13 x 11 x 6.5", works perfectly.*The Comptometer was invented by the American Dorr Eugene Felt and was pateented in 1887. Manufactured by the Felt and Tarrant Manufacturing Company of Chicago, the Comptometer was the first truly practical and commercially successful adding machine, with the first serious manufacture and sales occuring in 1888. it was the first succesfull key driven adding and calculating machine. "Key driven" means that just pressing the keys adds the numbers entered to the total - no other action is required - so it is very quick for adding long lists of numbers. The basic function of the Comptometer is addition. There is a column of keys (in general 1-9) for each decade. When a key is pressed, that number is added to that decade, with carry to the next higher decade, if applicable. Pulling the handle forwards clears the total to zero.
Comptometers were very fast in operation when adding up lists, such as required in accounting. Operators were specially trained to make use of the full keyboard and enter each number by pressing all the digits in one go using all fingers, as necessary, at once. In other words all the digits were entered in parallel, the mechanism being able to cope with this.
In contrast a modern electronic calculator only has 10 digit keys so the digits of each number have to be entered one at a time, serially, which is slower.
Comptometers were widely used into the late 1970s and were ousted by advances in the use of computers for accounting rather than the development of electronic calculators. People trained in the proper and swift use of Comptometers often kept using them for many more years since for adding up lists they were quicker than a standard ten-key electronic calculator.
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