"He went shopping and found the perfect casing -- a macaroni box. He bought some meat skewers, staples, elastic bands and string. He didn't finish the device until New Year's Day, 1885. But, the finished product was the prototype of the comptometer -- the first successful key driven calculator.
Felt had solved the over-rotation problem by providing positive stops that were activated by the keys themselves. He knew his machines must calculate faster than accountants, many of whom could mentally add four columns of figures at one time."

David Weil, Computer Museum of America.

Source: Computer Museum of America.

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