James Clerk Maxwell devises a theory to…

James Clerk Maxwell devises a theory to unify partial theories on electricity and magnetism in 1865.

In this year, he resigns the chair at King's College London and returns to Glenlair with his wife Katherine.

Around 1862, while lecturing at King's College, Maxwell had calculated that the speed of propagation of an electromagnetic field is approximately that of the speed of light.

He had considered this to be more than just a coincidence, and commented "We can scarcely avoid the conclusion that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena." (J J O'Connor and E F Robertson, James Clerk Maxwell, School of Mathematics and Statistics, University of St. Andrews, Scotland, November 1997)

Working on the problem further, Maxwell had shown that the equations predict the existence of waves of oscillating electric and magnetic fields that travel through empty space at a speed that could be predicted from simple electrical experiments; using the data available at the time, Maxwell had obtained a velocity of 310,740,000 m/s.

In his 1864 paper "A dynamical theory of the electromagnetic field", Maxwell had written, "The agreement of the results seems to show that light and magnetism are affections of the same substance, and that light is an electromagnetic disturbance propagated through the field according to electromagnetic laws". (Maxwell, James Clerk (1865). "A dynamical theory of the electromagnetic field" (PDF). Philosophical Transactions of the Royal Society of London 155: 459–512. (This article accompanied a December 8, 1864 presentation by Maxwell to the Royal Society.))

Maxwell also introduces the concept of the electromagnetic field in comparison to force lines that Faraday had discovered.

By understanding the propagation of electromagnetism as a field emitted by active particles, Maxwell could advance his work on light.

At this time, Maxwell believes that the propagation of light requires a medium for the waves, dubbed the luminiferous aether.

Over time, the existence of such a medium, permeating all space and yet apparently undetectable by mechanical means, will prove more and more difficult to reconcile with experiments such as the Michelson–Morley experiment.

Moreover, it seems to require an absolute frame of reference in which the equations are valid, with the distasteful result that the equations changed form for a moving observer.

These difficulties will inspire Albert Einstein to formulate the theory of special relativity, and in the process Einstein will dispensed with the requirement of a luminiferous aether.