chapter  5
29 Pages

Station 2 (1861–1862)

On physical lines of force
ByGiora Hon, Bernard R. Goldstein

In Station 2 Maxwell stated that he had changed his approach, but kept the unifying concept of lines of force. The contrived analogy of Station 1 was merely instrumental in showing that a mathematical formulation of electromagnetic phenomena could be based on the concept of lines of force. But in Station 2 Maxwell’s goal was to account for the phenomena: he now presupposed a mechanical hypothesis to explain the phenomena. The change in the underlying methodology is dramatic. Maxwell discarded the instrumental approach and sought a hypothesis at the micro-level to account for the phenomena at the macro-level. The question now was what tensions in, or motions of, a medium are capable of producing mechanically the observed electromagnetic phenomena? Maxwell’s mechanical point of view involved illustrations based on the molecular-vortex hypothesis. The change is then from analogical argumentation to causal reasoning. While in the first paper he took a formal-analogical approach, in the second he took a physical approach, that is, a theory closely related to a mechanical illustration. Recent commentators take this illustration to be a model but, in fact, Maxwell called it “hypothesis” and treated it as such. In that sense the methodology of Station 2 is traditional. Maxwell made two fundamental discoveries in Station 2: the displacement current and that light is an electromagnetic phenomenon, and Maxwell linked them to Faraday’s unifying concept of lines of force. These two discoveries substantially changed this domain of physics.