CH. XIII] WIRING FOR AN ELECTRIC CURRENT 497 



current must travel to get back to the dynamo. In figure 265 if a 

 wire were put across the circuit at the points s. c. instead of the 

 current extending entirely around the circuit, it would take the 

 shorter course. Short circuits are undesirable for two reasons : 

 (i) the current is not available where wanted; (2) it may be 

 dangerous. 



689. Ground. With many electric circuits such as with 

 street railway circuits, one terminal of the dynamo is permanently 

 connected with the earth. If now the wire connected to the other 



FIG. 266. AN ELECTRIC CIRCUIT WITH A SINGLE GROUND. 

 C D The two poles of the dynamo. 

 G Generator (dynamo). 



B I A conductor extending from the electric circuit to the ground (g 1 ). 

 If all the rest of the circuit is insulated this will do no harm, but see fig. 267. 

 g 1 The earth into which the conductor, B , extends. 

 A Arc lamp. 

 R Rheostat. 



terminal of the dynamo should also become connected with the 

 earth, as through a water or a gas pipe, current would wholly or in 

 part take that path back to the dynamo. 



When any part of the circuit is connected with the earth it is 

 called a "ground." 



In case the dynamo and circuit are entirely insulated from the 

 earth, a single ground will result in no flow of current outside the 

 wire. If, however, two points in a circuit are connected to the 

 earth the effect will be the same as if the two points of the circuit 

 were connected to each other, by an additional wire (fig. 266, 267). 



690. Insulation of wires. To avoid short circuits and the 

 consequent danger to men and animals and also the danger from 



