476 Prof. Bumstead and Mr. McGougan on Emission of 



negative charge away from the metal than it receives from 

 the incident stream *. 



It seemed possible that the diminution in the current 

 with an increasing field might be due to an effect upon 

 the electrons emitted by the plate, rather than upon the 

 radiation itself — for example, by increasing the reflexion, or 

 the number of secondary electrons from the case. In order 

 to test this possibility, ihe brass plate (corresponding to E 

 in fig. 1) was enclosed in a tin box, whose top was made of 

 wire gauze in order to permit the a-rays and the new 

 radiation to reach the plate. It was insulated from the case 

 and could be charged by means of an external electrode. The 

 box was kept charged to 40 volts while po-itive potentials up 

 to 1000 volts were applied to the case ; in this way the field 

 in the vicinity of the plate, E, remained practically constant 

 while the soft radiation had to pass through a variable field. 

 The results were not essentially different from those shown 

 in fig. 4. 



In order to determine whether or not this soft radiation 

 was peculiar to polonium, experiments were made in which 

 the active deposit of thorium (obtained from a preparation 

 of meso-thorium) was used. It was much less active than 

 the polonium and more time was necessary for each reading; 

 on account of the decay of the activity it was not practi- 

 cable to wait until the changes due to the removal of the gas 

 layer had ceased before beginning the readings. It was thus 

 impossible to obtain as satisfactory numerical results as with 

 the polonium, but there could be no doubt about the existence 

 of the soft radiation. It produced a greater effect in pro- 

 portion to that due to the a-rays than in the case of polonium; 

 the ratio was about twice as great. On the other hand, the 

 diminution produced by an opposing electrical field was not 

 as great as with the polonium ; with 940 volts on the case, 

 the effect of the soft radiation was about one-half as great 

 as with 80 volts. A similar change of potential with the 

 polonium reduced the effect to one-third. 



Returning to fig. 4, it will be observed that Curve I., which 

 was supposed to be due to the a-rays alone, shows a diminution 

 of the current as the potential on the case is increased, which 

 is similar to that of Carve III. but much less in amount. 

 This was at first difficult to explain ; it is quite evident that 



* See also Gehrts, Ann. d. Phys. xxxvi. p. 1001 (1911), where it is shown 

 that, when electrons with a velocity corresponding- to 200 volts fall upon 

 a copper plate, the secondary electrons carry away from the plate more 

 than twice the charge brought to it by the incident electrons. 



