Page:Outlines of Physical Chemistry - 1899.djvu/216

 ��OUTLINES OF PHYSICAL CHEMISTRY

��The telephone 1 is placed in the bridge c d> and d is a movable contact which slides along the graduated resis- tance a b.

This resistance a b generally consists of a platinum- iridium wire about 0*2 mm. in diameter and 1 metre long, stretched along a board which is provided with a milli- metre scale. An alloy of copper and manganese, or one of copper and nickel, may equally well be used for this resistance.

As the resistance of solutions varies within rather wide limits, the form of the cell s must be modified to suit the circumstances. For measurements where the conductivity is small the form shown in fig. 47 is used, but in those cases where the conductivity is fairly large the form shown in fig. 48 should be used. The electrodes, which have a diameter of 2 to 4 centimetres, are made of platinum and should be covered with a layer of platinum -black hy electrolysing a dilute solution of hydrochloro-platinic acid

��1 The choice of a suitable telephone is of the^very greatest impor- tance (under the circumstances a galvanometer cannot be used).

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