CH. i.] ACCELERATED CHARGE 7 



and a magnetic field are superposed there cannot 

 be static equilibrium at that place, energy must flow 

 through the medium ; and the rate of transfer of 

 energy the amount conveyed per second through 

 unit area is equal to 1/4 TT times the vector product of 

 the intensity of the two fields : that is to say, it is 

 measured by the area of the parallelogram bounded 

 by lines representing the two fields in magnitude and 

 direction ; a quantity commonly expressed as 



V(EH), or [EH], or as EH sin 0, 



where is the angle between E and H. 



The direction of propagation of energy is normal 

 to that same area, and its ' sense ' or sign depends 

 upon the sense of the two fields. If both were 

 reversed, the sense of the transmission of energy 

 would continue unchanged : and its amount remains 

 constant so long as the fields are constant, that is 

 so long as the current is steady. Another way of 

 expressing the facts is to say that the space in which 

 two fields are superposed is full of momentum ; and 

 that the moment of momentum ^appropriate to a pole 

 m and a charge e is simply em. 



Accelerated Charge. 



One more statement : 



So far we have dealt with the case of steady rest or 

 steady motion ; but what about the intermediate 

 stages, the stages of starting and stopping ? What is 

 the condition of things after the charge has begun to 

 move but before it has attained a constant speed, and 

 again when the brake is applied and the speed is 

 decreasing, or when the direction of motion is chang- 

 ing? What phenomena are observable during the 

 epoch of acceleration or retardation of speed or 



