104 ELECTRONS AND IONS [CH. ix. 



the fact that at moderate vacua the target gets 

 red-hot. 



The ratio of the radiation power to the total power, 

 is as the dimensions of an electron to the distance 

 light would travel during the period of the stoppage : 

 taking the acceleration as uniform. So to get all 

 the energy radiated it is necessary to stop a pellet 

 moving with a tenth the speed of light in something 

 like a tenth of its own diameter. 



