Page:Outlines of Physical Chemistry - 1899.djvu/214

 196 OUTLINES OF PHYSICAL CHEMISTRY

Bemark. — The battery circuit aefb remains open whilst the trials are being made to find the suitable r, and, by means of the spring contact-maker f, the current is allowed to pass through only for a time sufficient to allow galvanometric observations to be made.

The Wheatstone bridge is also useful for making copper or brass wire resistances equal to standard ones. (For details see Ostwald's * Manual of Physico-chemical Measurements/ English translation by Walker.)

The MEASUREMENT OF THE INTENSITY OP A CURRENT

can be made directly with a tangent or an ordinary galvano- meter. With the ordinary galvanometer the deflection of the needle, so long as it is of no great amplitude, is pro- portional to the intensity of the current. For greater deflec- tions, however, the relation is more complex.

But this direct measurement is not of much use, for we

know that o = —, and there are excellent methods for deter-

R

mining both e and r.

The Conductivity of Electrolytes

This subject is of such importance that it has become a sort of ' Leitmotiv ' in more than one treatise on theore- tical chemistry. The conductivity of electrolytic solutions stands in direct relationship with certain phenomena which have already been discussed, particularly with the osmotic pressure and with the depression of the freezing point. As these properties of solutions have only recently been investigated, it is not hard to understand that they play rather a preponderating rdle in the considerations of modern theorists.

A current can only pass through a salt solution and continue to electrolyse it if it has an electromotive force sufficient to overcome the adverse potentials developed by the polarisation of the electrodes. If, however, the polarisation effects are neutralised or reduced to a negli-

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