82 THE THEORY OF IONIZATION OF GASES 



Again, if the case of point discharges be considered it 

 will be seen that when the point is positive the supply of 

 negative ions cannot be kept up by the action of positive 

 ions at the negative electrode. For this electrode might 

 be large and so distant from the point, that the force at 

 its surface would be too small to give the positive ions a 

 velocity of impact sufficient for the production of negative 

 ions. In this case it is obvious that the effect of the 

 positive ions must be attributed to their action on the 

 molecules of the gas, and the difference between the 

 sparking potentials for positive and negative points 

 follows immediately from this theory, as has been 

 explained above. Another fact to be noticed is that 

 a positive ion makes only one collision with the electrode, 

 whereas it makes a large number of collisions with 

 molecules of the gas, and there is no reason for supposing 

 that a negative ion is more easily set free from a metal 

 than from the gas molecules. There is thus definite 

 evidence to show that molecules of the gas are ionized by 

 the positive ions, and for pressures above the critical 

 pressure a consistent explanation may be obtained of a 

 large number of phenomena on the theory that this is the 

 predominating effect produced by the positive ions. 



On the other hand, at pressures below the critical 

 pressure the negative ions forming the cathode rays 

 make up an integral part of the current, so that for 

 these low pressures the ions coming from the negative 

 electrode should be taken into consideration, and when 

 they introduce appreciable effects the sparking potentials 

 might be expected to depend on the metal of the electrode. 



36. Examination of some other theories of the sparking 



potential. 



Several expressions for the sparking potential in 

 terms of the pressure have been given by physicists, 



