Pro Calculo! Adder

Adders like this one were designed to help consumers with addition, but did not actually add automatically. The surface of the metal instrument has seven slots that reveal part of seven flat notched metal bands below. To enter a digit, one pulls down a band with the metal stylus. The hooked shape of the slots exposed a notch in an adjacent band, making it possible to carry or to borrow digits. This adder also has a zeroing bar at the base. It fits into a dark brown paper case.

Instruments of this type appeared as early as the 1600s, and sold commercially from the 1890s into the 1970s. They sold in Germany from the invention of the “Trick” in 1911. Otto Meuter patented a variation on this device that sold as the Arithma from 1920. Meuter received a fixed fee for each Arithma produced. With inflation, this sum soon was minute.

Meuter decided to form another company with J. Bergmann and to market adders known as the Pro Calculo! and the Correntator. These sold widely in the 1920s. For example, the trade magazine Typewriter Topics reported that 15,000 ProCalculo! adders sold in 1926. In 1928, the product was renamed the Produx.

References: Typewriter Topics, 59, February, 1925, p. 84. One model, offered by Pittsburgh Typewriter & Supply, sold for $3.00.

Typewriter Topics, 67 (November, 1927), p. 50-51. New style adders introduced.

Martin Reese, Historische Buerowelt, 43 (September 1995).

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