II.

Electromagnetism and Light

Rejecting explanations (modelled on the theory of gravitation) in terms of forces acting at a distance, Michael Faraday had interpreted electricity and magnetism in terms the electromagnetic “field”, defined by imaginary lines of force. William Thomson (Lord Kelvin) had shown that these ideas could be expressed in mathematical terms.

Initially guided by Thomson, Maxwell developed Faraday’s work. He first illustrated the geometry of lines of force by the physical analogy of streamlines in a fluid (see Fluid Mechanics). Seeking a theory of the field grounded on the mechanics of an ether, a medium for transmission, he found its basis in Thomson’s 1856 proposal that the Faraday effect—the rotation of polarized light in a magnetic field—could be explained by the rotation of vortices in an ether. In his paper “On Physical Lines of Force” (1861-1862) Maxwell set out an ether model of rotating vortices (representing magnetism) separated by “idle wheel” particles (whose motion represents the flow of an electric current).

The modification of the ether model to encompass electrostatics unexpectedly led to his electromagnetic theory of light. He showed that a disturbance in the electric or magnetic field should lead to a disturbance travelling as a wave through space. He demonstrated the close agreement between the velocity of these waves and the measured velocity of light. He developed the established theory that light was propagated by an ether by asserting that this ether was electromagnetic, and he thus unified optics and electromagnetism.

Maxwell had from the first emphasized that his “idle wheel” ether model was conjectural, and in 1864 he discarded this model as a temporary scaffolding for his theory. He achieved a more general presentation of his electromagnetic theory of light in terms of the transmission of energy through the ether. He retained mechanical foundations by grounding the general equations of the electromagnetic field (the forerunners of what are now known as the four “Maxwell equations”, as reformulated in the 1880s by Oliver Heaviside and Heinrich Hertz) on general equations of dynamics. He expounded this theory in his Treatise on Electricity and Magnetism (1873).

The production of electromagnetic waves by Hertz in 1887 led to the acceptance of Maxwell’s theory of the electromagnetic field. In the 20th century it came to be detached from its formulation in terms of ether.