Processes of Ionization in Gases. 853 



been attributed to ions generated in the gas, it might appear 

 from these recent experiments that a large proportion of 

 the additional electrons were really set free from the ne- 

 gative electrode by radiation emitted from molecules of 

 the gas. 



2. When ultra-violet light from an external source falls 

 on the negative electrode, a constant or saturation current 

 is obtained with certain forces but large increases of current 

 are obtained with larger forces ; and the question arises, 

 whether an appreciable number of the additional electrons 

 which make up these larger currents come from the negative 

 electrode. 



If the additional electrons were set free from the electrode 

 by radiations from the gas, the ratio of the currents with 

 large forces to the saturation currents would depend on 

 the photo-electric sensitiveness of the electrode, and large 

 variations in these ratios would be obtained by altering 

 the state of the surface or by using electrodes of different 

 metals. 



In the earlier experiments which were made to test the 

 theory of ionization by collision, the ratios of the currents 

 were always found to be the same, although the continued 

 use of the same electrodes affected very considerably the 

 photo-electric activity. 



As, however, no definite record of the semitiveness of 

 the plate to ultra-violet light was kept, it was thought 

 desirable to make some experiments with plates of widely 

 different activities and to see if there were any changes 

 in the ratios of the larger currents to the saturation currents 

 corresponding to changes in the sensitiveness of the plates 

 to ultra-violet light. 



3. If X be the electric force and p the pressure of the 



X 



gas, then for the smaller values of the ratio — the currents 



P 



between parallel plates are given by the formula n = n e ax 3 

 where a. is the number of pairs of fresh ions produced by 

 each electron in moving through 1 cm., x is the distance 

 apart of the plates, and njn is the ratio of the. current for 

 the given electric force to the saturation current. 



This equation is obtained on the supposition that the initial 

 ionization at the surface of the negative electrode is all duo 

 to the external source of ultra-violet light. If part o( the 

 ionization is due to radiation from the gas the equation will 

 not be accurately true, and in particular the values of x 



