Artifact Spotlight – Thacher’s Calculating Slide Rule

Before the development of digital calculators, multiplication, division and other complicated mathematical operations were commonly accomplished with slide rules such as this Thacher’s Calculating Instrument. In 1881, “computing engineer” Edwin Thacher of Pittsburg, Pennsylvania, received a patent for an improvement in slide rules. Working with London design firm W.F. Stanly & Co., Thacher developed a slide rule with scales equivalent to 57-66 feet in length instead of the 10 inch length of common slide rules. In addition, Thacher’s Calculator enabled calculations to four or five decimal places, making it a valuable tool in the disciplines of science, engineering, business and commerce.

CF&I’s engineers used this slide rule in the early 1900s to calculate the volume of gas produced in the Blast Furnaces. This slide rule includes scales for multiplication, division and is able to calculate squares and square roots. In addition, the instructions printed on the base of the item ensured accuracy to ±0.0001. There were no trigonometric scales on this rule. The scales are printed on paper sheets which are pasted to a drum. The drum is held in place with a brass frame, which is affixed to a wooden base. The user is able to roll the wooden handles at each end, which in turn, roll the drum to line up the scales to make the calculations. The slats that encompass the drum are also able to rotate for various calculations. Because of the tiny printing on the drum and slats, the original design of this rule also had a metal extension affixed to the base, to which a thumbscrew and magnifying glass were once attached.  It was donated to the Steelworks Center in 2003.

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