CH. ix.] COLLISION 103 



The " power " of the blow is 



whereas the radiation power is 



2 fu*V_pe*u*. 

 ' \2l) ~~ = 6vP ' 



,i f radiating power a u 2a 



therefore 6F = T . - = , 



total power I v vt 



where t is the time of stoppage, and v is the velocity 

 of light. 



Hence effective radiating power depends chiefly on 

 very sudden stoppage, and on the speed being 

 near that of light. If the velocity is a tenth that 

 of light, and if an electron can be stopped in some- 

 thing like its own diameter, about 10 per cent, of 

 the energy will go in radiation, and the rest will 

 take other forms, presumably heat. But it would 

 take immense " power " to effect such a stoppage as 

 that : not less than two or three thousand kilowatts 

 for each electron. So probably a stoppage within 

 atomic dimension is all that can be expected, and 

 that could be managed by something like 50 watts. 

 But then an exceedingly small fraction of the whole 

 only about one millionth would in that case take 

 the form of X-rays ; their wave-shell then having a 

 thickness comparable with molecular magnitude : 

 whereas in the previous case it was incomparably 

 thinner, and therefore far more penetrating. Both 

 thicknesses however are very small compared with 

 the wave-lengths of ordinary light. 



As the velocity diminishes, more and more of the 

 energy takes the form of heat ; which agrees with 



