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PROCEEDINGS 



CAMBRIDGE PHILOSOPHICAL SOCIETY. 



February 11, 1856. 



A paper was read by Mr. Maxwell of Trinity College, " On Fara- 

 day's Lines of Force," in continuation of a former paper (Proceedings 

 of the Society, Dec. 10, 1855). 



This paper was chiefly occupied with the extension of the method 

 of lines of force to the phamomena of electro-magnetism, by means 

 of a mathematical method founded on Faraday's idea of an " electro- 

 tonic state." 



In order to obtain a clear view of the phenomena to be explained, 

 we must begin with some general definitions of quantity and inten- 

 sity as applicable to electric currents and to magnetic induction. It 

 was shown in the first part of this paper, that electrical and mag- 

 netic phenomena present a mathematical analogy to the case of a 

 fluid whose steady motion is affected by certain moving forces and 

 resistances. {"The purely imaginary nature of this fluid has been 

 already insisted on.] Now the amount of fluid passing through any 

 area in unit of time measures the quantity of action over this area ; 

 and the moving forces which act on any element in order to over- 

 come the resistance, represent the total intensity of action within the 

 element. 



In electric currents, the quantity of the current in any given direc- 

 tion is measured by the quantity of electricity which crosses a unit 

 area perpendicularly to this direction ; and the intensity, by the 

 resolved part of the whole electromotive forces acting in that direc- 

 tion. In a closed circuit, whose length =1, coefficient of resistance 

 = k, and section = 0, if F be the whole electromotive force round 

 the circuit, and I the whole quantity of the current, 

 I C 2 F 



The laws of Ohm with respect to electric currents were then ap- 

 plied to cases in which the conducting power of the medium is dif- 

 ferent in different directions. The general equations were given and 

 several cases solved. 



In magnetic phaenomena, the distinction of quantity and intensity 

 is less obvious, though equally necessary. It is found, that what 

 Faraday calls the quantity of inductive magnetic action over any 



No. XII. — Proceedings of the Cambridge Phil. Soc. 





