chap, xv.] MEDICAL BATTERIES. 151 



the movement of the sledge increases the number of 

 cells in circuit without breaking the circuit. Opening 

 and closing shocks are, therefore, avoided. The 



B 



sledge has binding screws for the attachment of wires. 

 It also has an arrangement c which acts both as key 

 and commutator. When the handle is perpendicular 

 no current is passing; when it is turned back the right- 

 hand binding screw is positive ; when it is turned 

 fortvard the left-hand screw is positive. Thus the 

 circuit may be interrupted and tbe direction of the 

 current altered. 



By this arrangement, therefore, any strength of 

 current may be used, and the current may be inter- 

 rupted or sent in any desired direction. 



The Leclanche battery is the one now largely 

 in use for medical purposes. It is compactly put 

 up in boxes containing twenty, forty, or more 

 cells. Each cell is about the size of a two ounce 

 bottle, and is divided into two compartments by a 

 porous partition, on one side of which is zinc in sal- 

 ammoniac solution, and on the other gas carbon and 

 native pyrolusite moistened with water (page 23). Fig. 

 79 is a representation 

 of the cover of such a 

 battery, s h ow i 11 g a 

 dial plate a, with a 

 series of brass Studs, Fig. 79. The Leclanclie Battery. 



each stud having a 



number marked 011 it. A hand can be turned over 

 the dial and caused to make contact with one stud 

 after another. When it makes contact with a stud 

 the number attached indicates the number of 

 elements in circuit, the binding screws for connecting 

 with wires being always the same. The contact-point 

 of the hand should be so made that when exactly over 

 one stud it does not touch the stud 011 either side, but 

 when being moved to a new position it ought to he 



