162 
CHEMISTRY: MACINNES AND ABLER 
energy, 2FE, in which F is the Faraday equivalent and E the overvoltage, we 
obtain: 
7 (2) 
2FE =— 7 
4/37rr3^ 
pr 
Substituting 75.6 dynes per sq. cm. for y and appropriate values for the other 
terms, equation 4 becomes, for 25°C. and one atmosphere 
E = 
2.87 X 10-2 
(3) 
For platinized platinum electrodes this relation was found to hold quanti- 
tively. For instance, in one experiment the bubbles were found to have 
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FIG. 2 
radii of 0.017 mm., corresponding to an overvoltage of 1.6 mv. The ob- 
served values fluctuated between 1.9 and 1.6 mv. the latter potential, according 
to the theory, being that corresponding to the fully formed bubble. For 
electrodes of other metals difficulty was encountered in measuring the bubbles, 
but they were found to be smaller than those for platinized platinum in all 
cases observed. 
If the bubble radius does not change with the pressure (as was found to 
be the case, up to 16 atm. pressure, with platinized platinum electrodes) 
the overvoltage (according to equation 2) should increase as the pressure 
decreases, and vice versa. This prediction was found to be verified in some 
