Page:A history of the theories of aether and electricity. Whittacker E.T. (1910).pdf/211

 at any point measures the intensity of the magnetic field at that point.

Faraday constantly thought in terms of lines of force. "I cannot refrain," he wrote, in 1851, "from again expressing my conviction of the truthfulness of tho representation, which the idea of lines of force affords in regard to magnetic action. All the points which are experimentally established in regard to that action—i.e. all that is not hypothetical—appear to be well and truly represented by it."

Faraday found that a current is induced in a circuit either when the strength of an adjacent current is altered, or when a magnet is brought near to the circuit, or when the circuit itself is moved about in presence of another current or a magnet. He saw from the first that in all cases the induction depends on the relative motion of the circuit and the lines of magnetic force in its vicinity. The precise nature of this dependence was the subject of long-continued further experiments. In 1832 he found that the currents produced by induction under the same circumstances in different wires are proportional to the conducting powers of the wires—a result which showed that the induction consists in the production of a definite electromotive force, independent of the nature of the wire, and dependent only on the intersections of the wire and the magnetic curves. This electromotive force is produced whether the wire forms a closed circuit (so that a current flows) or is open (so that electric tension results).

All that now remained was to inquire in what way the electromotive force depends on the relative motion of the wire and the lines of force. The answer to this inquiry is, in