A fuse is usually a short piece of lead or some alloy which melts at a low temperature, and which is inserted in the circuit so that the current must flow through it. If too much current flows through the fuse it will become hot and melt, because of its low melting-point, thus interrupting the circuit and shutting the current off until the cause which occasioned the surplus current to flow can be ascertained. Fuses are rated according to the amount of current which is required to "blow" them out, and therefore are called 1, 3, 5, or 10 ampere fuses, as the case may be. [Illustration: Fig. 98.—Simple Fuses. _A_, Fuse-Block with plain Wire Fuse. _D_, Fuse-Block with Mica Fuse in position.] When a fuse burns out in a trolley car or in a light or power circuit, it is because a greater amount of current is trying to pass than the circuit can safely carry. If a fuse were not placed in such a circuit so as to shut the current off before the danger point is reached, any electrical device might become "burned out," or in extreme cases, the wires become so hot as to cause a serious fire. Figure 98 shows several simple forms of fuses which the experimenter may easily make to protect the batteries, etc., from short circuits. The simplest possible fuse consists merely of a small piece of lead wire or a strip of thick tinfoil held between two binding-posts mounted upon a wooden block. The same form of fuse may be made from a strip of mica about two and one-half inches long and one-half an inch wide. A strip of thin sheet-copper is bent around the ends of the mica strip. A piece of fuse wire is stretched between the two copper contacts and fastened to each with a drop of solder. Fuse wire of any desired ampere-carrying capacity can be obtained from most electrical supply houses. Such a fuse is held in a mounting as shown by _D_. The contacts are made from sheet-copper or brass. They should spring together very tightly, so as to make perfect contact with the copper ends on the mica strip. Lightning-Arresters Lightning-arresters are used to protect all wires which run into a building from outdoors, especially telegraph or telephone wires, so that static electricity due to lightning will not damage the instruments. Lightning-arresters may be constructed in many ways and of different materials, but there are only two types for which the average experimenter will have any use. [Illustration: Fig. 99.—Lightning-Arrester and Ground-Wire Switch.] The arrester shown in Figure 99 is the type known as "lightning-arrester and ground-wire switch." It is used principally on telegraph lines. It consists of three pieces of sheet-brass about one-sixteenth of an inch thick, and shaped as shown by _A_, _B_, and _C_ in Figure 100. The metal pieces are mounted on a wooden block with a narrow space of about one-thirty-second of an inch separating them. [Illustration: Fig. 100.—Home-made Lightning-Arrester.]