Mr maxwell, on FARADAY'S LINES OF FORCE. 47 



must therefore admit the existence of certain forces capable of keeping up a constant current 

 in a closed circuit. Of these the most remarkable is that which is produced by chemical 

 action. A cell of a voltaic battery, or rather the surface of separation of the fluid of the 

 cell and the zinc, is the seat of ati electro-motive force which can maintain a current in 

 opposition to the resistance of the circuit. If we adopt the usual convention in speaking 

 of electric currents, the positive current is from the fluid through the platinum, the conducting 

 circuit, and the zinc, back to the fluid again. If the electro-motive force act only in the 

 surface of separation of the fluid and zinc, then the tension of electricity in the fluid must 

 exceed that in the zinc by a quantity depending on the nature and length of the circuit 

 and on the strength of the current in the conductor. In order to keep up this diffierence 

 of pressure there must be an electro-motive force whose intensity is measured by that difference 

 of pressure. If F be the electro-motive force, / the quantity of the current or the number 

 of electrical units delivered in unit of time, and K a quantity depending on the length and 

 resistance of the conducting circuit, then 



F^lK='p-'p, 



where p is the electric tension in the fluid and p' in the zinc. 



If the circuit be broken at any point, then since there is no current the tension of 

 the part which remains attached to the platinum will be p, and that of the other will 

 be 'p . p —p, or F affbrds a measure of the intensity of the current. This distinction of 

 quantity and intensity is very useful*, but must be distinctly understood to mean nothing 

 more than this : — The quantity of a current is the amount of electricity which it transmits 

 in unit of time, and is measured by / the number of unit currents which it contains. 

 The intensity of a current is its power of overcoming resistance, and is measured by F 

 or IK, where K is the resistance of the whole circuit. 



The same idea of quantity and intensity may be applied to the case of magnetism "I". 

 The quantity of magnetization in any section of a magnetic body is measured by the 

 number of lines of magnetic force which pass through it. The intensity of magnetization 

 in the section depends on the resisting power of the section, as well as on the number of lines 

 which pass through it. If k be the resisting power of the material, and S the area of 

 the section, and / the number of lines of force which pass through it, then the whole 

 intensity throughout the section 



When magnetization is produced by the influence of other magnets only, we may put 

 p for the magnetic tension at any point, then for the whole magnetic solenoid 



F = I j^dx = IK^p-p'. 



• Exp. Res. Vol. III. p. 519. f Exp. Res. (2870), (3293). 



