On November 1, 1881, Edwin Thatcher a computing engineer for the Keystone Bridge Company in Pittsburgh, Pennsylvania, received U.S. Patent No. 249,117 on an improved slide rule. Thacher was a graduate of Rennselaer Polytechnic Institute and spent much of his career designing railway bridges, inventing his slide rule to assist with his calculations.

The slide rule was invented sometime between 1620 and 1630, shortly after John Napier’s publication of the concept of the logarithm. In 1620 Edmund Gunter of Oxford developed a calculating device with a single logarithmic scale. Two years later in 1622 William Oughtred of Cambridge combined two handheld Gunter rules to make a device that is recognizably the modern slide rule.

Edwin Thatcher’s slide rule is notable for its cylindrical form, although his was not the first slide rule of a cylindrical form factor. Thacher’s rule, though it fit on a desk, was equivalent to a conventional slide rule over 59 feet long. It had scales for multiplication and division and another scale, with divisions twice as large, for use in finding squares and square roots.