The purpose of a fuse is to break a circuit if,and when, the current exceeds a certain limit. It is usually a soft metal wire, with a low melting-point; and its gauge is such that the wire shall reach that melting-point before the copper conductor which it protects can become dangerously lf?t?g CrctfAr 4 3 1 one of them, have blown. Before the lamps can be used again the fuses must be replaced, and for this contingency a supply of fuse wire of the proper current-carrying capacity should be kept at hand.
Where the fuse connections consist merely of two screw terminals between which the wire is stretched, (he main switch must be opened before anything further is done. This will at once plunge the whole house into darkness.
Should, for example, the two conductors of a circuit come into contact, the current would instantaneously increase many times. But in a fraction of a second one fuse or both fuses of the circuit would blow (i.e.. melt) and the current cease, owing to gaps having been created, before damage had been done. The fuses, therefore, are very important fittings.
If one lamp suddenly goes out, its filament has probably failed. But if several lamps are extinguished simultaneously, one may assume that the circuit fuses, or this reason it is a great advantage to have the fuses mounted inside holders which can safely be removed and replaced without cutting off the current. Some types of holder make the replacement of a fuse a matter of a few moments only.
On no account yield to the temptation to substitute an odd piece of copper or iron wire – such as a hairpin – for fuse-wire if the supply of the latter has run out. To do so might seriously endanger the whole circuit. Keep the ends of the blown fuse as a sample for a fresh supply.