2 THE THEORY OF IONIZATION OF GASES 



attains a maximum value, which is not exceeded unless 

 very large forces are used. It is possible, however, by 

 reducing the pressure of the gas, to make the ions 

 travel with sufficient velocity to generate others by 

 collisions with molecules, even when the potential 

 differences employed are small, and thus with a few 

 hundred volts to obtain large increases in the current. 1 

 The curve, figure 1, showing the connection between 

 the current and electric force in a gas at low pressure, 

 may be taken as illustrating this effect. 



In the first stage, A B, the current between the plates 

 increases with the electro-motive force. The rate of 

 increase diminishes as the force increases, and the current 

 tends to attain a maximum value. 



In the second stage, B C, the current remains prac- 

 tically constant and shows only small variations for 

 large changes in the force. If the ions are produced by 

 the action of Eontgen rays or Becquerel rays, the con- 

 stant value is attained when the force is sufficiently great 

 to collect all the positive and negative ions on the 

 electrodes, but before this value is reached an appreciable 

 number of the ions is lost by recombination. Again, the 

 ions may be produced by the action of ultra-violet light 

 on the negative electrode. In this case, if the force is 

 too small, some of the ions do not reach the positive 

 electrode, but diffuse through the gas to the negative 

 electrode. 



In the third stage, C D, when the force is still further 

 increased, there is a large increase in the conductivity. 

 This can be explained on the hypothesis that new ions 

 are generated by collisions, at first practically by negative 



1 Nature, Yol. Ixii., August 9, 1900. 



