104 ON FARADAY'S LINKS OF FORCE. 



one another, and leave a single current running round the bounding line. So 

 that the magnetic effect of a uniformly magnetized shell is equivalent to that 

 of an electric current round the edge of the shell. If the direction of the current 

 coincide with that of the apparent motion of the sun, then the direction of 

 magnetization of the imaginary shell will be the same as that of the real mag- 

 netization of the earth*. 



The total intensity of magnetizing force in a closed curve passing through 

 and embracing the closed current is constant, and may therefore be made a 

 measure of the quantity of the current. As this intensity is independent of the 

 form of the closed curve and depends only on the quantity of the current which 

 passes through it, we may consider the elementajry case of the current which 

 flows through the elementary area dydz. 



Let the axis of x point towards the west, z towards the south, and y 

 upwards. Let x, y, z be the coordinates of a point in the middle of the area 



, then the total intensity measured round the four sides of the element is 



ffe 



- 



Total intensity = - dy dz. 



The quantity of electricity conducted through the elementary area dydz is 

 ".'/'/</:, and therefore if we define the measure of an electric current to be the 

 total intensity of magnetizing force in a closed curve embracing it, we shall have 



_dfr_d yi 

 ^~ dz dy ' 

 , dy, da, 



= ~ 



c= _ 

 dy dx ' 



* See Experimental Researches (3205) for the relations between the electrical and magnetic circuit. 

 considered as mutually embracing curves. 



