Dorr E. Felt of Chicago created the first comptometer (1885) using an old macaroni box at the age of twenty-two. The Comptometer is claimed to be the first successful keyboard adding machine on the market. It is a portable, full keyboard, direct key driven machine designed for high speed calculations involving all branches of mathematics. It is duplex in operation in that it will add in more than one column at a time, each column having the capacity to add, receive, and carry simultaneously. Totals are shown in dials below keyboard; denominational readings are facilitated by dial wheels of two colors, and by decimal markers; there is no printing mechanism. Dials are reset by a clearing lever at side of the machine. The keyboard design offers good feedback, and the operation is quiet compared to some of the clinkier models. Later enhancements to the machine included the addition of the multiplex keys, which permit simultaneous depression of keys in several different columns. Additionally, the triple-clear signal was added to eliminate the danger of the machine not being set to zero before beginning a new operation. As soon as a new operation is commenced, the operator will be reminded by the sound of a bell. If only half-zeros appear in the windows and the operator feels a slight resistance, the machine has been properly set to zero. Depending on the model, Comptometers generally ranged in weight from 17 1/4 pounds to 24 1/4 pounds, and could be purchased for $300 - $400.
Comptometer
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