Radiation and Ionization of Helium. 



761 



positive ions to which they gave rise were prevented from 

 passing into the main tube and consequently from reaching 

 the electrometer. The lower filament was not used in these 

 experiments, and the arrangement of the electric fields V 3 

 and V 4 was similar to that employed in the method of 

 obtaining an R curve already described. Radiation produced 

 by direct electron collisions with helium atoms in the side 

 tube cannot illuminate the electrodes A or B in the main 

 tube so as to give a photoelectric current, and the design of 

 the apparatus makes it exceedingly unlikely that anything 

 but a negligible amount of radiation could be reflected so as 

 to reach these electrodes. Any electrons liberated photo- 

 electrically from the under side of the gauze L were pre- 

 vented from reaching the electrometer by the fields V 8 , V 2 , 

 and V 3 , which were all arranged to oppose them. In these 

 experiments, the detection of a photoelectric current by the 

 Fig-. 6. — R curve. 

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 Volts 



electrometer can therefore only mean that radiation, pro- 

 duced by electron impacts with helium atoms in the upper 

 part of the side tube, is being handed on from atom to atom 

 into the main tube. The curve in fig. 6 shows the results 

 obtained by observing the current to the electrometer for 

 different values of the voltage V 5 . It will be seen from the 

 Phil Mag. S. 6. Vol. 4,2. No. 251. Nov. 1921. 3 E 



