426 Sir J. J. Thomson on 



which is about 2 x 10 s cm./sec. ; for some of these e/m = 10 4 - 

 Hence i£ V is the potential-difference required to give a 

 charge e when carried by a mass m, this velocity 



or V= J x 10- 4 x 4 x 10 lc = 2 x 10 12 



= 20,000 volts, 



whereas the actual potential-difference between the terminals- 

 was only 3000 volts; hence the positive particles acquire a 

 much greater velocity than they could get by falling through 

 the whole potential-difference between the terminals of the 

 discharge-tube. 



This result has such important relations with the question 

 as to how the positive rays originate, that it is necessary to 

 consider very carefully any sources of error that may lurk in 

 the method by which it was obtained. The method involves 

 the assumption that the magnetic and electric forces acting 

 on the positive rays after they have passed through the cathode 

 remain unaltered, though the pressure in the discharge-tube 

 increases from the value it had when the potential- difference 

 w r as 40,000 volts, to the value it had when this difference 

 was reduced to 3000 volts. There seems no reason to suppose 

 that the magnetic force should be affected by the pressure : 

 the case of the electric force, however, is not so simple. In 

 the experiments a fixed potential-difference was established 

 between two parallel plates, and the positive rays were de- 

 flected as they passed through the space between the plates;, 

 the electric force acting on them was assumed to be the 

 potential-difference between the plates divided by the distance 

 between them. Now the positive rays as they pass through 

 the gas between the plates ionize it; and if the ions were 

 sufficiently numerous, the aggregation of positive ions in one 

 place and negative ones in another might appreciably modify 

 the force between the plates, and make the force midway 

 between them less than the value which it is assumed to 

 possess when deducing the values of ejm and v. Thus it 

 might be possible for the electric force on the positively 

 charged particles to be appreciably less at the higher pres- 

 sures than the assumed value. If this were the case, the 

 values of v and ejm deduced on this assumption would be too* 

 large. It seems very unlikely a priori that if this change in 

 the electric force occurs, it should adjust itself with such 

 nicety as to leave the boundary of the phosphorescent patch 



