Mr. Maxwell on Faraday^s Lines of Force. 317 



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 =/, coefficient of resistance 

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

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



Oil) aS'^N"! 81 ttB98 j" J[_/y5._p j_C^F jnogdo &iii ooiiia Jhw 

 Jlid odj ^0 dlmr ■: C^ ' ik ' '^ >: 1:0 ioo^^i i noun {q& 



"^ftSiWi^sW^hmwith respect to electric curreilfi'.T^fe^llenip- 

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

 ferent in different directions. The general equations were given aud 

 S^eral cases solved. ' y^' ^ ^ , '/' .' 



^^in magnetic phsenomena, the distinction of quantify and' infceiisiiy' 

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

 Faraday calls the quantity of inductive magnetic action over any 

 surface, depends only on the number of lines of magnetic force which 

 pass through it, and that the total electromotive effect on a conduct- 

 ing wire will always be the same, provided it moves across the same 

 system of lines, in :, whatever manner it does so. But though the 

 quantity of magnetic action over a given area depends only on the 

 number of lines which cross it, the intensity depends on the force 

 required to keep up the magnetism at that part of the medium ; and 

 this will be measured by the product of the quantity of magnetiza- 

 tion, multiplied by the coefficient of resistance to magnetic induction 

 in that direction. 



The equations which connect magnetic quantity and intensity are 

 similar in form to those which were given for electric currents, and 

 from them the laws of diamagnetic and magnecrystallic induction 

 may be deduced and reduced to calculation. 



We have next to consider the mutual action of magnets and elec- 

 tric currents. It follows from the laws of Ampfere, that when a 

 magnetic pole is in presence of a closed electric circuit, their mutual 

 action will be the same as if a magnetized shell of given intensity 

 had been in the place of the circuit and been bounded by it. From 

 this it may be proved, that (1) the potential of a magnetized body on 

 an electric circuit is measured by the number of lines of magnetic 

 forc^ due to the magnet which pass through the circuit. (2) That 

 the total amount of work done on a unit magnetic pole during its 

 passage round a closed curve embracing the circuit depends only on 

 the quantity of the current, and not on the form of the path of the 

 pole, or the nature or form of the conducting wire. 



The first of these laws enables us to find the forces acting on an 

 electric circuit in the magnetic field. Give the circiiit any displace- 



