Faraday. 189 



discovery was attracting attention, he wrote an Historical 

 Sketch of Electro- Magnetism* as a preparation for which he 

 carefully repeated the experiments described by the writers he 

 was reviewing ; and this seems to have been the beginning of 

 the researches to which his fame is chiefly due. 



The memoir which stands first in the published volumes of 

 Faraday's electrical workf was communicated to the Royal 

 Society on November 24th, 1831. The investigation was 

 inspired, as he tells us, by the hope of discovering analogies 

 between the behaviour of electricity as observed in motion in 

 currents, and the behaviour of electricity at rest on conductors. 

 Static electricity was known to possess the power of " induction " 

 i.e., of causing an opposite electrical state on bodies in its 

 neighbourhood ; was it not possible that electric currents might 

 show a similar property ? The idea at first was that if in any 

 circuit a current were made to flow, any adjacent circuit would 

 be traversed by an induced current, which would persist exactly 

 as long as the inducing current. Faraday found that this was 

 not the case ; a current was indeed induced, but it lasted only 

 for an instant, being in fact perceived only when the primary 

 current was started or stopped. It depended, as he soon 

 convinced himself, not on the mere existence of the inducing 

 current, but on its variation. 



Faraday now set himself to determine the laws of induction 

 of currents, and for this purpose devised a new way of repre- 

 senting the state of a magnetic field. Philosophers had been 

 long accustomed? to illustrate magnetic power by strewing iron 

 filings on a sheet of paper, and observing the curves in which 

 they dispose themselves when a magnet is brought underneath. 



Published in Annals of Philosophy, ii (1821), pp. 195, 274; iii (1822), 

 p. 107. 



t Experimental Researches in Electricity, by Michael Faraday : 3 vols. 



* The practice goes back at least as far as Niccolo Cabeo ; indeed the curves 

 traced by Petrus Peregrinus on his globular lodestone (cf . p. 8) were projections 

 of lines of force. Among eighteenth-century writers La Hire mentions the use of 

 iron filings, Mem. de 1'Acad., 1717. Faraday had referred to them in his electro- 

 magnetic paper of 1821, Exp. Res. ii, p. 127. 



