148 ELEMENTARY LESSONS ON [CHAP. in.. 



frequent in the telegraph service to link thus together 

 two or three hundred of the flat Daniell's cells ; and 

 they are usually made up in trough-like boxes, containing 

 a series of 10 cells, as shown in Fig. 77. 



To diminish the internal resistance of a cell the follow- 

 ing expedients may be resorted to : 



(i.) The plates may be brought nearer together, so 

 that the current shall not have to traverse so thick a 

 stratum of liquid. 



(2.) The size of the plates may be increased, as this 

 affords the current, as it were, a greater number of 

 possible paths through the stratum of liquid. 



(3.) The zincs of several cells may be joined together, 

 to form, as it were, one large zinc plate, the coppers 

 being also joined to form one large copper plate. Cells 

 thus joined are said to be united " in compound circuit," 

 or " for quantity," to distinguish this method of joining 

 from the joining in simple series. Suppose four similar 

 cells thus joined, the current has four times the available 

 number of paths by which it can traverse the liquid 

 from zinc to copper ; hence the internal resistance of the 

 whole will be only J of that of a single cell. But the 

 E.M.F. of them will be no greater thus than that of 

 one cell. 



It is most important for the student to remember that 

 the strength of the current is also affected by the resist- 

 ances of the wires of the external circuit ; and if the 

 external resistance be already great, as in telegraphing 

 through a long line, it is little use diminishing the internal 

 resistance if this is already much smaller than the resist- 

 ance of the line wire. 



The E.M.F. of the single-fluid cells of Volta and Smee 

 is marked as doubtful, for the opposing E.M.F. of polar- 

 isation sets in almost before the true E.M.F. of the cell 

 can be measured. The different values assigned to other 

 cells are accounted for by the different degrees of con- 

 centration of the liquids. Thus in the Daniell's cells 



