Radiation and Ionization of Helium. 759 



the 204 volts point is, in these circumstances, indicated by 

 the beginning of an increasing positive current, denoting the 

 occurrence of ionization, while the 21*2 volts point is not 

 distinguishable owing to the steepness of the slope of the 

 curve. The detection of ionization at 20'4 volts in impure 

 helium while only radiation is detectable at this voltage in 

 the pure gas is in agreement with the experiments of Horton 

 and Bailey already referred to. 



The curve of fig. 3 shows that the negative current 

 detected between 20*4 volts and 21*2 volts is much smaller 

 than the positive current which is detected as soon as 

 21'2 volts is passed. The relative smallness of the current 



Fig 1 . 5. (I+R curve). 

 Pure helium, ;j = 0'50 mm. 























!/ 











> 















i 























/ 

























f 























/ 























J 



/ 





















> 



/, 



-' 



i 













^ 



^ 



Y& 









































1 

























1 



















&£ti 



VO-^v 



^H?H 



J 







2 



5 



2 



8 



2 



7 



























eoo 



between 20*4 volts and 21*2 volts is also illustrated by 

 the curve of fig. 5, which was obtained with the final field 

 V 4 reversed in direction so that radiation and ionization both 

 gave positive currents, the arrangement thus resembling that 

 used by Franck and Knipping. Pure helium was streaming- 

 through the apparatus, and the average pressure during the 

 observations was O50 mm. The first clearly marked bend 

 in the curve is at 21*2 volts — four volts before the ionization 

 bend which is also shown. The small positive current 

 measured at 19*5 volts is due to the collection o( positive 

 ions produced by the bombardment of the gauze C by the 



