Although the name 'Comptometer' is actually a trade name of the Felt and Tarrant Manufacturing Company of Chicago, it was commonly used as a generic name for these types of adding machines. The original design was patented in 1887 by Dorr Eugene Felt, a U.S. citizen.
A Comptometer is a mechanical (or electro-mechanical) adding machine. It was the first adding device to be operated solely by the action of pressing keys arranged vertically and horizontally.
Although primarily designed for adding, division, multiplication and subtraction could also be performed. Special comptometers with varying key arrays (with from 30 to well over 100 keys) were produced for a variety of purposes, including calculating currencies, time and Imperial measures of weight.
In the hands of a skilled operator, comptometers can add numbers very rapidly, since all the digits of a number could be entered simultaneously using as many fingers as is required, making them much faster than using an electronic calculator. Consequently, in specialist applications they remained in use in limited numbers into the 1990s, but with the exception of a handful of machines, have now all been superseded by the use of computer software.
From Wikipedia's definition of a comptometer