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



am not aware of any experiments in which the positive electricity 

 carried by these rays has been transferred to a conductor and 

 detected by the electrification of the conductor. I only succeeded 

 in doing this after a lengthy investigation in the course of which 

 some properties of polonium and radium, which seem interesting 

 and important, came to light. 



The first experiments were made with polonium. This body 

 is supposed only to give out a rays so that it seemed especially 

 suitable for this purpose; it was found however that polonium 

 gives out in addition to a rays large quantities of slowly moving 

 cathode rays. 



The polonium (obtained from Sthamer) was deposited on two 

 discs, one bismuth, the other copper. The method used was as 

 follows. A gold-leaf electroscope A (Fig. 1) was placed in B, a 

 large glass vessel coated with tinfoil, connected with the earth. 

 The leaves of the electroscope were supported by a stick of fused 

 quartz G, and were charged by the wire D. The polonium discs 

 were placed in the side tube E, in the earlier experiments the 

 disc was about 4 cm. from the electroscope. G is a tube contain- 

 ing hard charcoal, it was added for the purpose of obtaining high 

 vacua by Dewar's method of immersing charcoal in liquid air; for 

 the experiments we are describing to succeed the vacuum has to 

 be exceedingly good, and I have found Dewar's method of the 

 greatest possible assistance, giving in a few hours better vacua 

 than I previously obtained by several days' pumping. 



The vessel containing the electroscope and polonium was set 

 up between the poles of a powerful electro-magnet which when 

 excited gave a field of about 1200 C.G.S. units between the polo- 

 nium and the electroscope. 



In these experiments the electroscope was charged alternately 

 with positive and negative electricity, and the rate of collapse of 

 the leaves measured by a reading microscope provided with a 

 micrometer eye-piece. The potential to which the electroscope 

 was charged was + 180 volts. If the polonium gave out only 

 a rays we should expect the rate of leak to be much more rapid 

 when the electroscope is charged with negative electricity than 

 when charged with positive. It was found however that exactly 

 the opposite effect occurred, the leak being almost negligible 

 when the charge in the electroscope is negative and very appreci- 

 able when the charge is positive. The following observations are 

 samples of a large number all showing the same effect. 



