CLERK MAXWELL'S ELECTROMAGNETIC THEORY The Rede Lecture for 1923 BY H. A. LORENTZ CAMBRIDGE AT THE UNIVERSITY PRESS 1923 CLERK MAXWELL'S ELECTROMAGNETIC THEORY WHEN I had the honour to be invited to deliver a lecture in the Rede Foundation, I thought I might perhaps present to you a brief review of the electromagnetic theory of your great physicist James Clerk Maxwell. The choice seemed the more appropriate as it is now exactly fifty years ago that the work which raised him at once to the very first rank of investigators of all ages, the Treatise on Electricity and Magnetism, was published. In this work it was proved beyond all doubt that electric and magnetic actions can be conceived as being transmitted through a medium and the theory was crowned by the wonderful revelation that light is an electromagnetic phenomenon. Maxwell's theory was also a great simplification. Indeed, before his time there was much uncertainty and confusion in this part of physics and many contending theories were in the field. In electrodynamics, for instance, we had the laws of Ampère and Grassmann for the actions between elements of current, and, when we went further, we found the speculations of Weber, Riemann and Clausius about the mutual actions of particles of electricity. In connexion with these theories there was a good deal of discussion on the phenomena that were to be expected in the case of closed and in that of open circuits. It was thought in those days that the current in a wire by means of which a metallic conductor is charged, ends on that conductor, and even the discharge current of a condenser was considered not to be closed; there was a gap in the circuit, because we had no idea that something is going on in the insulating layer between the coatings.