Page:Text-book of Electrochemistry.djvu/141

 ��CONDUCTIVITY OF ELECTROLYTES. chap.

��which has now been removed from the circuit, is the same as that of the wire, between /•; and ki, which has been introduced into the circuit. The resistance of the wire lli is determined beforehand by means of a Wheatstone bridge.

By measuring the resistance of columns of liquid of different lengths it is found that this is proportional to the length. If the quantity of liquid in the trough be changed, the cross section of the liquid column is changed, and it is found that the resistance is inversely proportional to the cross section.

These facts prove that the laws of resistance are the same for electrolytic solutions as for metals. The resistance of salt solutions, however, decreases as the temperature rises, about 2*4 per cent, per rise of temperature of one degree in the neighbourhood of IS'', whilst that of the metals increases with rising temperature. If the concentration of the zinc sulphate is varied, the resistance changes so that it becomes not quite double when the concentration is halved.

Change of Conductivity with Dilution.— Let us assume that in the trough T (Fig. 30) the zinc sulphate solution is so dilute that the number of salt molecules is negligible

compared with the number

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��of water molecules, and let this solution fill the vessel to the level mmi. The re- sistance, or its reciprocal value the conductivity, of this solution depends both on the number of zinc and sulphate ions present and on their specific powers of transporting electricity under the influence of a certain fall of potential.

This power of the ions depends only on the galvanic friction which they experience against the surrounding liquid. Since this surrounding liquid is water — the number of zinc sulphate molecules being, by supposition, small, and consequently not able to exert any power on the galvanic

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