276 THE ELECTRIC TELEGRAPH. 



36. Conducting Powers of Fluids. In constructing a 

 battery for any given work, it is necessary frequently to 

 consider the resistance of the fluids used. Wheatstone, 

 Horseford, and others, have invented apparatus for the 

 determination of the conducting powers of solutions, &c. 

 That of Horseford is the best and simplest. It consists of 

 an oblong wooden trough, varnished inside with shellac. 

 On the top are two cross-bars of wood, with guides, over- 

 lapping the sides, to keep them straight. To each of these 

 cross-bars is attached a plate of platinum, only so much 

 smaller than the interior section of the trough as to allow 

 it to be moved freely, with its cross-bar, from end to end. 

 Usually one of the bars, with its platinum plate, is fixed, 

 and the other movable. A divided scale on the upper edge 

 of the trough facilitates the observation of the distance 

 between the plates. Copper wires are soldered to the pla- 

 tinum plates, and serve as connections with the measuring 

 apparatus. When the apparatus is to be used, it is placed 

 as level as may be upon a table, and filled to a convenient 

 height with the solution whose resistance is to be measured. 

 If the relation of the resistance to the distance between the 

 plates is to be determined, the solution is poured in, and 

 the resistances measured with various distances. By this 

 measurement it becomes evident that the resistance of a 

 conductor is directly proportional to its length. By keeping 

 the distance between the plates unaltered, and varying the 

 height of the solution in the trough, the experimenter may 

 convince himself of the truth of another law, namely, that 

 the resistance of a conductor is inversely proportional to its 

 transverse section. 



But the experiments which are most important are the 

 conducting powers of solutions of the various salts and of 

 the acids. For these measurements, of course, the distance 

 between the plates and the height of the solution in the 

 trough must be constant through the whole series. 



Becquerel has determined the conducting powers of some 

 of the concentrated solutions, and also of the same diluted 

 with water. When the conducting power of pure silver is 





