200 Prof. Townsend on the Conductivity produced in 



ebonite plug through which a brass tube U was passed, and 

 the junction between the tube and the ebonite was made air- 

 tight. The tube T fitted exactly into the tube U and could 

 be moved up and down in it ; by this means the distance 



Fig. 1. 



jf ^l 



w 



between the two plates A and B could be varied, and a scale 

 S ruled on the tube T gave a means of determining the 

 distance between the plates. A piece of rubber tubing was 

 put over the upper end of the tube U, so as to prevent air 

 getting into the apparatus between the two tubes. 



The pressure of the gas inside the apparatus was found by 

 a MacLeod gauge. 



The electrical connexions were made in the usual manner, 

 The zinc plate A ; which was supported on ebonite legs, was 



