ox FARADAY'S LINES OF FORCE. 207 



LAW V. TJie total electro-magnetic potential of a closed current is measured 

 by the product of the quantity of the current multiplied by the entire electro-tonic 

 intensity estimated in the same direction round the circuit. 



Any displacement of the conductors which would cause an increase in the 

 potential will be assisted by a force measured by the rate of increase of the 

 potential, so that the mechanical work done during the displacement will be 

 measured by the increase of potential. 



Although in certain cases a displacement in direction or alteration of inten- 

 sity of the current might increase the potential, such an alteration would not 

 itself produce work, and there will be no tendency towards this displacement, 

 for alterations in the current are due to electro-motive force, not to electro- 

 magnetic attractions, which can only act on the conductor. 



LAW VI. The electro-motive force on any element of a conductor is measured 

 by the instantaneous rate of change of the electro-tonic intensity on that element, 

 whether in magnitude or direction. 



The electro-motive force in a closed conductor is measured by the rate of 

 change of the entire electro-tonic intensity round the circuit referred to unit 

 of time. It is independent of the nature of the conductor, though the current 

 produced varies inversely as the resistance ; and it is the same in whatever 

 way the change of electro-tonic intensity has been produced, whether by motion 

 of the conductor or by alterations in the external circumstances. 



Tn these six laws I have endeavoured to express the idea which I believe to 

 be the mathematical foundation of the modes of thought indicated in the Ex- 

 perimental Researches. I do not think that it contains even the shadow of a 

 true physical theory ; in fact, its chief merit as a temporary instrument of 

 research is that it does not, even in appearance, account for anything. 



There exists however a professedly physical theory of electro-dynamics, which 

 is so elegant, so mathematical, and so entirely different from anything in this 

 paper, that I must state its axioms, at the risk of repeating what ought to 

 be well known. It is contained in M. W. Weber's Electro-dynamic Measure- 

 ments, and may be found in the Transactions of the Leibnitz Society, and of the 

 Royal Society of Sciences of Saxony*. The assumptions are, 



* When this was written, I was not aware that part of M. Weber's Memoir is translated in 

 Taylor's Scientific Memoirs, VoL v. Art. xiv. The value of his researches, both experimental and 

 theoretical, renders the study of his theory necessary to every electrician. 



