552 Mr. J. J. E. Durack on the Specific Ionization 



with a plate of aluminium L 0*083 mm. thick, and made air- 

 tight by screwing down a thicker plate of aluminium over it 



Fig- 1. 



7~o EtECT/roMETef? 



7o pump a ■ M C L too 



and melting an elastic glue into the crevices. A ring of ebonite 

 RR separated the brass box CC from the lead plate. Above 

 the plate L and 13*5 mm. from it was placed another plate 

 pp\ the back of this plate was of lead, which was turned so 

 as to form a thin rim of lead on the side nearer L; an alu- 

 minium plate similar to L was then placed over the rim and 

 fastened to it so that there was an air-space between the 

 aluminium and the lead ; pp was connected to a quadrant 

 electrometer with a rod passing through the ebonite plug E 

 and shielded from electrostatic effects. 



The space inside CC was in communication with a pump 

 and McLeod gauge. 



All joints between metal and ebonite and between metal 

 and glass were made air-tight with sealing-wax. 



The plate PP was 8 cms. diameter, and the apparatus is 

 drawn to scale in fig. 1. 



During the time the observations were being made CC and B 

 were kept connected to earth, and PP to one pole of a battery 

 of storage-cells, the other pole being to earth. By keeping 

 B connected to earth while P is charged to a high potential 

 the ions formed in the air between B and P are destroyed, 

 and consequently prevented from diffusing out and, may be, 

 finding their way to the electrometer. 



It is known from the experiments of Rutherford* that an 

 aluminium plate 0'08 mm. thick is sufficient to absorb all the 



* See Rutherford and Miss Brooks, Phil. Mag. ser. 6, vol. iv. p. o. 



