This five and a half inches circular slide rule, was fabricated by the British company Fearns Calculators, and distributed (?) by Sole Australian Distributors Sonnerdale Richardson David Brown Ltd., company based in Stanmore, New South Wales, Australia, what is the inscription on the rule's sheath. 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 twenty black color decimal-inches-millimeters conversion tables are engraved. These two-columns by ten-rows tables are arranged radially and divided in two sections with ten tables each: Millimeters to Inches and Inches to Millimeters. The number of digits per column varies between these two sections. In the Millimeters to Inches section tables have three digits for the millimeters (except Table 0 with two) and six digits for the inches. In the Inches to Millimeters section tables have four digits for the inches and five digits for the millimeters (except tables 1 & 2 with six). Each of these ten tables sets gives a sequence of hundred values, from 0 to 99 in the first column.
There are no scales on this rule to perform arithmetic calculations. On the front face is only engraved a double scale for the direct decimal-inches - millimeters conversion for any value. On the back face there are two examples of how to use the rule, and a table with the equivalences of fractions of an inch (the smallest is 1/64), to decimal-inches and millimeters.
This rule has also a clear plastic indicator with two printed windows to facilitate the readings on the rule. The outer window is labeled Precision Dimensions, and it has imprinted two radial lines of red dots that work as movable decimal points, In this window you can read on the tables the direct conversion decimal-inches into millimeters, and vice-versa. The inner window is labeled Rule Dimensions, and has imprinted lines with the labels inches - millimeters that help to make conversion on the two imprinted scales.
On the front face there are also some paragraphs with brief instructions of how to use the rule. On the back face of the rule there are two examples of rule's operation, (showing this is a very easy to use rule), and a table with the equivalences of fractions of an inch (the smallest is 1/64), to decimal-inches and millimeters.
This rule has no the variety of text-colors other FEARNS Rules have, but it is still aesthetic. There are no marks or engravings on this rule indicating the exact manufacturing date, only the manufacturing country:
References.
Mathematical Instruments. A Private Collecttion: FEARNS-MEARS Circular Calculators. http://www.mathsinstruments.me.uk/page61.html