Gases on Contact Potential Difference between Metals. 165 



respect to the cylinder. Initially this displacement appeared 

 to be quite reversible. Thus in a particular test starting 

 with a pressure of 10 -4 mm. or less, the admission of 0*201 

 mm. of hydrogen displaced the curve '35 volt to the left, on 

 pumping the hydrogen out it moved back '33 volt (i. e. to 

 within 0*02 volt of its original position). A typical set of 

 curves obtained after a few hours heating is shown in fig. 2. 



Fig. 2. 



•PRESSURE BELOW lo" M.M. Hg 



X PRESSURE OF H z 0-0117 MJ*I. Hg 



o •• •• o-oz7 



Q n o .« 0-049 •• " 



<3 * - 0-078 - 



- •• o. n z ■ • 



© - - 0-153 - - 



VOLTS. 



+ -4 



The temperature for the high vacuum curve in this case 

 was about 1460° K. The gas pressures at which the 

 various curves were taken are marked on the figure. The 

 displacements are very nearly proportional to the pressure 

 of the admitted hydrogen, as is shown in the plot fig. 3. 

 After heating for some time the curves became somewhat 



