carried by the a and ft Rays of Radium. 201 



Origin of the Slow-Speed Electrons. 



We have seen that J. J. Thomson has shown that a number 

 of slow-speed electrons are emitted with the a. particles from 

 ;i, plate of radio-tellurium. The experiments described above 

 show that this is equally the case with radium. The mag- 

 nitude of the current due to these electrons is sufficient to 

 completely mask the effect due to the charge carried by the 

 a particles, and in some cases is twenty times as great. The 

 question now. arises, whether these electrons are projected 

 from the radioactive matter itself, or are emittted from the 

 plates on which the a particles impinge. The fact that the 

 current without a magnetic field is greater when the lower 

 plate is negative than when it is positive shows that a greater 

 number of electrons are emitted from the lower plate. If 

 the electrons were only emitted from the lower plate, the 

 positive current should greatly exceed the negative. The 

 results indicate that these electrons arise from both the Upper 

 und lower plate, but to a greater extent from the latter. This 

 is borne out by the experiments when the radium film de- 

 posited on the glass plate is covered with thin aluminium-foil. 

 The negative current is still the greater, but the difference is 

 not nearly so marked. Since the electrons are probably not 

 projected with sufficient velocity to pass through even one 

 thickness of aluminium-foil, it seems probable that the electrons 

 liberated from the radium itself and the glass plate would be 

 absorbed in the aluminium-foil. The experiments as a whole 

 indicate that the electrons escape both from the radium plate 

 and from the surface in which they are absorbed. It seems 

 probable that these electrons constitute a type of secondary 

 radiation which results from the impact of the "a. particles on 

 matter. Since half the a particles emitted from the radium 

 are projected into the lower plate and the other half are pro- 

 jected upwards, it is to be expected that more would escape 

 from the lower than from the upper plate. It is difficult 

 to settle whether the radium itself emits any of these 

 slow-speed electrons at the moment of expulsion of the a 

 particle. There is no reason, however, to doubt that such 

 electrons would be liberated by the bombardment of the 

 radium by the a. particles projected from its own mass. 



The existence of this type of secondary radiation, set up 

 when the a particles pass through matter, readily explains 

 some results obtained by Mme. Curie in her experiments 

 on the absorption of the a rays from polonium. The ioni- 

 zation current between two parallel plates was compared 

 when two screens of different materials, placed over the 



