62 THE THEORY OF IONIZATION OF GASES 



is due to the multiplication of the few ions which under 

 all conditions are present in the gas. Theoretically any 

 small number of ions would be sufficient to initiate a 

 discharge when the plates are at the sparking distance, 

 S corresponding to the electric force, and that a few 

 ions are being continually generated in a gas even when 

 it is contained in a closed vessel has been shown by 

 Geitel 1 and C. T. K. Wilson. 2 But the number of such 

 ions is very small, and being generated throughout the 

 volume of the gas, they are not on the average as 

 efficient for starting a spark as ions coming from the 

 negative electrode, each of which traverses the whole dis- 

 tance between the electrodes. This accounts for the small 

 difference of two or three volts between the ordinary 

 sparking potential and that obtained when ultra-violet 

 light of small intensity falls on the negative electrode. 



1 Geitel, Physikalische Zeitschr., ii., 1900. 

 8 0. T. E. Wilson, Cam. Phil. 8oc., xi., 1900. 



