carried by the a and (3 Rays of Radium. 195 



presence by the electric method, as the ionization produced 

 by the a particles would probably mask that produced by the 

 electrons. The electron should be readily deflected in a 

 magnetic field, and experiments are at present in progress 

 to examine whether the a rays show any trace of positive 

 charge when the rays are exposed to a strong magnetic field/' 

 If a strong magnetic field is applied parallel to the plane 

 of the plates, any slow moving electrons, which escape from 

 the plates, will describe curved paths and return to the plates 

 from which they set out. 



Owing to work in other directions, experiments of this 

 character were not begun till November 1904. The apparatus 

 of fig. 1 was placed between the pole-pieces of a large 

 electromagnet, when a striking alteration of the values of the 

 current between the two plates was observed. The currents 

 in both directions were much reduced in value, and the upper 

 plate was found to gain a positive charge, whether the 

 lower plate was charged positively or negatively. 



While these experiments were in progress, an abstract 

 of a paper by Professor J. J. Thomson (Proc. Camb. Phil. 

 Soc. Nov. 14, 1904) appeared in 'Nature' (Dec. 15, 1904). 

 In this an account was given of some experiments made 

 with a view of detecting the positive charge carried by 

 the a rays. A plate of radio-tellurium, which emits only 

 ex. rays, was used. It was found that a number of slow 

 moving electrons were emitted from the active plate, which 

 could readily be bent by a magnetic field. I did not receive 

 a copy of the paper itself until my experiments were com- 

 pleted, and was not aware until after a publication of my 

 results in ' Nature ' (March 2, 1905) that Prof. Thomson had 

 previously succeeded in detecting the positive charge carried 

 by the a rays. 



As the method employed by J. J. Thomson is especially 

 well suited to show the presence of the electrons expelled 

 with the a particles, a brief account will be given of his 

 experiments. A plate of radio-tellurium was placed in a 

 vacuum-tube 3 cms. away from a metal plate connected with 

 a gold-leaf electroscope. When a very good vacuum was 

 obtained, the electroscope was observed to leak very rapidly 

 if positively charged, and very slowly if negatively. The 

 positive leak was at least 100 times that of the negative. 

 This result indicated that the plate of radio-tellurium expelled 

 a number of slow moving electrons, which gave up their 

 charge to the electroscope. This was confirmed by placing 

 the apparatus in a strong magnetic field. The positive leak 

 was almost stopped, showing that the electrons had been bent 

 away by the action of the magnetic field. The small negative 



02 



