17G Mr Richardson, On an attempt to detect radiation, etc. 



The apparatus had now assumed its most sensitive form, since 

 the capacity of the charged cylinder and parts connected with it 

 was only three times that of the electroscope. In a slightly 

 different form of apparatus, used for the experiments at ordinary 

 pressures, and insulated entirely with sulphur, the normal leak 

 was reduced to about 01 scale division per hour ; and in the last 

 described apparatus, used for low pressures the leak was never 

 greater than 1 scale division per hour. The apparatus was there- 

 fore much better insulated than that with parallel plates when 

 there was a normal leak of 2 divisions per hour. Experiments 

 were made at ordinary pressures with aluminium and steel wires. 

 The following numbers, fur an aluminium wire, indicate the sort of 

 readings obtained. 



Pressure 



Initial 



Eeading of 



movable leaf 



Ph 



DQ J 



S3 "3 



CD ^ 



"I 



fc-l tf 



HI " 



1-3 H 

 ai 

 ft 





Atmospheric 



46-0 



44-2 



1-8 



lSkrs. 



o-i 



No current 

 through wire 



j) 



44-2 



44-0 



0-2 



3 „ 



0-07 



Current passing 

 through wire 



From a large number of similar observations it was evident 

 that no appreciable amount of ionisation was produced in air at 

 ordinary pressures. It was still possible however that an appre- 

 ciable amount might be produced at low pressures. For if a few 

 ions were produced under ordinary circumstances, these, if the 

 pressures were lower, would produce more by their motion in the 

 electric field, i.e. when they collided with the molecules of the 

 gas 1 . Thus the method might be expected to become more 

 sensitive when employed at low pressures. A large number of 

 observations were therefore taken (using an aluminium wire) at 

 pressures varying from 3 to - l mm. Two of the observations 

 taken are subjoined : — 



1 J. S. Townsend, Phil. Mag., Feb. 1901. 



