*)•> L. Page — Fundamental Relations of Electrodynamics. 



This expression gives all the forces hetween currents, and 

 also the induced current phenomena due to moving a closed 

 circuit through a so-called magnetic field. The induced cur- 

 rent effects produced in a secondary circuit by variation of the 

 current in the primary are very simply treated as follows : 



Faraday's Law. 



Whenever a charged hody is accelerated, it is obvious that 

 the lines of force will he kinked. If the charged hody is 

 accelerated only for a very short time, these kinks will travel 

 outwards in the form of a pulse. Now this pulse must have 

 the same velocity relative to the system of the field inside the 



Fig. 3. 



-* 



■(n»f<tt)t 



pulse as it has relative to the system of the field outside the 

 pulse. These two systems, however, may be chosen arbitrarily. 

 Therefore the pulse must have the same velocity relative to all 

 systems. The only velocity to satisfy this condition is the 

 velocity of light. Hence the velocity of the pulse must be c* 



*This reasoning may be objected to on the ground that the pulse may 

 expand as it moves outward : i. e., the outside of the pulse may have 

 a greater velocity than the inside. But if this was true under certain 

 conditions, it would be necessary to assume that the reverse was true under 

 certain other conditions. So we would be forced to the most improbable 

 conclusion that the inside of the pulse might outstrip and pass through the 

 outside of the pulse. 



