Abstract

Maxwell’s derivation of an equality between the speed of wave propagation c in a hypothetical electromagnetic medium and the ratio of electrostatic and electromagnetic units of electrical quantity ν was historically his most important argument for the electromagnetic theory of light. He argued that it provided strong grounds for believing that light was an electromagnetic wave and the optical and electromagnetic ether were two different names for the same thing. Acceptance of this identity, Maxwell knew, substantiated his field-theoretic approach to electricity and magnetism at the expense of Continental action-at-a-distance theories. This study begins by problematizing the equality between ν, ostensibly a numerical ratio, and c, a canonical physical quantity. We are thereby drawn into a critical examination of the evolution of Maxwell’s practices of representing physical quantities, units, and their dimensions, with the expectation of shedding light on the nature of physical constants, units, and dimensions in modern scientific practice.