296 



The Hon. B. J. Strutt 



[April 24, 



Mr. Wilson made a series of measurements of this electrical leakage 

 through various gases, and he came to the interesting conclusion 

 that the rates of leak were in the same ratio to one another as those 

 which I had found for the same gases under the action of Becquerel 

 rays. As a rule, the leakages were proportional to the densities of 

 the gases, but as in the case of Becquerel rays, hydrogen was an ex- 

 ception, giving about twice as great a leakage as it ought to, if 

 this law were exactly obeyed. 



This curious agreement naturally suggested 

 that the leakage ordinarily occurring was due 

 to the same cause as the leakage under Bec- 

 querel rays. In other words, that the walls of 

 the vessel containing the electroscope were 

 giving off rays of this kind, although of course 

 only to a very slight extent. In order to test 

 whether this was really the case, I carried out 

 a series of experiments on the rate of leak 

 from a charged wire, when surrounded by cylin- 

 ders of a uniform size, but of different materials. 

 It soon became apparent that the rate of dis- 

 charge depended on the nature of the surround- 

 ing wall. 



I will now show you a diagram of the 

 apparatus used (Fig. 2). a is a charged wire 

 in the axis of the cylindrical vessel h. The 

 walls of h could be lined with any desired ma- 

 terial by inserting a cylinder c c composed of 

 it. This could be done by removing the glass 

 plate d at the end, which was cemented on. 

 The vessel could be exhausted through the 

 stopcock / if desired, e was a drying bulb, 

 containing phosphoric anhydride. The wire 

 a passed air-tight through the brass cap </, 

 cemented on to the neck A of 6. h was made 

 of lead glass, on account of the superior insula- 

 ting qualities of this kind of glass. The cap 

 g carried a brass strip h, to which the gold 

 The whole was surrounded by a vessel m, as 

 iron wire, attached to a platinum wire o sealed 

 The iron wire could be brought into contact 



Fig. 2. 



leaf I was attached, 

 shown, n was an 

 through the glass, 

 with k by means of an external magnet, in order to charge the system. 

 m could be exhausted through the stopcock p, and dried by means 

 of the phosphoric anhydride contained in q. The position of the 

 gold leaf was read by a microscope with micrometer eye-piece, focussed 

 upon it. 



Before making an experiment, the insulation, which is all-im- 

 portant, was tested, m was permanently exhausted, and the stopcock 

 p closed, h was also exhausted for the time, and a charge given to 



