28 



Fig. 8. 



electrodes. For introducing gas to the chamber a metal plug 

 was provided, which could be screwed into the opening be- 

 low the mica, and while air was being exhausted from the 

 chamber a small screw, operated from the outside, enabled 

 a by-pass to be opened between the space on either side of 

 the mica sheet. The lower electrode, C , consisted of a brass 

 gauze insulated from the case, attachment to it being made 

 from the outside by an insulated wire. The upper electrode, 

 2), consisted of a brass plate 8 cm. diameter, supported by 

 a rod passing through sulphur insulation and being connected 

 to one set of plates of a sliding condenser as in the experi- 

 ments of Langevin.(ii) 



An aluminium semaphore, G , enabled the a rays to be 

 entirely shut off, when necessary, during the swing of the pen- 

 dulum without reducing to any great extent the ^ and y radia- 

 tion which entered the chamber. A plate of lead, //, about 

 2 mm. thick, attached to the pendulum, was arranged so as 

 to swing between the radium and the opening in the ioni- 

 zation-chamber. In this lead plate was cut a hole 1*5 cm. 

 square, and the amplitude of swing was such that the a rays 



(n) Ann. de Chim. and Phys., 7me ser., tome 28-39, 1903. 



