SWITCHES, FUSES, CIRCUIT BREAKERS 



[CH. XIII 



or special plant should be enclosed in a metal box where it can be 

 easily got at, but not where the naked metal parts might inad- 

 vertently become short-circuited. 



It is necessary also to put the switch in such a position that 

 when it is opened it will not close of itself by gravity. If the 

 switch is in a vertical position it must be placed with the hinge 

 below, so that gravity will tend to open it, never to close it (fig. 

 277). 



If the switch is horizontal, then the hinge should be tight enough 

 so that the blades will remain in any position in which they are 

 placed. For a double-pole, double-throw switch for two lamps see 

 fig. 162. 



A knife switch has an appreciable amount of naked metal 

 exposed. It therefore makes a short circuit easily possible. For 

 use with projection apparatus, especially if high amperages are to 

 be used as with opaque projection and with moving pictures it is 



Base 



AW 



FIG. 277. OPEN KNIFE SWITCH IN A VERTICAL 



POSITION, WITH THE HANDLE BELOW so THAT 



THERE IS NO DANGER OF THE SWITCH 



CLOSING BY GRAVITY. 



L W Line wires from the electric supply (fig. 

 270) to the switch. 



A W Arc lamp wires from the switch to 

 the arc lamp. A rheostat is inserted in one of 

 them (fig. 270). 



5 C Spring clamps pressing against the switch 

 blades when the switch is closed, thus making 

 good metallic contact. 



Base The insulating base of the switch. It 

 is held in position by two or more screws. 



Hg Hinges of the switch blades. 



5 B Switch blades. When the switch is 

 closed these blades make a continuous circuit, 

 and when the switch is open the circuit is 

 broken. 



Cb Cross-bar of insulating material to 

 which the switch blades and the handle are 

 attached. 



H Handle for opening and closing the switch. 

 It is of insulating material 



