results of Faraday, but established the electro-magnetic theory of light, since verified experimentally by Hertz. His first researches thereon were published in 1864. In 1871 appeared his great Treatise on Electricity and Magnetism. He constructed the electro-magnetic theory from general equations, which are established upon purely dynamical principles, and which determine the state of the electric field. It is a mathematical discussion of the stresses and strains in a dielectric medium subjected to electro-magnetic forces. The electro-magnetic theory has received developments from Lord Rayleigh, J. J. Thomson, H. A. Rowland, R. T.
Glazebrook, H. Helmholtz, L. Boltzmann, O. Heaviside, J. H.
Poynting, and others. Hermann von Helmholtz turned his
attention to this part of the subject in 1871. He was born in 1821 at Potsdam, studied at the University of Berlin, and published in 1847 his pamphlet Ueber die Erhaltung der Kraft. He became teacher of anatomy in the Academy of Art in Berlin. He was elected professor of physiology at Königsberg in 1849, at Bonn in 1855, at Heidelberg in 1858. It was at Heidelberg that he produced his work on Tonempfindung. In 1871 he accepted the chair of physics at the University of Berlin. From this time on he has been engaged chiefly on inquiries in electricity and hydrodynamics. Helmholtz aimed to determine in what direction experiments should be made to
decide between the theories of W. Weber, F. E. Neumann,
Riemann, and Clausius, who had attempted to explain electro-dynamic
phenomena by the assumption of forces acting at a distance between two portions of the hypothetical electrical fluid,–-the intensity being dependent not only on the distance, but also on the velocity and acceleration,–-and the theory of Faraday
and Maxwell, which discarded action at a distance and assumed
stresses and strains in the dielectric. His experiments favoured the British theory. He wrote on abnormal dispersion, and created analogies between electro-dynamics and hydrodynamics. Lord Rayleigh compared electro-magnetic problems
with their mechanical analogues, gave a dynamical theory of diffraction, and applied Laplace's coefficients to the theory of
radiation. Rowland made some emendations on Stokes' paper
on diffraction and considered the propagation of an arbitrary electro-magnetic disturbance and spherical waves of light. Electro-magnetic induction has been investigated mathematically by Oliver Heaviside, and he showed that in a cable it is
an actual benefit. Heaviside and Poynting have reached
remarkable mathematical results in their interpretation and development of Maxwell's theory. Most of Heaviside's papers have been published since 1882; they cover a wide field.