4 Prof. Rutherford and Miss Brooks : Comparison of 



Electrostatic disturbances were completely eliminated by 

 covering the electromagnet and wires leading to it with tinfoil 

 connected to earth. 



There was always a small current observed between the 

 plates on account of the spontaneous ionization of the air in 

 the testing vessel when the uranium oxide was removed to a 

 distance. 



The layer of uranium oxide was covered with several thin 

 layers of aluminium of sufficient thickness to completely 

 absorb all the a radiation. The open end of the lead vessel 

 was covered with thin aluminium-foil. In that case the rate 

 of leak of the electrometer was due to ionization produced 

 between the plates by the /3 radiation together with the ions 

 spontaneously produced by the air itself. The latter was 

 accurately determined before the lead vessel containing the 

 oxide was placed between the poles of the electromagnet. 



As the magnetic field was increased, the rate of leak 

 observed by the electrometer steadily diminished, until with a 

 strong field the rate of leak was reduced almost to that due 

 to the spontaneous ionization of air. This diminution of the 

 rate of leak between A and B is due to the curvature of 

 the path of the rays by the magnetic field before they reach 

 the testing vessel. Since the rate of leak, due to the action 

 of the /3 radiation, with a strong magnetic field is reduced to 

 a small fraction of its value when no magnetic field is acting, 

 we may conclude that the $ radiation is composed almost 

 entirely of rays deviable by a magnetic field. 



A comparison experiment with radium showed that the (3 

 rays of uranium were deflected to about the same extent as 

 the radium rays for the same strength of field. 



No action of a magnetic field on the a radiation of uranium 

 was observed. Both radium and uranium resemble one 

 another in emitting two types of radiation, one of which is 

 deviated in a magnetic field, and the other not. 



Absorption of the /3 Radiation by Substances. 



Since the (3 radiation of uranium is acted on by a magnetic 

 field to almost the same extent as radium rays, we may 

 conclude that the deviable rays are due to negatively charged 

 particles emitted with high velocities ; for Becquerel has 

 shown that some of the radium rays move with a velocity of 

 at least 1*6 x 10 10 cms. per second. The penetrating power 

 of the j3 rays is greater than that of the similar radiation for 

 radium in our possession. It readily passes through 2 rains. 

 of glass before complete absorption. 



Lenard, in his well-known experiments on cathode-rays. 



