68 



THE THEORY OF IONIZATION OF GASES 



above. He assumed that the positive and negative ions 

 in the gas have equal charges, the same as the atomic 

 charges on ions in liquid electrolytes, a hypothesis which 

 subsequent experiments have confirmed, and attributed 

 the positive charge in the gas to the difference in mass 

 between the positive and negative ions. 



28. Increase of force at cathode accompanied by decrease 



of potential required to maintain a current between 



the electrodes for pressures above the critical pressure. 



The decrease in the potential difference between the 



electrodes that accompanies the increase of the electric 



force at the cathode may also be determined theoretically 



by finding the potential required to maintain a current 



in a field made up of two parts in each of which the 



force is constant, the value of the constant being 



greater in the part near the negative electrode. 



Let a and ft have the values ai and ft, corresponding to 

 a force Xi, through a space of thickness a on the side of 

 the cathode, and the values 0% and /3 2 , corresponding to a 

 force X-2, in the rest of the field of thickness b. 



Negative 



electrode 



x=o 



X, 



Positive 



electrode 



x=l. 



In this case the condition for the maintenance of the 

 current between the plates becomes 



I 



__-i 



Oi A 



In order to apply this formula to a definite case, let the 

 gas between the electrodes be hydrogen at 10 millimetres 



