Page:Outlines of Physical Chemistry - 1899.djvu/217

 CONDUCTIVITY OF ELECTEOLYTES

��between them. 1 The conducting wires are soldered on to the electrodes, and are protected from contact with the solution by means of a rubber or glass tube.

Method. — The capacity of the electrolytic cell is first determined, which is done by introducing a solution of known specific conductivity (for example, a ^ normal solu- tion of potassium chloride). After starting the coil the movable contact is moved along the graduated resistance a b (fig. 46) until the telephone becomes silent, or at least

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��until the maximum silence-point is reached. At this point the resistances are in the following ratio :

r : a d = s : d b.

For the resistance wanted, therefore, we have :

"R.db

��s =

��and for the conductivity :

s

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ad R.db

��1 The object of this precaution is to increase the surface of the electrodes and thus minimise polarisation effects, which is particu- larly important for the distinctness of the telephone,

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