140 S. E. PIERCE. 



To get relative values of fed for different substances it is 

 necessary to prepare ionisation vessels of similar shape but 

 of different materials. Following Professor Bragg's method, 

 values relative to that for lead have been obtained by com- 

 paring the ionisation in a lead cylinder, due to a stream of 

 7 rays from some radium bromide placed beneath it, with 

 ionisations under similar conditions when the cylinder was 

 completely lined with other metals. 



The lead cylinder is 15 cm. long and 9*6 cm. in internal 

 radius. It is closed with lead plates of the same thickness 

 as that of the cylinder (0*5 cm.). The linings are made of 

 the same form to fit closely inside the lead chamber. The 

 top plates both of the lead and of the linings are provided 

 with a hole to admit the copper wire electrode. This is 

 supported axially in the chamber by a plug of sulphur in 

 which is embedded an earthed guard ring. 



A battery of small storage cells is attached to the lead 

 cylinder raising its potential to about 400 volts, which was 

 found to be sufficient to saturate the largest current 

 obtained. The axial electrode is connected to a key by 

 means of which it may be joined to earth and thence to 

 one pair of quadrants of a Dolezalek electrometer, the other 

 pair being earthed. The electrode is also connected to the 

 inside cylinder of a cylindrical condenser, of which the 

 outer coating is attached to a variable source of potential. 

 By varying the potential of the outer cylinder the inner 

 cylinder and its connections may be kept at zero potential 

 while they are receiving a charge from the ionisation 

 chamber. The current in the ionisation chamber will then 

 be proportional to the rate at which the potential on the 

 outside cylinder is altered. 



In order to vary the potential of the outer cylinder of 

 the condenser it was connected to a sliding contact on a 

 potentiometer wire wound helically on an ebonite drum. 



