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Before the rise of scientific calculators and computers in the 1960s and 70s, cylindrical slide rules were used to complete multiplication, division and other complex mathematical operations.

This particular slide rule — known as Thacher’s Calculating Instrument — was donated to the university by the late William J. Hanna (ElEngr’43; MS’48; PhD’51), an engineering professor who taught at CU Boulder for 46 years. It was purchased in the early 1920s.                        

Edwin Thacher, a computing engineer who designed railway bridges for Pittsburgh’s Keystone Bridge Company, first patented this slide rule in 1881. The model pictured above was produced by Keuffel & Esser Co. in New York in 1902-03 and cost $35. Today, its estimated value is $1,050 and it is part of the CU Heritage Center collections. 

Complex Math

More elaborate slide rules allowed for calculations such as square roots, exponentials, logarithms and trigonometric functions.                         

Better Accuracy

Slide rules increased the speed and accuracy of mathematical calculations previously done by hand.

Mid-Century Decline

Slide rule use declined as computers and scientific calculators became more widely available to technical workers during the 1960s and 1970s. 

A Versatile Tool

With its range of accuracy (the manufacturer claimed up to five significant figure precision), this scale appealed to engineers, architects, businesspeople and scientists. 

The Size

This specific cylinder is 4 inches in diameter and 18 inches in length.

Photo by Mona Lambrecht