Cathode Rays emitted by Substances exposed to 7 Rays. 619 



distance of the chamber B from the radium so arranged that 

 the leak in B partially compensated for that in A. 



The substance under investigation, if in the form of 

 powder, was tightly packed into the little dish a, the upper 

 surface of the powder being made flush with the rim. The 

 dish was of a circular form and made of thin sheet zinc, its 

 diameter was 5'6 cm. and its depth 4 mm. This dish was 

 then placed into another dish b, into which it fitted closely. 

 The dish b was then placed in the centre of the aluminium 

 plate c, and this placed on the table d, three strips of ebonite 

 fixed to the table serving to keep the aluminium plate in a 

 fixed position. By means of the screw e the table was then 

 raised till the ebonite blocks /, f l9 attached to the aluminium 

 plate, touched the gauze, the distance of the aluminium plate 

 from the gauze in this position being 11 mm. The leak was 

 then taken. 



The aluminium plate c, when placed on the table d, made 

 contact with an earthed wire; an electric field therefore 

 existed between the gauze and plate, which prevented the 

 diffusion of ions through the gauze into the chamber. 



Bv partially compensating for the leak in the chamber A 

 by means of the leak in the chamber B, the observed leak 

 was under control, and the radium could be placed quite 

 near to the chamber A. The secondary radiation from the 

 substance in the dish a was thereby rendered intense, and 

 the change in the leak when a different substance was sub- 

 stituted for the one in the dish was therefore much greater 

 in comparison with the observed leak than would have been 

 the case with one chamber only. In fact, with one chamber 

 only, the change in the leak with change of substance would 

 be, in many cases, so small that it would be difficult to 

 measure. 



The leak was allowed to run into the electrometer till the 

 deflexion was approximately equal to some fixed deflexion of 

 convenient magnitude, and the current then broken, and the 

 first and second swings of the needle read. The position of 

 rest of the needle was obtained by means of a formula given by 

 the writer *, and knowing the time of leak, the leak for any 

 given time could be calculated. The disturbing effect of the 

 change of capacity of the electrometer with deflexion was 

 avoided in this manner. 



Since the leak due to the secondary radiation from a given 

 substance depended on the position of the radium, the sensi- 

 tiveness of the electrometer, &c, it became necessary to 

 devise a method of comparison of the secondary radiation 



* Phil. Mag. Oct. 1906, p. 276. 



