oj Radiation by Electron Impacts in Helium. 565 



filament side of the gauze, and the ions do not reach the 

 electrode E. The rise in the R-Y„ curve as V a increases 

 is due to the fact that, whereas lower and fewer ions reach 

 the electrode E, the radiation from impacts and reconciliation 

 still strikes it. This rise in R is therefore due to a peculiarit v 

 of the apparatus, and does not indicate a real increase in fche 

 actual proportion of radiation. If the curves were extended 

 to higher values of V a , the successive maxima at 20-volt, 

 intervals would be shown. 



The question of the effect of radiation or ionization pro- 

 duced by impacts against the gauze has frequently been 

 raised. Horton and Miss Davies* concluded that the 

 platinum gauze emits positive ions, this emission beginning 

 at about 13 volts and increasing as V a is increased. Tin's 

 point was tested by thoroughly baking out and evacuatino- 

 the apparatus, and making tests similar to those made in the 

 presence of helium. Since the effects were exceedinglv 

 small, the thermionic current was increased to the limit of 

 safety for the filament. The results, after correcting V a for 

 emission velocities, are shown in fig. 6. There is a very 



Fig. 6. p=00 mm. ; I 20 =205 (10)~' ; amp. 



80 



^60 



1 50 



T 40 

 o 



30 



10 

 





























A 



























/ 



/ 



























/ 



r 



























/ 





























/ 





























4: 



q 































j 













.+-- 











^ 







R 





































12 



16 



32 



36 



20 24 28 

 V a (Volts) 



small effect beginning at about 15 volts and increasing 

 considerably beyond 20 volts. The effect is seen to be 

 almost entirely one of ionization. These ionization currents 

 are so small in comparison with those observed with helium 

 in the apparatus that they are entirely negligible except in 

 the work at 0005 mm. and 0*001 mm. pressure. In these 

 cases corrections to the observed values of E were made by 



" Roy. Soc. Proc. A. xcvii. j>. 23 ( 1920). 



1.20 

 1.10 

 1.00 



