322 Jin ni stead and McGougan — Emission, of Electrons by 



velocities. As the positive potential on the case is increased, 

 more and more of these electrons are withheld from reaching 

 the brass plate. The fact that this radiation communicates a 

 positive charge to the plate is not an obstacle to the hypoth- 

 esis that it consists of electrons. Since the early experiments 

 of Lenard it has been known that electrons, moving with 

 velocities corresponding to several hundred volts, when they 

 fall upon a metal, cause the latter to emit secondary electrons, 

 which may carry a larger 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 reflection, or the number of secondary 

 electrons from the case. In order to test this possibility the brass 

 plate (corresponding to E in fig. 3) 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 positive 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 practicable 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 proportion 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 pro- 

 duced 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 sim- 

 ilar change of potential with the polonium reduced the effect to 

 one-third. 



* See also Gehrts (Ann. d. Phys., xxxvi, 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. 



