Radiation and Ionization of Helium. 765 



the main tube at the same time that the lower filament 

 maintained a supply of electrons with energies between 

 204 volts and 21'2 volts, was investigated with the (I — It) 

 arrangement of the electric fields used in the test just 

 described. As in that experiment, both filaments were 

 maintained at constant temperatures throughout^ and the 

 presence or absence of radiation from the side tube was 



Fig. 7. (I-R) curves. 

 Pure helium, p = , 590 mm. 



controlled by altering the accelerating potential difference 

 V 5 . The curves in figure 7 illustrate the results obtained. 

 The curve drawn with a continuous line gives the variation 

 of the measured current as the energy of the electrons 

 emitted by the lower filament was increased in steps of one- 

 tenth of a volt by increasing the potential difference \ L , the 



