310 Prof. J. J. Thomson on Cathode Rays. 



the region outside the plates; in this region the magnetic 

 force will be in the opposite direction to that between the 

 plates, and will tend to bend the cathode rays in the opposite 

 direction : thus the effective value of H will be smaller than 

 the value used in the equations, so that the values of m/e are 

 larger, and those of v less than they would be if this correction 

 were applied. This method of determining the values of m/e 

 and vis much less laborious and probably more accurate than 

 the former method ; it cannot, however, be used over so wide 

 a range of pressures. 



From these determinations we see that the value of m/e is 

 independent of the nature of the gas, and that its value 10-7 

 is very small compared with the value 10 ~ , which is the 

 smallest value of this quantity previously known, and which is 

 the value for the hydrogen ion in electrolysis. 



Thus for the carriers of the electricity in the cathode rays 

 ■m/e is very small compared with its value in electrolysis. 

 The smallness of m/e may be due to the smallness of m or the 

 largeness of e, or to a combination of these two. That the 

 carriers of the charges in the cathode rays are small compare! 

 with ordinary molecules is shown, I think, by Lenard's results 

 as to the rate at which the brightness of the phosphorescence 

 produced by these rays diminishes with the length of path 

 travelled by the ray. If we regard this phosphorescence sis 

 due to the impact of the charged particles, the distance 

 through which the rays must travel before the phosphorescence 

 fades to a given fraction (say 1/e, where e= 'I'll) of its original 

 intensity, will be some moderate multiple of the mean free 

 path. Now Lenard found that this distance depends solely 

 upon the density of the medium, and not upon its chemical 

 nature or physical state. In air at atmospheric pressure the 

 distance was about half a centimetre, and this must be com- 

 parable with the mean free path of the carriers through air 

 at atmospheric pressure. But the mean free path of the 

 molecules of air is a quantity of quite. a different order. The 

 carrier, then, must be small compared with ordinary molecules. 



The two fundamental points about these carriers seem to 

 me to be (1) that these carriers are the same whatever the gas 

 through which the discharge passes, (2) that the mean free 

 paths depend upon nothing but the density of the medium 

 traversed by these rays. 



It might be supposed that the independence of the mass of 

 the carriers of the gas through which the discharge passes 

 was due to the mass concerned being the quasi mass which a 

 charged body possesses in virtue of the electric field set up in 



