Amos Mendenhall

Amos Mendenhall

In 1867, Amos Mendenhall of Cerro Gordo, Indiana, patented a practical calculator (US patent 67786). The patent model of the device (up to 1880, the US Patent Office required inventors to submit a model with their patent application) is still preserved in the National Museum of American History, Washington, D.C. (see the image below).

The practical calculator of Amos Mendenhall is a square metal box with overall measurements: 5.4 cm x 13 cm x 13.8 cm. The box has a boss in the centre inside, in which the shaft of a cylinder is to turn.

The <em>practical calculator</em> of Amos Mendenhall’ width=’450′ height=’571’></img></p><p>The <em>practical calculator</em> of Amos Mendenhall (© National Museum of American History, Washington, D.C.)</p><p>The <em>practical calculator</em> of Amos Mendenhall is an adder with a brass box which has a rotating disc inset in the top. There are 100 small holes around the rim of the disc. Outside the disc, on the top of the box, is a circular ring numbered from 01 to 99, with a gap at 00. Outside of this are three rings of holes, with 100 holes in each ring. These holes are to be used to hold markers indicating digits carried when the disc makes full rotations. On the side of the box is a rotating multiplication table.</p><p>Amos Mendenhall proposed two methods for recording numbers over 99. The first was a set of 9 holes around the edge of the fixed disc, into which the operator could place a pin. Whenever the rotating disc moved a full turn, the operator moved the pin up to the next hundreds digit. Mendenhall suggested a mechanism which would count the number of times the upper plate rotated, and hence give the hundreds place. If the operator rotated especially energetically, and arrived at higher numbers, he suggested a system of pins to be used to represent thousands and higher places.</p><p><img src=

The headstone of Amos Mendenhall in Buena Vista Cemetery, Winchester, Indiana (he shares a headstone with his niece, Leila Bell Mendenhall, who died only two days prior)

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