What's nu? A re-examination of Maxwell's 'ratio-of-units' argument, from the mechanical theory of the electromagnetic field to 'On the elementary relations between electrical measurements'.

This re-examination of the earliest version of Maxwell's most important argument for the electromagnetic theory of light-the equality between the speed of wave propagation in the electromagnetic ether and the ratio of electrostatic to electromagnetic measures of electrical quantity-establishes unforeseen connections between Maxwell's theoretical electrical metrology and his mechanical theory of the electromagnetic field. Electrical metrology was not neutral with respect to field-theoretic versus action-at-a-distance conceptions of electro-magnetic interaction. Mutual accommodation between these conceptions was reached by Maxwell on the British Association for the Advancement of Science (BAAS) Committee on Electrical Standards by exploiting the measurement of the medium parameters-electric inductive capacity and magnetic permeability-on an arbitrary scale. While he always worked within this constraint in developing the 'ratio-of-units' argument mathematically, I maintain that Maxwell came to conceive of the ratio 'as a velocity' by treating the medium parameters as physical quantities that could be measured absolutely, which was only possible via the correspondences between electrical and mechanical quantities established in the mechanical theory. I thereby correct two closely-related misconceptions of the ratio-of-units argument-the counterintuitive but widespread notion that the ratio is naturally a speed, and the supposition that Maxwell either inferred or proved this from its dimensional formula.