Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/274

 This simple result may be regarded as the beginning of the theory of electric oscillations.

Thomson was at this time much engaged in the problems of submarine telegraphy; and thus lie was led to examine the vexed question of the "velocity of electricity" over long insulated wires and cables. Various workers had made experiments on this subject at different times, but with hopelessly discordant results. Their attempts had generally taken the form of measuring the interval of time between the appearance of sparks at two spark-gaps in the same circuit, between which a great length of wire intervened, but which were brought near each other in order that the discharges might be seen together, In one series of experiments, performed by Watson at Shooter's Hill in 1747-8, the circuit was four miles in length, two miles through wire and two miles through the ground; but the discharges appeared to be perfectly simultaneous; whence Watson concluded that the velocity of propagation of electric effects is too great to be measurable.

In 1834 Charles Wheatstone, Professor of Experimental Philosophy in King's College, London, by examining in a revolving mirror sparks formed at the extremities of a circuit, found the velocity of electricity in a copper wire to be about one and a half times the velocity of light. In 1850 H. Fizeau and E. Gounelle, I experimenting with the telegraph lines from Paris to Rouen and to Amiens, obtained a velocity about one-third that of light for the propagation of electricity in an iron wire, and nearly two-thirds that of light for the propagation in a copper wire.

The first step towards explaining these discrepancies was made by Faraday, who early in 1854 showed experimentally that a submarine cable, formed of copper wire covered with