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CH. XIII] SWITCHES, FUSES, CIRCUIT BREAKERS 519
§ 717. Fuses. — A fuse is a wire of low melting point forming part of the circuit. If the current becomes too great this fuse is melted, thus making a gap in the line. The fuse is then said to burn out or to "blow." If the current becomes much too great as in a short circuit the fuse will "blow" instantly, if however, the current is only slightly larger than the fuse is designed for — as for example, when striking the arc in an arc lamp — the fuse will not "blow" instantly, and if the overload is only for a short time it will not melt at all. If the overload continues for some time, however, the fuse will get hotter and hotter until its melting point is reached, when it will melt and open the circuit. This property of the fuse is of great advantage when using arc lamps, for the temporary overload in lighting the arc lamp is unavoidable.
§ 718. Location and installation of fuses. — Like the switch, the fuses should be placed in the path of all the wires of a circuit — i. e., with a two-wire system two fuses, and with a three-wire system, three fuses, etc. The wiring of a fuse block is the same as for a switch (§ 712).
There is always a switch in the supply box from the electric lighting system or from the private dynamo. In this box are also fuses to open the circuit in case of accidental short-circuiting. The fuse block, whether for cartridge fuses or for plug fuses should be selected with care to make sure that it is of the right capacity for the maximum current and conforms to the standard code. The fuses are plainly marked, so there need be no mistake.
One should not use fuses of higher capacity than the line was designed for, for fear of fire or other accident.
If the supply box is some distance from the arc lamp, many careful operators have fuses as well as a switch at the supplementary supply box in the operating room, when a conduit or fixed wires are carried from the main supply to the operating room. The fuse nearest the arc lamp is preferably of somewhat less capacity than the ones farther away, then if a fuse is blown it will be the handiest one to renew.
§ 719. Fuses and the wattmeter. — If but a single meter is used to measure the current for arc lights, house lights, heating appara