Station 3 (1865)
Station 3 presents a dynamical theory of the electromagnetic field in which the displacement current and light as an electromagnetic phenomenon are taken as assumptions. The argumentative structure was reversed: results of Station 2 were now presupposed as phenomena. The theory of the electromagnetic field is dynamical because it assumes a field surrounding electrified and magnetic bodies, in which matter in motion subject to forces produces electromagnetic phenomena. In Station 3 the medium does the work and therefore the mechanical scheme of the hypothesis of molecular vortices of Station 2 is superfluous. Maxwell no longer appealed to a specific mechanism, and remarked that dynamical illustrations, such as a flywheel, are to be considered merely as assisting the reader to understand the phenomena. The goal was to construct a satisfactory formal theory: “General equations of the electromagnetic field.” Thus, the overriding concern was not to offer an explanatory mechanism by which the phenomena are produced. The methodology of Station 3 consists in translating the phenomena directly into mathematical equations. In general, no hypothesis was involved, and no explanatory mechanism was sought. Maxwell identified two kinds of energy that can exist in the medium: “actual” and “potential.” The fundamental property of the medium, namely, its elasticity, becomes then essential, since in virtue of this elasticity both kinds of energy can be stored in the medium. In this novel approach, which assumes an energetic medium, a specific mechanical scheme is unnecessary, for the medium takes on the physics of electromagnetism.