VELOCITY OF ELECTRICITY, the rate at which electricity is propagated through a conductor. The velocity of electricity through a conducting wire is in all cases very great when compared with the velocities of moving bodies, such as the velocity of a railway train or of a projectile; but the velocity of electricity, any more than the velocity of a planet, of a railway train, or of a bullet, is not a certain number of miles per second fixed for all cases; it varies for a number of causes, and to such an extent that while Wheatstone found the velocity of static electricity through copper wire to be 288,000 miles per second, in the Atlantic cable of 1858 it was found to be 3,000 miles per second. Faraday showed that a submarine cable acts precisely as a leyden-jar, that the water serves as the outside coating, and that just as a leyden-jar takes time to become charged and to discharge, so a cable through which a sharp signal is sent from one end delivers the signal at the other end more or less prolonged; that is, the deflection of 'the receiving needle is gradual, reaching a maximum and then at the same rate coming back to rest. Sir William Thomson showed that the more delicate the receiving instrument, the more instantaneous is the first appearance of the current at the receiv ing end of the cable. He gives three reasons for the retardation of the electric current: (1) Charge and electrical accumulation in a con ductor subjected in any way to the process of electrification. (2) Electromagnetic induction,
or electromotive force, excited in a conductor by variations of electric currents either in adjacent conductors or in different parts of its own length. (3) Resistance to conduction through a solid. The first successful attempt to find the velocity of electricity was made by Wheatstone with the revolving mirror (his invention), which has been so successfully employed by Foucault to discover the velocity of light.
The velocity of a voltaic current when the earth forms part of the circuit has been de termined by the United States Coast Survey to average 16,000 miles per second. The velocity of an electric current through any conductor depends upon the resistance of the conductor, so that in economic practice the question of velocity becomes a problem of resistances.
In the use of electric meters for the meas urement of inductances and capacities it is essential that the current employed shall have a uniform speed. This is secured by the use of a device known as a regulator' Several forms of this instrument are in use, but the principle is the same in all — the introduction or elimination of resistances in the field circuit by short-circuiting. For extensive data as to resistance of metals, glasses, porcelains, etc., at various temperatures above and below zero. Consult the "Smithsonian Physical Tables'