Faraday. 191 



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 the 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."f 



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 firstj 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 



* Exp. Res., 3174. 



t Some of Faraday's most distinguished contemporaries were far from sharing 

 this conviction. " I declare," wrote Sir George Airy in 1855, " that I can hardly 

 imagine anyone who practically and numerically knows this agreement " between 

 observation and the results of calculation based on action at a distance, "to hesitate 

 au instant in the choice between this simple and precise action, on the one hand, 

 and anything so vague and varying as lines of force, on the other hand." Cf. 

 Bence Jones's Life of Faraday, ii, p. 353. 



I Exp. Res., 116. Ibid., 213. 



