Circular slide rules

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Circular slide rules

Circular slide rules use two dimensions (a flat circle) and are often more compact than linear slide rules of similar accuracy. Some have the scale spread around multiple turns in order to achieve a longer scale and hence greater accuracy. The multiple turns can be either concentric rings like the Fowlers long scale, or a continuous spiral like the Ross Precision Computer.

  • Single turn - Scale lengths 100 to 630 mm (4 to 25"). Significant digits: 3 to 4
  • Concentric - Scale lengths 100 to 630 mm (4 to 25"). Significant digits: 3 to 4
  • Spiral - Scale lengths 100 to 630 mm (4 to 25"). Significant digits: 3 to 4

Halden Calculex

Halden's calculex circular slide rule, double sided, the inner disc is rotated by the thumb nut in the centre. The cursors are marked on the glass faces which can also be rotated. Mine is complete with leather wallet and a copy of 'Revised Rules, Tables and Formulae for Patent Circular Slide Rule (Halden Calculex)' - a 94 page booklet which is also housed in a pocket in the case. On one side there are A, B,& C scales, whilst on the other are A with B reversed.

D Scale length: 126 mm. Significant digits: 2.7 to 3.7.

Fowlers Calculator

A compact "pocket watch" style of circular slide rule.

D Scale length: 188 mm. Significant digits: 2.9 to 3.7.

Loga 30sT

Highly functional circular slide rule.

D Scale length: 302 mm. Significant digits: 3.1 to 4.

Fowlers Long Scale

Fowler's double sided Long Scale Calculator, complete with velvet lined nickel plated case. Known also as a 'vest pocket' model, these double sided calculators were regarded as comprehensive and accurate. It measures 2.5 inches in diameter. Full instructions with worked examples can be found on

D Scale length: 980 mm. Significant digits: 3.7 to 4.

Gilson Atlas spiral slide rule

Vintage (c.1940s) Gilson spiral slide rule measures 8 3/8" in diameter. Metal Marked on one side "ATLAS Slide Rule" and Copyrighted Made in USA Gilson Slide Rule Co. Stuart, Ca. on the other.

D Scale length: 15708 mm. Significant digits: 4.8 to 5.

Fowlers Magnum Long Scale

A bigger, and hence more accurate, version of the Fowlers Long Scale. Diameter 120 mm. The long scale has 6 turns making a scale length of 1.2 metres.

D Scale length: 1508 mm. Significant digits: 3.8 to 4.6.

Fowlers Jubilee Magnum Extra Long Scale

Fowler's Jubilee Magnum Extra Long Scale calculator, designed in 1948 by Harold Fowler to mark the 50th anniversary of the founding of the firm by his father William Henry Fowler in 1898. This was the largest circular calculator made by Fowler's, measuring 120 mm in diameter, and it has a 2 metre long spiral scale extending over ten turns.

D Scale length: 2859 mm. Significant digits: 4.1 to 4.7.

Ross Precision Computer

The Ross Precision Computer is a very interesting spiral slide rule. The materials are metal, glass, and celluloid all in a leather case, with intruction book.

D Scale length: 15394 mm. Significant digits: 4.8 to 5.

Copyright 2012, Dave Hoyer.