What Happened on October 11th
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 20th century to teach the efficient operation of this machine.