CHAPTER VIII. 

 DETERMINATION OF THE MASS OF AN ELECTRON. 



So far, all the measurements quoted have resulted in 

 a consensus of certainty respecting our knowledge of 

 e/m. for gaseous conduction and radiation ; and the 

 measurements made on the cathode rays in a Crookes's 

 tube, or near a plate leaking in ultra-violet light, 

 have likewise given us a knowledge of their velocity, 

 and shown that it is about one- thirtieth of the velocity 

 of light : more or less according to circumstances. 

 But so far no direct estimate has been made of either 

 e or m separately. The difficulty of making these 

 measurements is great, because we are dealing with an 

 aggregate of an enormous and unknown number of 

 these bodies. It would not be difficult to make a 

 determination of the aggregate mass of a set of pro- 

 jectiles, say Nra, where N is the number falling on a 

 target in a given time, by means of the heat which 

 the blow generates ; or better, perhaps, by the 

 momentum which they would impart to a moving 

 arm after the fashion of a ballistic pendulum ; 

 provided their velocity u were known, as in this 

 case it is. The aggregate energy ^ Nmu 2 , or the 

 aggregate momentum Nmtt, could thus be found ; but 

 howls m to be separated from N ? 



Again, if the particles are collected in, a hollow 



