Page:Outlines of Physical Chemistry - 1899.djvu/215

 gible minimum, then the smallest electromotive force can produce a current in the solution.

In order to determine the specific resistance of a solu- tion in ohms, we consider a cubical mass of the solution, the length of whose side is 1 centimetre. If the resistance

of this is r, then its reciprocal - = I represents the conduc- tivity. And if I be multiplied by the number of cubic centimetres of solution which contains one gram -molecule of dissolved substance, we obtain the molecular conductivity, that is, the conductivity which we should observe if the quantity of solution containing one gram-molecule of dissolved substance were placed between two large elec- trodes 1 centimetre apart.

In order to apply these definitions let us take a fiftieth litres). According to Kohlrausch 1 the specific conduc- tivity is 0*002244, and the molecular conductivity there- fore 0*002244 x 50,000=112*2. These numbers are for 18° ; at 25° the molecular conductivity is 129*7.

Instruments. — Polarisation effects can be avoided, as Kohlrausch has discovered, by using an alternating current of high frequency. The apparatus used is represented diagrammatically in fig. 46, and consists essentially of a Wheatstone bridge in which the galvanometer is replaced by a telephone and, in addition, a small induction-coil b of very rapid vibration, which serves to transform the current derived from the battery p. Those coils constructed for medical use are admirably adapted for this work.

The resistance-box b contains resistances of 1, 10, 100, 1,000 ohms, and is used for putting in ac a resis- tance of about the same size as that of the electrolytic cell s.

1 Kohlrausch measures resistances in Siemens's units. Never- theless we shall continue to use the numbers 0*002244 and 112*2 as if they were based on a resistance measured in ohm s, and yet the comparativeness of our results will hardly be impaired.

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