138 ELEMENTARY LESSONS ON [CHAP. in. 



168. No single battery fulfils all these conditions, 

 however, and some batteries are better for one purpose 

 and some for another. Thus, for telegraphing through 

 a long line of wire a considerable internal resistance in 

 the battery is no great disadvantage ; while, for producing 

 an electric light, much internal resistance is absolutely 

 fatal. The electromotive-force of a battery depends on 

 the materials of the cell, and on the number of cells 

 linked together, and a high E.M.F. can therefore be 

 gained by choosing the right substances and by taking 

 a large number of cells. The resistance within the cell 

 can be diminished by increasing the size of the plates, 

 by bringing them near together, so that the thickness 

 of the liquid between them may be as small as possible, 

 and by choosing liquids that are good conductors. Of 

 the innumerable forms of battery that have been invented, 

 only those of first importance can be described. Batteries 

 may be classified into two groups, according as they 

 contain one or two fluids. 



SINGLE-FLUID CELLS. 



169. The simple cell of Volta, with its zinc and copper 

 plates, has been'already described. Cruickshank suggested 

 to place the plates vertically in a trough, producing a 

 more powerful combination. Dr. Wollaston proposed 

 to use a plate of copper of double size, bent round so as 

 to approach the zinc on both sides, thus diminishing the 

 resistance. Smee, as we have seen, replaced the copper 

 plate by platinised silver, and Walker suggested the use 

 of plates of hard carbon instead of copper or silver, 

 thereby saving cost, and at the same time increasing the 

 electromotive-force. The simple bichromate cell (Fig. 

 72) is the only single-fluid cell free from polarisation, 

 and even in this form the strength of the current falls 

 off after a few minutes' working, owing to the chemical 



