This five and a half inches circular slide rule, was fabricated by the British company Fearns Calculators, by the early 1970's, and distributed by Sole Australian Distributors Sonnerdale Richardson David Brown Ltd., company based in Stanmore, New South Wales, Australia. Fearns Calculators Ltd. specialized in the production of high quality engineering circular slide rules, one of them in this collection: the Fearns Tank Capacity Calculator.
In the 1950's Fearns Calculators were associated with M.H. Mears & Co, as Fearns, Mears & Co. but in 1962 they dissolved the association continuing each with their own specialized and different range of calculators. Fearns Calculators appears to have ceased trading in the early 1980s.
Fearns Calculators fabricated all plastic slide rules. This circular rule was fabricated with white plastic where color blue scales are engraved; it has also a plastic clear concentric movable disc with a red scale and arrows engraved on. A clear plastic indicator with a hairline is screwed at the disk's center, rotates around and over, and is kept "in tension" on the rule with a little metallic clip, hooked to the rule's edge.
This rule has only two single logarithmic scales D-C, this last one engraved on the movable clear disk. On the rules body is also engraved a double scale for direct Fahrenheit-Centigrade degrees and table of for conversion of eights, sixteenths and twelfths to decimal. On the outer part of the front face there are 105 marks corresponding to different imperial-metric units conversions; this is really a "spread" unit conversion table, distributed such that the corresponding numerical conversion may be calculated using scales D-C.
On the back of the rule is engraved the a numbered index for the 105 unit conversions, a little table for common unit conversions, a reference of the abbreviations used, and quick instructions of how to use the rule, what is quite simple. This rule is an excellent aide for the engineering and scientific work.
The quality in its construction as well as the scales colors and design, makes this rule very aesthetic tool...
References.
Mathematical Instruments. A Private Collecttion: FEARNS-MEARS Circular Calculators. http://www.mathsinstruments.me.uk/page61.html