Faraday. 193 



equator, a current would steadily flow through it. Experiments 

 confirmatory of these views were made by Faraday himself ;* 

 but they do not strictly prove his hypothesis that the lines of 

 force remain at rest ; for it is easily seenf that, if they were to 

 rotate, that part of the electromotive force which would be 

 produced by. their rotation would be derivable from a potential, 

 and so would produce no effect in closed circuits such as Faraday 

 used. 



Three years after the commencement of Faraday's researches 

 on induced currents he was led to an important extension of 

 them by an observation which was communicated to him by 

 another worker. William Jenkin had noticed that an 

 electric shock may be obtained with no more powerful source of 

 electricity than a single cell, provided the wire through which 

 the current passes is long and coiled ; the shock being felt when 

 contact is broken. J As Jenkin did not choose to investigate 

 the matter further, Faraday took it up, and showed that the 

 powerful momentary current, which was observed when the 

 circuit was interrupted, was really an induced current governed 

 by the same laws as all other induced currents, but with this 

 peculiarity, that the induced and inducing currents now flowed 

 in the same circuit. In fact, the current in its steady state 

 establishes in the surrounding region a magnetic field, whose 

 lines of force are linked with the circuit ; and the removal of 

 these lines of force when the circuit is broken originates an 

 induced current, which greatly reinforces the primary current 

 just before its final extinction. To this phenomenon the name 

 of self-induction has been given. 



The circumstances attending the discovery of self-induction 



Exp. Res., $ 218, 3109, &c. 



t Cf. W. Weber, Ann. d. Phys. lii (1841) ; S. Tolver Preston, Phil. Mag. xix 

 (1885), p. 131. In 1891 S. T. Preston, Phil. Mag. xxxi, p. 100, designed a crucial 

 experiment to test the question ; but it was not tried for want of a sufficiently 

 delicate electrometer. 



% A similar observation had been made by Henry, and published in the Amer. 

 Jour. Sci. xxii (1832), p. 408. The spark at the rupture of a spirally-wound 

 circuit had been often observed, e.g., by Pouillet and Nobili. 



Exp. Res., 1048. 



O 



