Slide rules for chemistry

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.

Chemical element conversions
Image 1 Chemical element conversions, c.1820s (Wh.6195). Image © the Whipple Museum.


In chemical reactions, given compounds always combine in the same weight ratios. Tools for calculating proportions between various chemical reactants have existed since at least the work of alchemists in the Early Modern period.

As chemistry later emerged as a professional discipline, special chemical slide rules were developed to aid practical work. By the early nineteenth century the term 'chemical equivalents' was used to defined the proportions in which the common chemical substances (either elements or compounds) combined.

In 1814 the Englishman William Hyde Wollaston proposed using a slide rule from which known chemical equivalents could be read. On this chemical slide rule, distances are proportional to the logarithms of the combining quantities. This makes it straightforward to compute what weights of different chemical substances combine to produce the weight of a given product (or conversely how much of each material results from decomposition). Wollaston's 1814 slide rule used oxygen = 10 as its base value, the figure chosen as a matter of convenience.

Wollaston was yet to be convinced by John Dalton's XR  new atomic theory, calling it "purely theoretical" and, therefore, of no use to the "formation of a table adapted to most practical purposes". Not desirous of "warping my numbers according to an atomic theory", Wollaston thus made oxygen the decimal unit of his scale because oxygen was the most common reactant in numerous important chemical reactions and the number ten was a simple figure to manipulate in calculations.

The Whipple Museum's example (image 1) was made by Martin Ehrmann, Professor of Chemistry at the Universities of Vienna and Olomouc (in Moravia). It too used oxygen = 10 as its base unit, the rule listing 175 entries for elements and compounds in total (compared to Wollaston's original 94).

Mikey McGovern

Mikey McGovern, 'Slide rules for chemistry', Explore Whipple Collections, Whipple Museum of the History of Science, University of Cambridge, []

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