to 10 and then from 1 to the square root of 10). The E scales are identical to the adjacent F scales. Written in pencil on the back of the slide and underneath the slide is the number 1097.

The bottom of the base on both sides is marked: Patented July 2, 1901. The top edge of the rule is marked: THE ENGINEERS (/) SLIDE RULE. It is also marked: From E. M. Scofield, Bridge Engineer. (/) Sta. A, – Youngstown, – Ohio. (/) Rule sent prepaid on receipt of price. (/) Factory, – Youngstown, Ohio. The paper pasted on this edge also has the formula ex/f, a description of how to solve this formula with the instrument, and tables of equivalents. The bottom edge of the rule gives seventeen more formulas and instructions. It is marked: DIRECTIONS FOR SCALES A • B • C & D (/) FOR "Op." READ "OPPOSITE" (/) FOR "—" READ "ON SCALE" (/) FOR "Ans." READ "FIND ANSWER" (/) FOR LAST SIX PROBLEMS REVERSE SLIDE.

Edson Mason Scofield (1867–1939) developed this rule in 1891, while he worked for Edwin Thacher (inventor of the cylindrical slide rule bearing his name; see, for example, MA.312866). Thacher applied for a patent on the design in 1900 and assigned a half-interest to Scofield when the patent was issued in 1901. According to the instrument, Scofield may have distributed the rule himself. It was also sold by Eugene Dietzgen Company of Chicago from about 1901 through at least 1931. The price for model 1787 ranged from $5.00 to $7.00. By 1926, Dietzgen marked the rule with its company name. This instrument is identical to MA.325990.

John N. Welsh (1904–1999), the donor of this slide rule, was a chemical engineer who obtained his B.S. from the Carnegie Institute of Technology in 1925. He was associated with Hall Laboratories, Inc. of Pittsburgh (later CALGON) from 1939 through at least 1968. By 1972, when this slide rule was briefly placed on exhibit, Welsh was living in Florida.

References: Edwin Thacher, "Slide Rule" (U.S. Patent 677,817 issued July 2, 1901); Catalogue & Price List of Eugene Dietzgen Co., 7th ed. (Chicago, 1904), 173; Catalog of Eugene Dietzgen Co., 12th ed. (Chicago, 1926), 177; Conrad Schure, "The Scofield-Thacher Slide Rule," Journal of the Oughtred Society 3, no. 1 (1994): 20–25; Peter M. Hopp, Slide Rules: Their History, Models, and Makers (Mendham, N.J.: Astragal Press, 1999), 161; Who's Who in Engineering (New York and West Palm Beach: Lewis Historical Publishing Co., 1964), 1996.

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