No actual measurement ... was required: Maxwell and Cavendish's null method for the inverse square law of electrostatics

TL;DR

This paper examines Maxwell and Cavendish's null experiment for the inverse square law, highlighting its role in establishing the law through null methods and Maxwell's methodological innovations.

Contribution

It reveals how Maxwell used null tests to reinforce the inverse square law and developed a doctrine of method emphasizing the absence of actual measurement.

Findings

Maxwell's null method reinforced the law's validity.

Maxwell's doctrine of method prioritized null tests over direct measurement.

The historical analysis shows the importance of null methods in 19th-century electromagnetism.

Abstract

In 1877 James Clerk Maxwell and his student Donald MacAlister refined Henry Cavendish's 1773 null experiment demonstrating the absence of electricity inside a charged conductor. This null result was a mathematical prediction of the inverse square law of electrostatics, and both Cavendish and Maxwell took the experiment as verifying the law. However, Maxwell had already expressed absolute conviction in the law, based on results of Michael Faraday's. So, what was the value to him of repeating Cavendish's experiment? After assessing whether the law was as secure as he claimed, this paper explores its central importance to the electrical programme that Maxwell was pursuing. It traces the historical and conceptual re-orderings through which Maxwell established the law by constructing a tradition of null tests and asserting the superior accuracy of the method. Maxwell drew on his developing 'doctrine of method' to identify Cavendish's experiment as a member of a wider class of null methods. By doing so, he appealed to the null practices of telegraph engineers, diverted attention from the flawed logic of the method, and sought to localise issues around the mapping of numbers onto instrumental indications, on the grounds that 'no actual measurement ... was required'.