64 THE THEORY OF IONIZATION OF GASES 



sparking potential. The difference between these two 

 potentials arises from the same effects as produce the 

 difference between the sparking potentials for negative 

 and positive points. 



It is necessary to point out that there are limits to the 

 ranges of values of the current, electric force, and 

 pressure for which the collision theory, as it is here 

 discussed in a simple form, can be expected to give a 

 complete explanation of the experimental results. When 

 the current is large the gas becomes heated, and the 

 number of molecules in the path of the discharge is 

 diminished ; also a certain proportion of the positive and 

 negative ions recombine, and this introduces a further 

 complication. When the pressures are very low, and 

 large forces are required to produce a discharge, radia- 

 tions, such as cathode rays and Kontgen rays, are 

 emitted, which contribute to the formation of ions and 

 thus facilitate the discharge. A comparison of the 

 sparking potentials for cylinders and parallel plates 

 shows that radiation effects are probably of importance 

 when the pressure is somewhat below the critical 

 pressure. The investigations will therefore be confined 

 at first to pressures above the critical pressure and to 

 the smaller currents, for which it is highly improbable 

 that there are any other processes of ionization taking 

 place that add appreciably to the number of ions 

 generated by collisions. 



In order to examine the experimental results, and to 

 see how far they may be explained by the theory, it is 

 necessary to find the condition that a given field of force 

 should suffice to maintain a current when the supply of 

 ions is kept up by the effects of collisions. An expression 

 may be found for this condition in the general case by 



