[fierhellbr] to determine MINIMUM SPARK POTENTIALS 



109 



In the experiments which have been described all the exhaustions 

 were made by means of a Toepler-Hagan pump and the gas was admitted 

 to the chamber by opening a tap in a tube leading to a supply of 

 hydrogen. 



The introduction into the laboratory of a Gaede exhausting air 

 pump during the investigation, however, suggested at once the superiority 

 of this piece of apparatus over the older form of pump for the purposes 

 of the present investigation. 



For with this pump the gas could not only be easily and rapidly 

 withdrawn from the discharge chamber but it could also by a reversal 

 of the pump's action be just as easily and Avith full control be again 

 admitted back into the chamber. 



TABLE IV 

 Above Qritical Pressure 



A point of special interest in connection with the present investiga- 

 tion is the confirmation it affords of some conclusions drawn by Mr. J. 

 A. Brown i from his measurements on the potential required to maintain 

 a current in a gas. With the arrangement described above the difference 

 between the Weston voltmeter reading and that of the electrostatic volt- 

 meter for a selected current gave a measure of the corresponding 



1 Brown. Phil. Mag., Sept., 1906. 



