192 ON FARADAY'S LINES OF FORCE. 



If we make 



da db dc 



\edS=lir\\\pdxdydz, 



where the integration on the right side of the equation is effected over every 

 part of space within the surface. In a large class of phenomena, including all 

 cases of uniform currents, the quantity p disappears. 



Magnetic Quantity and Intensity. 



From his study of the lines of magnetic force, Faraday has been led to 

 the conclusion that in the tubular surface* formed by a system of such lines, 

 the quantity of magnetic induction across any section of the tube is constant, 

 and that the alteration of the character of these lines in passing from one 

 substance to another, is to be explained by a difference of inductive capacity 

 in the two substances, which is analogous to conductive power in the theory 

 of electric currents. 



In the following investigation we shall have occasion to treat of magnetic 

 quantity and intensity in connection with electric. In such cases the magnetic 

 symbols will be distinguished by the suffix 1, and the electric by the suffix 2. 

 The equations connecting a, b, c, k, a, ft, y, p, and p, are the same in form as 

 those which we have just given, a, b, c are the symbols of magnetic induction 

 with respect to quantity; k denotes the resistance to magnetic induction, and 

 may be different in different directions ; a, ft, y, are the effective magnetizing 

 forces, connected with a, b, c, by equations (B) ; p is the magnetic tension or 

 potential which will be afterwards explained ; p denotes the density of real 

 magnetic matter and is connected with a, b, c by equations (C). As all the 

 details of magnetic calculations will be more intelligible after the exposition of the 

 connexion of magnetism with electricity, it will be sufficient here to say that 

 all the definitions of total quantity, with respect to a surface, the total intensity 

 to a curve, apply to the case of magnetism as well as to that of electricity. 



* Exp. Res. 3271, definition of " Sphondyloid." 



