Page:A Treatise on Electricity and Magnetism - Volume 1.djvu/461

On the Electric Resistance of Electrolytes.

363.] The measurement of the electric resistance of electrolytes is rendered difficult on account of the polarization of the electrodes, which causes the observed difference of potentials of the metallic electrodes to be greater than the electromotive force which actually produces the current.

This difficulty can be overcome in various ways. In certain cases we can get rid of polarization by using electrodes of proper material, as, for instance, zinc electrodes in a solution of sulphate of zinc. By making the surface of the electrodes very large compared with the section of the part of the electrolyte whose resistance is to be measured, and by using only currents of short duration in opposite directions alternately, we can make the measurements before any considerable intensity of polarization has been excited by the passage of the current.

Finally, by making two different experiments, in one of which the path of the current through the electrolyte is much longer than in the other, and so adjusting the electromotive force that the actual current, and the time during which it flows, are nearly the same in each case, we can eliminate the effect of polarization altogether.

364.] In the experiments of Dr. Paalzow^[1] the electrodes were in the form of large disks placed in separate flat vessels filled with the electrolyte, and the connexion was made by means of a long siphon filled with the electrolyte and dipping into both vessels. Two such siphons of different lengths were used.

The observed resistances of the electrolyte in these siphons being $R_{1}$ and $R_{2}$, the siphons were next filled with mercury, and their resistances when filled with mercury were found to be $R_{1}'$ and $R_{2}'$.

The ratio of the resistance of the electrolyte to that of a mass of mercury at 0°C of the same form was then found from the formula

$\rho ={\frac {R_{1}-R_{2}}{R_{1}'-R_{2}'}}$.

To deduce from the values of $\rho$ the resistance of a centimetre in length having a section of a square centimetre, we must multiply them by the value of $r$ for mercury at 0°C. See Art. 361.

1. Berlin Monatsbericht, July, 1868.