912 Prof. H. A. Bumstead on Emission of Electrons 



the rays passed. This difference persisted when the ease 

 itself was lined with aluminium foil, so it was not due to a 

 specific difference between aluminium and brass. It is 

 probable that the explanation of this difference as well as 

 the difficulty in obtaining negative saturation mentioned in 

 the last paragraph is to be found in the construction of the 

 cover and the apertures through which the rays enter the 

 exhausted chamber. Many of the rays, passing in a diver- 

 gent pencil through the aluminium foil which covers the 

 holes, strike the sides of the holes and do not reach the 

 electrode. They thus liberate a number of electrons from 

 the case and no corresponding ones from the electrode ; and 

 to draw all of these superfluous electrons from the small 

 apertures in the top of the cover requires a considerably 

 greater potential- difference than when they come from the 

 electrode. 



In order to find out whether the observed currents really 

 had their source in the metals and were not due to residual 

 gas or vapour, the foil was removed from the ring electrode. 

 (The apertures in the top of the cover were so placed that the 

 geometrical beam of rays fell entirely within the ring.) 

 Under these conditions, the current with the case charged 

 positively fell to 4 per cent, of its former value, while with a 

 negative potential on the case the current was 90 per cent, of 

 what it had been before. This slight falling off in the negative 

 current was doubtless due to the fact that the ring electrode 

 was less efficient in catching electrons from the case than 

 when it was covered with foil. Of the small positive current, 

 part at least was due to the rays which struck the brass rod 

 which supported the ring (see fig. 1), so that we may 

 be quite sure that the gas-effect contributes not more than 2 

 or 3 per cent, to the currents observed with this apparatus. The 

 absence of positive particles justifies' the further conclusion 

 that the effect produced by the a- rays on a metal consists in 

 the direct expulsion of electrons, and not o£ neutral pairs 

 which afterwards break up. As will be seen later, there is a 

 very close relationship between this metal effect and the 

 ordinary ionization of gases by a-rays ; and the result 

 just given renders it at least unlikely that neutral pairs are 

 emitted when gases are ionized by a-rays as has been some- 

 times assumed in the case of other ionizing agents. 



A further test of the relative importance of the metal and 

 gas effects was made by measuring the current at different 

 gas pressure? up to 0'2 mm. The result may be expressed 

 by the linear formula 



