Fearns started manufacturing special purpose scale rules for industrial application in 1947. They are well known for their circular scale rules- like this one- which was imported into Australia in the 70s when the country covered to the metric system. The rule is model #A7.
The circular slide rule comes in its original vinyl [circular] case – and has instructions for use printed on the back. I am very fond of scale rules, linear and circular- and so snapped up this example; it’s in very good condition and still useful today for quick and efficient metric conversion calculations.
In researching the slide rule I found a number of sites dedicated to the collection of slide rules- and circular slides rules- and indeed Fearns circular slide rules. These useful, technical devices are now very collectible and have a huge fan base. I am not alone in my adoration of slide rules!
The circular slide rule is for sale: $AUD40