lONIZATION BY NEGATIVE IONS 3 



ions alone, but as the force increases and the sparking 

 potential is approached, the positive ions also acquire 

 the property of producing others to an appreciable 

 extent. 



2. Variation of current with distance between the electrodes 

 when the force is constant. 



In the earlier experiments which were made to test 

 the theory the initial ionization was produced by the 

 action of Bontgen rays. The simplest conditions, how- 

 ever, are realized when the initial ionization consists of 

 negative ions set free from a metal surface by a beam of 

 ultra-violet light. 



When the light falls on a metal plate a number n Q of 

 negative ions are set free which can be made to travel 

 various distances through a gas under any required force 

 to a parallel plate positively charged. If no new ions are 

 produced by collisions the number reaching the positive 

 plate will be no, and the current will be independent of 

 the distance between the plates. If, however, the ions 

 produce others by collisions with the molecules of the 

 gas between the plates, the number reaching the positive 

 plate will increase and will depend on the distance 

 between the plates. In fact, if each negative ion set free 

 from the metal plate produces a new negative ions in 

 going through a centimetre of the gas, and if the new 

 ions produced in the gas have exactly the same property 

 of generating others by collisions, then the number that 



arrive at the positive plate will be n Q a where I is the 

 distance between the plates. For let n be number of ions 

 produced in a layer of thickness, x, measured from the 

 negative electrode, the number n including the original 



B 2 



