ROTOR logartárcsa

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In the beginning of 1930s, mostly wooden slide rules with straight scales were widely used, but existed circular slide rules too, which seemed to be an extra-sized pocket watch. These instruments had only the basic scales in most cases. Though the beginning and the end of the scales are meet, multiplication and division can be made without moving through the slide to the other side of the rule.

In the yeas of recession, the Telefongyár (Factory for telephones) in Budapest had to design and produce goods that were not belonging to their profile. The patent for a logar-tárcsa (circular slide rule) was dated from 8 February 1933 with ID 109576. This invention was unusual on its material, its size and its features. It was made of new silver or german silver (alloy of copper, zinc and nickel) and the diameter is 6.36 cm so makes it possible to measure in cms with rolling the slide rule. The planned version described in the patent has the stator (the outer disc) an engraved socket for the inner disc called rotor, but the version produced has two plain discs. Both versions were made for PR-purposes with company names engraved on the front. The production of Rotor was discontinued in 1938 due to increasing requirements of the army.

Slide rules manufactured in Hungary

• Sarolta Czakó-Károly Jenei: A Telefongyár története (History of Factory for telephones) 1876-1976. Telefongyár, Budapest, 1976. 299 pages
  
• 109576 sz. Szabadalmi leírás (Patent description): Logar-tárcsa
  
• A Telefongyár emlékoldala (Memorial website of Telefongyár)