758 



Prof. F. Horton and Miss A. 0. Davies on 



collecting electrode. Fig. 3 affords an example of the type 

 of curve obtained from observations of the variation of 

 the measured current with increasing accelerating potential 

 difference by this method. Such curves will be referred to 



Fig. 2. E. curve. 

 Pure helium, 

 P = 0*034 mui. 



Fig. 3. (I— R) curves. Fig. 4. (I — R) curve. 



Pure helium, 

 p=Q-o76 mm. 



Impure helium, 

 _p = 0-608 mm. 





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as (I— R) curves, since with the arrangement of fields used 

 ionization tends to give a positive current, while the photo- 

 electric effect of radiation tends to give a negative current. 

 It may be seen from the curve that a gradually increasing 

 negative current begins at 20*4 volts, indicating the pro- 

 duction of radiation at that point, while at 21*2 volts a sharp 

 reversal of the current takes place, and a few tenths of a 

 volt later the current has a relatively large positive value, 

 indicating that ionization has begun. 



For comparison with the curve of fig. 3, obtained with 

 pure helium at an average pressure of 0*576 mm., a similar 

 series of observations was taken when the helium was con- 

 taminated with a small amount of impurity by shutting off 

 the stream of gas and by lowering the liquid-air vessels so 

 that the carbon in the purifying tubes was not completely 

 covered by liquid air. The results of this series of obser- 

 vations are shown in fig. 4, from which it will be seen that 



