PRACTICAL OPERATION OF DYNAMOS. 2O/ 



prompt in action and when once adjusted, it is opened by a cur- 

 rent of a perfectly definite value. The electromagnetic circuit 

 breaker has the further great advantage over a fuse link in that 

 connections can be reestablished at once after the circuit has been 

 opened by an excess of current, whereas the replacing of a fuse 

 is a tedious operation and it is attended by serious risk as ex- 

 plained in Art. 85. 



The circuit breaker is a metal switch, single-pole or double- 

 pole, which is arranged to open automatically when the current 

 is in excess of a certain value. The closing of this switch puts 

 a strong spring in tension tending to open it. The switch is 

 kept closed by a latch which is controlled by a trigger, and this 

 trigger is released by an electromagnet (through which the main 

 current flows) when the current reaches a certain value. 



Inasmuch as a circuit breaker always opens when an excessive 

 current flows, it is evident that an intense arc will always flash 



Fig. 125. Fig. 126. 



across the break. It is necessary to thoroughly protect the 

 metal contacts of the circuit breaker from the destructive action 

 of this arc by having a pair of auxiliary contacts in parallel with 

 the main contacts and so arranged that the main contacts open 

 first (without an arc) and the auxiliary contacts open a moment 

 later (with an intense arc). 



