James Clerk Maxwell’s field theory of electromagnetism had important and previously unrecognized roots in the cable industry of the mid-nineteenth century. When he took up electrical physics in 1854, the subject was permeated by a concern with cable problems. Guided by William Thomson, Maxwell soon adopted Faraday’s field approach, which in 1861 he sought to embody in a mechanical model of the electromagnetic ether. Seeking evidence to bolster the electromagnetic theory of light to which this model had led him, Maxwell joined the British Association Committee on Electrical Standards, which had been formed in 1861 largely to meet the needs of the submarine telegraph industry. Maxwell’s work on the committee between 1862 and 1864 brought home to him the value of framing his theory in terms of quantities he could measure in the laboratory—particularly the “ratio of units”—rather than relying on a hypothetical mechanism. Maxwell’s shift from his mechanical ether model of 1861 to his seemingly abstract “Dynamical Theory of the Electromagnetic Field” of 1864 thus reflected the often overlooked role concerns rooted in cable telegraphy played in the evolution of his thinking.

Maxwell's great paper of 1865 on the theory of the electromagnetic field is analysed in detail, emphasising the revolutionary nature of its contents.Building on his discovery of the equations for the electromagnetic field, he reformulated the theory without recourse to a mechanical model of the field, although his thinking was still strongly influenced by Newtonian mechanics. He emphasised the primacy of fields as the vehicle for transmitting forces and abolished the Newtonian concept of action at a distance. Fields pervaded the aether, endowing empty space with physical properties. In the new formalism, he demonstrated again that light is electromagnetic radiation.