Re: Do any duplex circular slide rules have the front and back center disc connected?

Mark Davis

The Dempster RotaRule model 'A' and model 'AA'  circular slide rules have a single central disk that is functional on both the front and back of the rule. These slide rules were made primarily of "Vinylite" plastic, introduced by the company that later became Union Carbide about a year before the first RotaRule model 'A' was manufactured in 1928. This Oughtred Society article, by W. Richard Davis, provides a good deal of detail on the design and construction of the Dempster RotaRule models:

After the Dempster patents expired, a different manufacturer, B.C. Boykin, produced a very similar product in two models; the "RotaRule 510", and "RotaRule 560". The model 510 has decimally-divided trig scales, vs. the minute-second divisions on the model 560 trig scales.

A key feature of both the Dempster and B.C. Boykin designs are three-cycle spiral C/D scales, for an advertised scale length of 50 inches. Additionally, there is a 3-cycle spiral LL1/LL2/LL3 scale and a 3-cycle spiral LL01/LL02/LL03 scale in the inner rings on the disk, on the same side as the spiral C/D scales.

Mark Davis

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