

Electrons by Metals under Influence of Alpha Rays. 481 



was still a very appreciable effect from the secondary 

 radiation with an opposing' potential of." 1700 volts ; this 

 corresponds to a velocity of 24*4 x 10 8 . This is not far from 

 the maximum estimate which Wertenstein obtained with the 

 magnetic field, viz. 2*3 x 10 9 . 



It is plain that the existence of this radiation vitiates the 

 estimates which have hitherto been made of the velocities of 

 the slow 7 S-electrons. Thus, for example, an insulated source 

 of a- and S-rays, within an earthed enclosure, will lose more 

 swift electrons than it gains from the enclosure ; its 

 potential will rise until it can attract enough slow electrons 

 to balance this loss. The value of this steady potential will 

 not depend on the speed of the slow electrons so much as 

 upon the size and shape of the enclosure in relation to the 

 insulated source. If, as in Campbell's experiments *, the 

 apparatus is arranged so that all the electrons from either 

 of tw r o electrodes A and B strike the other one, there is 

 still a complication which renders impossible any simple 

 interpretation of the results. Comparatively slight changesf 

 in the velocity of the swift secondary electrons from A, for 

 example, may alter considerably the number of slow tertiary 

 electrons emitted by B. Thus false balances may be ob- 

 tained which have little relation to the speed of the slow 

 electrons. In fact it seems possible that the difficulties 

 which Campbell encountered when one of his electrodes w r as 

 covered with soot J, and w 7 hich led him to adopt the hypo- 

 thesis that the ^-electrons had no appreciable velocity, may 

 be accounted for in this way. 



When a metal is struck by a-rays it is evident that some, 

 at least, of the slow ^-electrons which result are not due to 

 the direct action of the a-rays, but are produced through the 

 intermediary of the swifter secondary electrons generated in 

 the metal itself near its surface. The question naturally 

 arises whether the whole of the effect may not be produced 

 in this indirect way. A consideration of the results exhibited 

 in Curve I. of fig. 4 show r s that this is not beyond the 

 bounds of possibility. In this experiment w r e have secondarv 

 rays produced by the same pencil of a-rays at two places — 

 the aluminium foil on the wheel and the plate corres- 

 ponding to E, fig, 1. The S-ray current from the 

 plate due to the a-rays is 1'24 ; let us assume that it is 

 produced as indicated above. The current produced by the 

 socondary rays from the foil (with 40 volts on the case) is 



* Phil. Mag. xxi. p. 280 (1911). 



t Se3 Gehrts, l. o. 



% Phil. Mag. xxiii. pp. . r ,4 et sea. (1912). 



