Comptometer

The comptometer was the first commercially successful key-driven mechanical calculator, patented in the USA by Dorr E. Felt in 1887.

A key-driven calculator is extremely fast because each key adds or subtracts its value to the accumulator as soon as it is pressed and a skilled operator can enter all of the digits of a number simultaneously, using as many fingers as required, making them sometimes faster to use than electronic calculators. Consequently, in specialized applications, comptometers remained in use in limited numbers into the early 1990s, but with the exception of museum pieces, they have all now been superseded by electronic calculators and computers.

Manufactured without interruption from 1887 to the mid-1970s, it was constantly improved. The mechanical versions were made faster and more reliable, then a line of electro-mechanical models was added in the 1930s. Notably, it was the first mechanical calculator to receive an all-electronic calculator engine in 1961, with the ANITA Mark VII model released by Sumlock Comptometer. This created the link between the mechanical calculator industries and the electronic.

Although the comptometer was primarily an adding machine, it could also do subtractions, multiplication and division. Its keyboard consisted of eight or more columns of nine keys each. Special comptometers with varying key arrays were produced for a variety of special purposes, including calculating currency exchanges, times and Imperial weights. The name comptometer was formerly in wide use as a generic name for this class of calculating machine.

The comptometer is the direct descendant of the key-driven machine of Thomas Hill patented in the USA in 1857 and of the Pascaline invented by Blaise Pascal in France in 1642. By just replacing the input wheels of the Pascaline by the columns of keys of Hill's machine, the comptometer was invented. Addition is performed exactly the same way, and both the Pascaline and the Comptometer make use of the 9's complement method for subtraction, but in the case of the comptometer it is the operator who must choose the right keys for the subtrahend (each key has its 9's complement written in miniature letter next to it).

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