280 Prof. J. J. Thomson on the Genesis of the Ions 



molecules, to produce other ions and corpuscles. This disso- 

 ciation may be directly due to the collision, or indirectly to 

 rays like Rontgen rays produced by the collision. Let us 

 take the case of a gas through which a few ions are dis- 

 tributed : then, if the energy or velocity communicated by the 

 electric field to these ions exceeds a certain critical value, 

 each of the original ions will produce other ions, and these 

 new ions will give rise to fresh ions, so that the number of 

 ions and the conductivity of the gas will rapidly increase; it will 

 go on increasing until the conductivity of the gas reaches such 

 a value that the strength of the electric field is reduced so 

 that the work done by it on an ion is equal to the critical 

 value; when this stage is reached each ion produces one and only 

 one successor, so that the number of ions remains constant, 

 and the gas, with the discharge passing through it, attains a 

 steady state. 



If the strength of the field had been so small that the work 

 done on an ion was not equal to the critical value, the number 

 of ions would not increase, and there M 7 ould be no discharge 

 through the gas beyond the very slight leak due to the 

 ions originally present in the gas. Thus this view explains 

 why a field of finite strength is required to send the discharge 

 through a gas. 



The maximum kinetic energy given by the field to an ion 

 will be measured by the work done on the ion during its free 

 path, for after a collision the disturbance in the energy and 

 direction of motion is so great that the work has as it were 

 to be begun over again ; while if the direction of the motion 

 is reversed the field, if it increased the kinetic energy of the 

 ion before the collision, will diminish it afterwards. Thus if 

 X is the mean free path, the kinetic energy given by the field 

 to the ion will be measured by F^X, where e is the charge on 

 the ion and F the electric force. The condition that discharge 

 should take place is that the maximum kinetic energy should 

 equal q, where q is a quantity depending on the nature of the 

 gas. Thus the condition for discharge is Fe\ = q, so that F, 

 the electric force required to produce discharge, varies in- 

 versely as the mean free path of the ions or corpuscles* 



Now X is inversely proportional to the density of the gas, 

 so that on this theory the electric force required to produce 

 discharge should in the same gas be directly proportional to 

 the density of the gas, a result which is approximately true 

 over a wide range of pressures.. Thus the theory we are 

 describing gives a very simple explanation of the fact that 

 it is easier to produce a discharge through a gas at a low 

 pressure than through one at a high. 



