This is a fine example of a watch-style circular slide rule, albeit a very large one – it is 87mm (about 3½ inches) in diameter. Click on the photo at right for a large version, or visit my slide rule collection for all the details.Using one of these slide rules is a bit more work than an ordinary linear slide rule (turning the knobs is slightly tedious), but they pack a lot of accuracy into a very small package. With its three-segment D scale, this one can multiply and divide with the same accuracy as a linear slide rule that is 49 cm (over 19 inches) long – not back for something that fits conveniently into your pocket!
The Fowler watch-style slide rules (they made many models) seem somehow quintessentially English to me: heavy, sturdy, practical, and yet quite sophisticated. I know of no American-made slide rule even remotely like this one, though several American manufacturers made circular slide rules. There were several English manufacturers of fine watch-style circular slide rules, at least one French manufacturer, and even the Soviets made a cheap imitation. But the English models have a unique style that, for whatever reason, was never duplicated outside of England…
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