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SWITCHES, FUSES, CIRCUIT BREAKERS
[Cn. 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 inadvertently 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
— Hg—
SB —
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).
S 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