Nature of an Electric Current.—The question, What is an electric current? is involved in the larger question of the nature of electricity. Modern investigations have shown that negative electricity is identical with the electrons or corpuscles which are components of the chemical atom (see [Matter] and [Electricity]). Certain lines of argument lead to the conclusion that a solid conductor is not only composed of chemical atoms, but that there is a certain proportion of free electrons present in it, the electronic density or number per unit of volume being determined by the material, its temperature and other physical conditions. If any cause operates to add or remove electrons at one point there is an immediate diffusion of electrons to re-establish equilibrium, and this electronic movement constitutes an electric current. This hypothesis explains the reason for the identity between the laws of diffusion of matter, of heat and of electricity. Electromotive force is then any cause making or tending to make an inequality of electronic density in conductors, and may arise from differences of temperature, i.e. thermoelectromotive force (see [Thermoelectricity]), or from chemical action when part of the circuit is an electrolytic conductor, or from the movement of lines of magnetic force across the conductor.
Bibliography.—For additional information the reader may be referred to the following books: M. Faraday, Experimental Researches in Electricity (3 vols., London, 1839, 1844, 1855); J. Clerk Maxwell, Electricity and Magnetism (2 vols., Oxford, 1892); W. Watson and S.H. Burbury, Mathematical Theory of Electricity and Magnetism, vol. ii. (Oxford, 1889); E. Mascart and J. Joubert, A Treatise on Electricity and Magnetism (2 vols., London, 1883); A. Hay, Alternating Currents (London, 1905); W.G. Rhodes, An Elementary Treatise on Alternating Currents (London, 1902); D.C. Jackson and J.P. Jackson, Alternating Currents and Alternating Current Machinery (1896, new ed. 1903); S.P. Thompson, Polyphase Electric Currents (London, 1900); Dynamo-Electric Machinery, vol. ii., “Alternating Currents” (London, 1905); E.E. Fournier d’Albe, The Electron Theory (London, 1906).
(J. A. F.)
[1] See J.A. Fleming, The Alternate Current Transformer, vol. i. p. 519.
[2] See Maxwell, Electricity and Magnetism, vol. ii. chap. ii.
[3] See Maxwell, Electricity and Magnetism, vol. ii. 642.
[4] Experimental Researches, vol. i. ser. 1.
[5] See Maxwell, Electricity and Magnetism, vol. ii. § 542, p. 178.
[6] See W.G. Rhodes, An Elementary Treatise on Alternating Currents (London, 1902), chap. vii.