Shoebox Comptometers - "Duplex"

Felt's selection of the term "duplex" was an unfortunate choice, as we shall see. Far better, and more accurate, would have been "multiplex" as used by Ernst Martin in his seminal work, "The Calculating Machines". This feature permitted simultaneous depression of keys in several different columns and significantly improved multiplication and division operations.

McCarthy used the term in this sense as well while Martin would occasionally refer to it as "so-called Duplex". In other words, there was "official" support for the term's usage in this context until at least 1925.

Multi-register calculators date back to the 1880s to the original Odhner design but these were neither key-driven (they were key-set) nor "lister" machines. Perhaps the term was also applied to such machines as well.

The earliest key-driven calculator having more than one register appears to be the Mechanical Accountant which had a second register for displaying the latest keyed item. Second (and sometimes, third) registers appeared on adding machines as well.

Probably as a result of this alternate use of the term, "duplex" was often used to refer to sub-totals on the SuperTotalizer model, thus creating some confusion with the original feature on earlier Comptometers.

By the late 1920s, electric driven machines had arrived and motorized buttons allowed for "direct" subtraction on two-register machines as well as semi-automated multiplication and division. Of course, earlier mechanical calculators with far different designs were able to perform all such operations (and more) but generally with less speed.

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