52 



THE THEORY OF IONIZATION OF GASES 



gas insulates well for a potential which is a few volts 

 less than that required to initiate a discharge. When 

 the applied potential is equal to or greater than the 

 sparking potential, the insulation breaks down, and a 

 comparatively large current, accompanied by a glow, 

 passes through the gas. For a fixed distance between 

 the electrodes the sparking potential changes with the 

 pressure of the gas. At high pressures the potential is 

 large; it diminishes with the pressure and reaches a 

 minimum for a certain critical value of the pressure. 



a 



I 



Pressure. 

 Figure 10. 



When further reductions are made in the pressure the 

 sparking potential rises rapidly, and attains very high 

 values for the lower pressures. 



The variations in the sparking potential may be illus- 

 trated by the curve, P Q B, figure 10, A being the 

 critical pressure and Q A the minimum sparking 

 potential. 



When parallel plates are used as electrodes the spark- 

 ing potential depends only on the quantity of gas between 

 the electrodes, which is measured by the product of the 

 pressure p and distance S between the electrodes. This' 



