144 ELECTRICITY 



the positive again (Fig. 105). This complete conducting 

 path is called the circuit. 



If there is any break in a circuit at any point from 

 beginning to end, no current will flow through any part of 

 the circuit. This matter is of great importance to man 

 in controlling electrical energy. For example, note in 

 Fig. 105 that the circuit is not complete, being broken 

 at the push button for the weak current will not go 

 through the air space. The circuit being broken at this 

 one point, no current will go through any part of it, and 

 the bell will not ring. But close the circuit by press- 

 ing the button, and the current travels at once through 

 every part of the conducting path and rings the bell. 



A completed circuit is said to be closed, or made; 

 when interrupted at any point by an insulator, it is said 

 to be open, or broken. 



165. Resistance of the Circuit. We have learned 

 that any conductor offers some resistance to the pas- 

 sage of a current ( 156). The resistance of a circuit is 

 divided into two classes, internal, or that offered by 

 the cell ; and external, or that of the wires, instruments, 

 or other outside conductors. External resistance depends 

 upon three things : the kind of substance, the length of 

 conductor, and its area of cross section. Other things 

 being equal, the greater the length of a conductor, or the 

 smaller its area of cross section, the more resistance it offers 

 to the current. Naturally a current traveling through a 

 greater length of conductor would meet more resistance ; 

 and, lengths being equal, passage along a small conductor 

 would, of course, be more difficult than over a larger. 



