jan 1, 1872 - Start of period of development of commercially successful mechanical calculators.
Frank Baldwin in the USA invents the pin-wheel calculator. This calculator used a set of wheels that had an adjustable number of teeth. These wheels, also called pinwheels, could be set by using a side lever which could expose anywhere from 0 to 9 teeth, and therefore when coupled to a counter they could, at each rotation, add a number from 0 to 9 to the result.
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