Outline of method and apparatus

Outline of Method and Apparatus.—For the purpose of measuring the resistance of a solution, one usually employs the Wheatstone bridge method, the arrangement of which is shown dia-grammatically in Fig. 53. 

The induction coil should be a small one, so that the amount of electricity which passes at each pulse, and therefore the polarization produced, is small. The hammer, also, should be a light one, so that it can be made to vibrate rapidly, and thus produce a high-pitched and more readily audible sound. The coil is most conveniently actuated by a single lead accumulator, or by a dry cell, and the strength of current should be so regulated that the sound of the coil is just distinctly audible. For this purpose a sliding resistance may be inserted in the circuit between the cell and the coil but it is better, especially 

The resistance box contains a number of wire resistances, the ends of which are attached to brass blocks on the top of the box, electrical connection between these being made by means of brass plugs (Fig. 55). By removing the plugs, resistance is placed in the circuit, the current being then made to pass through the wire. The value of the resistance inserted is marked opposite each plug-hole. 

Care must be taken that these plugs do not become dirty. When taken out, they should be placed on the top of the box or in the blind holes made for them, and not on the working bench. The plugs should be wiped from time to time with a cloth moistened with petroleum and when die box is not in use, they should all be inserted in their places. 

The conductivity vessel may have various forms, according to the liquid for which it is to be used. The two forms which come most generally into use are shown in Fig. 56. They con sist of cylindrical glass vessels, either of uniform diameter or narrowed at the foot, for use with liquids of greater conductivity. 

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