192 Faraday. 



Faraday's own words,* that "whether the wire moves directly or 

 obliquely across the lines of force, in one direction or another, it 

 sums up the amount of the forces represented by the lines it 

 has crossed," so that " the quantity of electricity thrown into a 

 current is directly as the number of curves intersected."t The 

 induced electromotive force is, in fact, simply proportional to 

 the number of the unit lines of magnetic force intersected by 

 the wire per second. 



This is the fundamental principle of the induction of 

 currents. Faraday is undoubtedly entitled to the full honour 

 of its discovery ; but for a right understanding of the progress 

 of electrical theory at this period, it is necessary to remember 

 that many years elapsed before all the conceptions involved in 

 Faraday's principle became clear and familiar to his contem- 

 poraries ; and that in the meantime the problem of formulating 

 the laws of induced currents was approached with success from 

 other points of view. There were indeed many obstacles to the 

 direct appropriation of Faraday's work by the mathematical 

 physicists of his own generation ; not being himself a mathe- 

 matician, he was unable to address them in their own language ; 

 and his favourite mode of representation by moving lines of 

 force repelled analysts who had been trained in the school of 

 Laplace and Poisson. Moreover, the idea of electromotive force 

 itself, which had been applied to currents a few years previously 

 in Ohm's memoir, was, as we have seen, still involved in 

 obscurity and misapprehension. 



A curious question which arose out of Faraday's theory 

 was whether a bar-magnet which is rotated on its own axis 

 carries its lines of magnetic force in rotation with it. Faraday 

 himself believed that the lines of force do not rotate J: on this 

 view a revolving magnet like the earth is to be regarded as 

 moving through its own lines of force, so that it must become 

 charged at the equator and poles with electricity of opposite 

 signs ; and if a wire not partaking in the earth's rotation were 

 to have sliding contact with the earth at a pole and at the 



* Exp. Res., 3082. t Ibid., 3115. % Ibid., 3090. 



