152 PHYSIOLOGICAL PHYSICS. |_Ch ap . xv. 



broad enough to make contact with the next stud 



O 



before leaving the preceding one, to prevent breaking 

 the circuit every time a change in the strength of 

 current is made. The cover has also a commutator 

 b for reversing the direction of the current, and a 

 key for interrupting it c. 



The description of these two forms of batteries 

 sufficiently indicates what is wanted to make a ser- 

 viceable battery. The following points should be 

 noted : 



(1) Batteries containing large cells are not required, 

 because, as explained in chapter iii., the resistance of 

 the body to be overcome is so great in proportion to 

 the resistance of the elements that the latter can 

 afford to be neglected. Consequently large plates 

 are of no advantage. For the same reason, (2) the 

 number of cells arranged in series should be multi- 

 plied, for with large external resistance this increases 

 the electromotive force. 



Therefore, in selecting a battery one should choose 

 a case containing forty or fifty small elements. 



There are two exceptions to this rule. If the 

 current is to be used for electrolysis, or for heating 

 purposes (cautery, for example), cells with large plates 

 are desirable, because in this case the external resis- 

 tance is small. 



(3) The battery should have an arrangement for 

 altering the strength of the current without breaking 

 the circuit. 



(4) It should be provided with a key for inter- 

 rupting the current ; and many are provided with an 

 arrangement for interrupting more frequently than 

 can be done by hand with a simple key, so as to give 

 a rapid series of shocks. 



(5) Besides the accessories mentioned, a galvano- 

 meter is often supplied together with a battery 

 for testing the strength of current. The best is a 



