The Electric Resistance of Electrolytes

231*.] 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 dura- tion 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.

232*.] In the experiments of Dr. Paalzow∗ 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 R1 and R2 , the siphons were next filled with mercury, and their resistances when filled with mercury were found to be R1 ′ and R2 ′ .

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 ρ= R1 − R 2 . R1 ′ − R 2 ′ To deduce from the values of ρ 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. 230.) The results given by Paalzow are as follow: ∗ Berlin Monatsbericht, July, 1868.RESISTANCE OF ELECTROLYTES. 227 Mixtures of Sulphuric Acid and Water. H2 SO4 H2 SO4 + 14 H2 O H2 SO4 + 13 H2 O H2 SO4 + 499 H2 O Temp.Resistance compared with mercury. 15°C 19°C 22°C 22°C96950 14157 13310 184773 Sulphate of Zinc and Water. ZnSO4 + 23 H2 O 23°C 194400 ZnSO4 + 24 H2 O 23°C 191000 ZnSO4 + 105 H2 O 23°C 354000 Sulphate of Copper and Water. CuSO4 + 45 H2 O 22°C 202410 CuSO4 + 105 H2 O 22°C 339341 Sulphate of Magnesium and Water. MgSO4 + 34 H2 O 22°C 199180 MgSO4 + 107 H2 O 22°C 324600 HCl HCl Hydrochloric Acid and Water.

• 15 H2 O 23°C 13626
• 500 H2 O 23°C 86679 233*.] MM. F. Kohlrausch and W. A. Nippoldt∗ have determined the resis- tance of mixtures of sulphuric acid and water. They used alternating magneto- 1 electric currents, the electromotive force of which varied from 12 to 74 of that of a Grove’s cell, and by means of a thermoelectric copper-iron pair they re- 1 duced the electromotive force to 429000 that of a Grove’s cell. They found that Ohm’s law was applicable to this electrolyte throughout the range of these electromotive forces. The resistance is a minimum in a mixture containing about one-third of sulphuric acid. The resistance of electrolytes diminishes as the temperature increases. The ∗ Pogg. Ann. cxxxviii, p. 286, Oct. 1869.228 RESISTANCE OF DIELECTRICS. percentage increment of conductivity for a rise of 1°C is given in the follow- ing table: Resistance of Mixtures of Sulphuric Acid and Water at 22°C in terms of Mercury at 0°C. MM. Kohlrausch and Nippoldt. Specific gravity at 18°5Percentage of H2 SO4Resistance at 22°C (Hg = 1) 0·9985 1·00 1·0504 1·0989 1·1431 1·2045 1·2631 1·3163 1·3547 1·3994 1·4482 1·50260 ·0 0 ·2 8 ·3 14·2 20·2 28·0 35·2 41·5 46·0 50·4 55·2 60·3746300 465100 34530 18946 14990 13133 13132 14286 15762 17726 20796 25574 Percentage increment of conductivity for 1°C 0·47 0·47 0·653 0·646 0·799 1·317 1·259 1·410 1·674 1·582 1·417 1·794