52 Prof. Thomson, On the positive electrification of a rays, 



strike against the electroscope, thus when the disc was tapped so 

 that it turned through an angle of 90°, from the position in 

 which the polonium faced the electroscope, the leak in the electro- 

 scope for a positive charge with the magnet off only fell from 

 45 to 22, although in the new position no rays at right angles to 

 the disc could reach the electroscope. The negative particles which 

 discharge the disc in this case come from the walls of the vessel 

 and are either diffusely reflected cathode rays originating from the 

 polonium or are due to secondary radiation excited by the rays 

 from the polonium. 



The absence of any appreciable leak in the preceding experi- 

 ments, when the electroscope is charged negatively, even when 

 the negatively charged cathode rays are prevented by the magnetic 

 field from reaching the electroscope, shows that the a rays must 

 lose their positive charge, or rather have it neutralized by the 

 negative charge on the cathode rays, as they pass through the 

 swarm of cathode rays. These experiments have given no indi- 

 cation of the positive charge on the a rays. I will now pass on to 

 some experiments by which the positive charge on these rays can 

 be detected. In these the polonium disc instead of being at a 

 distance of 4 or 5 cm. from the electroscope is placed as close to it 

 as possible: the distance in the experiments in which the following 

 results were obtained was 1 millimetre. 



Leak in scale Pl 



divisions in wiarge on Magnet 



five minutes electroscope 



31 + Off 



+ on 



9 - off 



5 — on 



It will be noticed that although with the magnet off the leak 

 is as before much greater when the electroscope is charged posi- 

 tively than when it is charged negatively, owing to the copious 

 emission of slow cathode rays by the polonium, yet now when 

 the magnetic field is on the negatively electrified body shows by 

 far the bigger leak, in fact there is no appreciable leak with the 

 positive charge. We see that now the a particles are able to 

 reach the electroscope and give up their charge to it, these 

 particles are but little affected by the magnetic field, while the 

 cathode particles are almost entirely stopped by it. The leak 

 when the electroscope is charged negatively cannot be wholly 

 due to secondary cathode rays emitted by the electroscope when 

 bombarded by the a rays from the polonium, for these cathode 

 rays unless moving with a velocity of about 10 10 cm./sec. would be 

 stopped by the magnetic field, although it is probable that when 

 no magnetic field acts upon the vessel, part of the leak, when the 



