186 



ELECTRICAL CONSIDERATIONS. 



The Du Bois Reymond key (Fig. 21, A) is one of the most efficient, 

 and is of historical interest. 



The simple switch depicted (Fig. 21, B) is quite as efficient and is 

 less costly. 



MAKE AND 



FIG. 21. A Du Bois Reymond, B Simple key. 



Whatever the key, it can be used in one of two ways. 



1. As a Make and Break device it is inserted into the circuit so 

 that when closed the circuit is completed, and when it is opened the 



circuit is broken. Always use it in 

 this manner when in circuit with a 

 Leclanche cell (Fig. 22). 



2. For short circuiting* (" short 

 ing"). In this case the circuit is never 

 broken, but the closure of the key is 

 so arranged as to offer a path of less 

 resistance to the current. 



The current is not entirely diverted 

 from the " longer " path, but practical!}" 

 so little passes that it comes to the 

 same thing. 



Remember that in a divided circuit the distribution of the current 

 is proportioned inversely to the resistance of the respective paths. 

 Less resistance more current ; more resistance less current. 



Commutator. (Fig. 23.) For the purpose of reversing the direction 

 of a current. PoliVn instrument is the best. 



The rocker consists of two portions joined across the middle line 

 l>\ the insulating piece R. 



SHORTING" 

 FIG. 22. 



