﻿724 Profs. Wellisch and Bronson on the Distribution of 



Moulin and Wheelock have shown (locc. citt.) that the ioniza- 

 tion produced by a particles is more readily saturated when 

 the pressure of the gas is reduced ; it was, therefore, of 

 interest to determine whether the percentage cathode activity 

 would follow the positive ionization current when the gas 

 pressure was reduced. For this purpose the emanation was 

 introduced into the testing vessel, which contained air at a 

 pressure of 260 mm., and the percentage cathode activity 

 was determined for various applied positive potentials. The 

 results are given below (Table II.) , and are exhibited as a 

 curve in fig. 2. 



Table II. 

 Emanation in air at a pressure of 260 mm. 



Potential in 



Percentage cathode 



volts. 



activity. 



80 



825 



150 



82-5 



775 



81-8 



790 



82-2 



mo 



81-8 



2250 



83-8 



2250 



84-2 



It is noticeable that although the percentage cathode 

 activity for 150 volts is greater than the corresponding value 

 for a pressure of 1 atmosphere, nevertheless when the higher 

 potentials are reached the values are smaller at the lower 

 pressure. 



When the ionization due to the emanation in equilibrium 

 with its activity and in air at a pressure of 260 mm. was 

 measured for various applied positive potentials, it was ob- 

 served that the alteration of ionization with potential was so 

 extremely slow as to suggest saturation. Over the range 

 for which the ionization could he measured this alteration 

 was too small to justify comparison with the figures given 

 for the percentage cathode activity ; the striking feature is 

 that both curves approach more nearly to the horizontal than 

 the corresponding curves for a pressure of 1 atmosphere. 

 The figures for the lower pressure show that for large 

 potentials the activity is farther from saturation than at 

 1 atmosphere ; it is therefore only reasonable to conclude 

 that in the case of ionization the percentage lack of saturation 

 is greater at the lower than at the higher pressure. 



