TONIZATION BY POSITIVE IONS 39 



from the negative electrode was cut off, and a continuous 

 discharge would ensue. The investigations show how 

 the potential required to produce a continuous discharge 

 may be found on the assumption that the whole ioniza- 

 tion is produced by collisions of positive and negative 

 ions in a gas, and, as will be seen, there is a very accurate 

 agreement over a large range of pressures between the 

 potentials thus calculated and the sparking potentials 

 determined experimentally. 



In making an investigation of the currents which 

 would be produced between parallel plates when both 

 positive and negative ions generate others by collisions, 

 the results of the experiments may be anticipated, and 

 it may be assumed that the positive and negative ions 

 produced by the impact of a positive ion with a neutral 

 molecule are identical with the positive and negative ions 

 produced by the impact of a negative ion. In applying 

 the theory, therefore, L t is necessary only to consider one 

 kind of positive ion and one kind of negative ion in each 

 gas, but the positive ions, unlike the negative ions, are 

 most probably different in different gases. 



If a number HQ of negative ions start. from the negative 

 plate and move through a distance I fiom the positive 

 plate they will generate others in the gas, and a number 



w e a ^ will reach the positive electrode. Thus ftof * a lj 



positive ions are produced in the gas and move in 

 the opposite direction. When these also ionize the gas 

 the total number n of negative ions that reach the 



positive electrode exceeds noe . 



Of the number n no of each kind generated in the 

 gas, let r be produced in the layer of gas between the 

 negative plate and a parallel plane at a distance x, and 



