Circular Slide Rules

Fowler's Long Scale Magnum Fowler's Universal Fowler's Jubilee Magnum Fowler's Long Scale
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Circular slide rules are typically about 2 to 10 inches in diameter. They are often used as conversion calculators for changing values in one type of unit to another. Some of the better ones - such as the Fowlers - are enclosed in metal cases with glass covers and have similar looks to a large pocket watch. The circular scales of some of the instruments allow for a longer scale length than the standard slide rule and hence improved accuracy.

The effective scale length of the Fowler "Jubilee Magnum" Extra Long Scale Calculator, for instance, is 79 inches (about 6.5 feet). Imagine getting on public transport with a slide rule that long sticking out of your briefcase. Fowler claims that calculations can be made accurately to 4 and sometimes 5 significant figures. Diameter approx 120 mm (4.7 inches).Circular slide rules were not as common as the standard type of slide rule. This most likely reason being due to the fact that they were more expensive and lacked the ease of use. They often appear in very good condition due to the fact that their owners got fed-up with fiddling with them and reverted back to a normal slide rule.