Fowler's "Magnum" Calculator

"Magnum" long scale calculator by Fowler's & Co., with leather case and instruction booklet.

The Fowler Calculator is a type of circular slide rule, a sort of mechanical calculating device, popular in the time before electronic calculators were developed.

Slide rules have a number of different moveable scales on them. By moving the scales on the slide rule, counting the number moved, and comparing the results between them, it is possible to do a range of calculations quite quickly, particularly multiplications, divisions, and conversions between different units. Many slide rules are shaped like straight rulers, but this one is pocket-watch shaped, with the scales adjusted by twisting the knobs on the edge.

It was made by Fowler & Co, a Manchester based company, originally established as the Scientific Publishing Company in 1898 by William Henry Fowler. The firm began offering pocket-watch calculators in 1898 and manufactured devices of this type for several decades, with William Fowler and his son Harold patenting several improvements. The firm offered a whole range of calculators, some intended for general use and others manufactured for the calculations common in specific industries.

This particular Fowler Magnum long-scale calculator was made around 1940 and intended to do a range of calculations. It is larger than other types of Fowler calculators, allowing it to be read with more accuracy. The long scale makes it easier to do larger calculations.