PRACTICAL OPERATION OF DYNAMOS. 209 



which blows out the arc almost instantaneously when the auxiliary 

 contacts are separated. In Fig. 127 the structure at the top is 

 the blow-out magnet, and the rod which is shown immediately 

 below the center of this blow-out magnet actuates the auxiliary 

 contacts which are hidden by the large flat front pole-piece of the 

 blow-out magnet. 



The main contacts of a circuit breaker are usually made of a 

 bundle of thin sheets of spring copper and a massive block or 

 blocks of copper against which the ends of the thin sheets are 

 pressed when the circuit breaker is closed, as shown in Fig. 127. 

 This figure also shows very distinctly the electromagnetic device 

 which actuates the trigger and opens the breaker. The coil of 

 this electromagnetic device consists of a massive copper bar wound 

 in the form of a helix. The rod which terminates in the small 

 handle at the lower left hand corner in Fig. 127 is for tripping the 

 trigger by hand when, it is desired to open the breaker. The large 

 handle is the lever for closing the breaker ; the figure shows the 

 breaker closed. The breaker can be adjusted to be opened by 

 any desired value of current within certain limits, by adjusting the 

 tension of the small helical spring shown at the left in Fig. 127. 



94. Grounds and ground detectors. A ground on one wire 

 of a pair of supply mains is unsafe because an accidental ground 

 on the other wire will complete the circuit. If this second ground 

 is through a human body, a personal injury may result, and if 

 either ground connection is through a wire or other conductor, 

 the conductor may become hot enough to start a fire. In any 

 case a ground connection on both mains involves a waste of power. 

 Since accidental ground connections frequently occur on electric 

 mains, a device called a ground detector is usually mounted upon 

 the switchboard in a generating station for showing the existence 

 of serious grounds on the line wires. 



A ground detector usually employed on low voltage lines (up 

 to five or six hundred volts) consists of two incandescent lamp 

 units * connected in series between the mains with the middle 



* Each unit consists of one or more lamps in series according to the voltage. 



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