of Radiation by Electron Impacts in Helium. 561 



its melting-point, while that due to the third cause was not 

 important except at the two higher pressures. Although the 

 corrections were 1 actually measured in some cases, it w;is 

 found much easier and just as satisfactory to simply assume 

 that the correct value of V a for the first break is 20'2 volts*, 

 and to adjust every curve to this point. 



Fio-. 3. p = 00o mm. ; I 20 = l 5 (10) ~ ( ' a 



csi 



3 50 



300 



2 50 



200 



7 E 



o 



"150 



100 



50 



7£ 



t 



4t ^t 



it 



U- ~C 



if 



u 



Jt- it 



— --^-— *-i\ Vn 



\% 



zs 



Jz ^^ 



J"^ ^ 





20 



22 



24 



V* 



26 



28 



(VdtsJ 



30 



32 



R 

 1.60 



1.40 



1.20 



1.00 



Figs. 2, 3. and 4 show that there are two " break " points 

 in the E-V a curves, indicating two critical potentials or 

 energies. The values of these critical potentials are 20*2 volts 

 and 25*5 volts respectively. The R-V a curves show that 

 between 20*2 volts and 25*5 volts the effect is due largely to 

 radiation, whereas the effect at 25'5 volts marks the setting in 

 of intense ionization. These results are, therefore, in entire 



* This value is chosen as a mean between the values given by suc- 

 cessive "breaks" in two electrode tubes, and by the corrected first 

 " break'' in tubes with three or more electrodes. 



