11th October 1887
Dorr E. Felt was granted a patent for the Comptometer.
He experimented with an adding device that he built in a "macaroni box." The comptometer, an adder, displayed a single register of results. Subtraction was carried out by nines-complement arithmetic, and multiplication by repeated addition. The comptometer was a commercial success for businesses, and "comptometer schools" were established early in the 1900s century to teach the efficient operation of this machine.
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