Slide rules for other applications

With the rise of the slide rule as an essential aid to calculation, a number of inventors began to create rules for specialist applications. With no international standards for their manufacture until the late 19th century, slide rules could be highly idiosyncratic. Rules made for specific purposes could have the support of professional groups, and instrument makers vied for their support by catering to specific needs.

Specific gravity and alcohol proof
Image 1 Hydrometer and brewer's slide rule, mid 19th century (Wh.2337). Image © the Whipple Museum.
McFarlane's Calculating Cylinder
Image 2 McFarlane's Calculating Cylinder, c.1835 (Wh.6381). Image © the Whipple Museum.
Lawrence Antenna Slide Rule
Image 3 Lawrence Antenna Slide Rule c. 1945 (Wh.6049). Image © the Whipple Museum.

Brewing

Brewing was important for the development of standardisation in industry as the process had to be monitored carefully and the final proof reported for taxation and the fixing of price. As is still done today, a hydrometer (Image 1) was used to measure the changing 'gravity' of the 'wort', which would start off high due to the presence of fermentable sugars and drop as fermentation continued.

A brewer's slide rule was used to convert these gravity measurements to others measurements, for example degrees Plato or proof, which specified the percentage of alcohol by volume. These scales were not linearly related, and the slide rule made it easier for the busy brewer to monitor progress. Improper calibration at any point could lead to customers getting more (or less) than they bargained for out of a pint!

Finance

The rise of modern commerce relied upon tables that showed changing commodity prices and compound currency exchanges. Any good capitalist would have his ready reckoner on hand when transacting a deal.

Slide rules were developed to perform financial calculations, such as working out interest, more handily. Rather than flipping through a book, a Victorian banker could align desired conversions using McFarlane's Calculating Cylinder (Image 2), a cylindrical rule. While not a logarithmic slide rule, it made use of rotating scales like in Oughtred's XR  Circles of Proportion, and inspired the design of further cylindrical rules, including ones that could fit into a shirt pocket.

Military

Galileo's XR  sector had been developed for use as an artillery instrument. With this in mind, it only makes sense that many slide rules were engineered for the purpose of calculating firing ranges.

However, slide rules could also serve other purposes on the battlefield. This example (Image 3) was produced by Lawrence Engineering Service, an American company that mass-produced millions of slide rules in the mid-20th century. It was commissioned by the Massachusetts Institute of Technology's Radiation Laboratory, and was used for calculating radar antenna beam patterns. This slide rule was considered so strategically important that it's use was classified until after World War II.

Mikey McGovern

Mikey McGovern, 'Slide rules for other applications', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, [http://www.hps.cam.ac.uk/whipple/explore/calculatingdevices/sliderules/sliderulesforotherapplications/, accessed 25 November 2017]

 
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