lONIZATION BY POSITIVE IONS 41 



and the value of n in terms of the other constants is 

 obtained by substituting for r its value n n when x=l, 

 I being the distance between the plates. Thus 



This gives the number of negative ions n arriving at 

 the positive electrode when n negative ions start from 

 the negative electrode, and both positive and negative 

 ions generate others by collisions with molecules of the 

 gas. 



It will be noticed that when the distance between the 

 plates I has a certain value S given by the equation 



the denominator of the fraction in the expression for n 

 becomes zero, so that n becomes infinite. This shows 

 that a current would continue to flow indefinitely after 

 the supply of the negative ions n Q from the surface of 

 the negative electrode ceases. The importance of this 

 conclusion in connection with sparking potentials will be 

 considered later. 1 



For distances between the plates shorter than S the 

 denominator of the fraction expressing n is positive, and 

 the current becomes zero after the light ceases to act on 

 the electrode. The values of n are then finite, but greater 



than the number of ions rc e a which would reach the 

 positive electrode if the negative ions alone produced 

 others by collisions. 



1 See section 23. 



