On this day in 1887 the comptometer, the first commercially successful key-driven mechanical calculator, was patented in the United States by inventor Dorr E. Felt. The comptometer was primarily an adding machine, but it could also subtract by nines-complement arithmetic, divide as well as multiply through repeated addition.
The comptometer had a keyboard of eight or more columns with nine keys each, and some models had varying key arrays specifically made for special purposes, such as currency exchanges and imperial weights. It was a commercial success, and schools were established that taught the efficient operation of the machine. It was manufactured without interruption from 1887 to the mid-1970s with constant improvements, and continued to be in limited use up until the early 1990s when it was superseded by electronic calculators and computers.
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