This is William Fowler’s Universal Calculator, one of a prolific line of circular slide rules produced in Manchester from the end of the 19th century and into the second half of the 20th. This one is 3.4 inches across; others ranged from 2.5″ to 4.7″. It was made between 1938 and 1948.
The Fowler is simple but effective. It has only two controls, one to rotate the scales under a fixed red radial hairline, and the other to rotate a transparent disc bearing a black hairline. By setting the angular separation of the two hairlines to represent a given multiplication factor (i.e. logarithm), one can then use this separation to multiply or divide any number by that factor.
There are also scales for reckoning cube roots, reciprocals, logarithms, sines and tangents. Being circular, they pack more length into less space, allowing a higher precision than a straight slide rule of similar compactness.
Mr. Fowler & Co. spared no attention to detail. If you look at the close-up below, you will note the marks sprinkled between the scales, which are designed to let you compute unit conversions (e.g. between meters and yards), and to locate useful constants (like Pi/4 , the square root of 2 and the natural log of 10).
This delicate instrument comes in a nice protective leather case. Then there is a somewhat tattered instruction manual, with directions and examples — and some unusual added touches. Thus, there is a long section on the history of logarithms, with due tribute to their 17th century inventor, John Napier, and to Sir Joseph Whitworth, “a pioneer of mechanical accuracy and efficiency”. When did you last open the manual of a notebook computer to find the photos of Charles Babbage, Alan Turing, or Marcian Hoff?… The last photo below shows a touching dedication to Fowler’s son, Harold, who evidently participated in the development of these devices. Again, we don’t see such thoughtful sentiment in the instructions of today’s computing hardware.
Exhibit provenance:
eBay, from a seller in Britain.
More info:
“The Fowler Calculators – a Catalogue Raisonné”, published in the “Journal of the Oughtred Society,” Vol. 11, No. 2, Fall, 2002.