20 



NEWBERY, The Theory of Overvoltage. 



keeps more or less constant about that maximum. At 

 higher current densities, it rises to a maximum and 

 then falls to a more or less variable minimum. The time 

 taken to reach this maximum varies from a few seconds 

 to as many days, and is less with greater current density. 

 The following table shows the variation in the overvoltage 

 of a plate of polished platinum with time, the current 

 densities being 300 mil amps, and 400 mil. amps, per sq. 

 cm. respectively. Platinum attains its maximum over- 

 voltage with a current density of about 350 mil. amps, per 

 sq. cm. under the conditions of the experiment so that 

 one of the current densities used was a little below and 

 the other a little above that necessary for producing the 

 maximum. 





Volt. 



Volt. 



Overvoltage after 1 min. 



0-076 



0*060 



55 5 " 



o'o8o 



0-068 



15 > 



0-088 



0'092 



n 3° » 



O'lOO 



0*104 



,, i hour 



O'l I 2 



0"I20 



» 6 » 



OI73 



0*236 



T ■? 



o"40i 



O"20O 



53 2 4 „ 



°'5 1 J 



0'I20 



After 5 mins. rest with no current ... 



o - o8o 





,, further 10 mins. with current... 



0"I22 



1 



After each series the platinum was examined under 

 the microscope ; in the first case it was hardly changed 



