236 POTENTIAL DIFFERENCE. chap.
'* bind " each other, so that fresh and comparatively large quantities of electricity collect on the plates in order to maintain the electrical equilibrium.
Now let the connecting wire be removed, and the plates separated from each other. The previously " bound " elec- tricities become free, and the more easily oxidisable metal is found to be positively electrified, and the " nobler " metal negatively. This explains why the metals can be arranged in an " electromotive series " (with reference to one and the same gas), and why the most easily oxidisable metals occur at the beginning of the series, and the least oxidisable at the end.
If we use other gases which act on the metals we obtain a different series {eg. with chlorine, hydrogen sulphide, etc.), as J. Brown (<9) has proved.
The actual potential difiTerence between two metals is ascertained by conducting a known quantity of electricity through the junction and determining the heat developed cal., the potential difference can easily be calculated in volts ; these differences of potential seldom reach so much as a few hundredths of a volt.
Pellaf s Method. — A fourth method of determining when the potential difference between mercury and a liquid in contact with it becomes zero was devised by Pellat (9), who observed, in a capillary electrometer, the polarised mercury surface, which could be increased in R (Fig. 44) by suction. The potential difference P could be altered as desired.
If there is a difference of potential between the mercury and the solution, and the surface of contact be suddenly increased, a current flows through a galvanometer, G, placed at P, to the newly formed parts of the surface, so as to charge these to the same potential £is the original parts. If, however, the contact surface is uncharged, no current is l)roduced. This occurs when P is equal to the potential difference, Hg [ HgaSOi in H2SO4. Pellat, by altering Pimtil this point was reached, obtained the value P = —0*97 volt.
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