=142. Safety Devices.= We have seen that when too large a current passes through a wire, the wire becomes heated and may even be melted. Buildings are wired to use certain currents, and if from any cause much more current than the regular amount should suddenly pass through the service wires into the house, the various smaller wires would become overheated, and perhaps melt or start a fire. An accidental short circuit, for example, would so reduce resistance that too much current would suddenly rush through the wires. There are several devices by which the over-heating of wires is obviated. [Illustration: Figs. 171 to 175.] Fig. 165 shows a _safety fuse_, or _safety cut-out_, which consists of a short length of easily fusible wire, called _fuse wire_, placed in the circuit and supported by a porcelain block. These wires are tested, different sizes being used for different currents. As soon as there is any tendency toward over-heating, the fuse _blows_; that is, it promptly melts and opens the circuit before any damage can be done to the regular conductors. Fig. 166 shows a cross-section of a _fuse plug_ that can be screwed into an ordinary socket. The fuse wire is shown black. Fig. 167 shows a _fuse link_. These are also of fusible material, and so made that they can be firmly held under screw-heads. For heavy currents _fuse ribbons_ are used, or several wires or links may be used side by side. Fig. 168 shows a _fusible rosette_. Fig. 169 shows two fuse wires fixed between screw-heads, the current passing through them in opposite directions, both sides of the circuit being included. Fig. 170 shows various forms of cut-outs. [Illustration: Fig. 176.] =143. Wires and Cables= are made in many sizes. Figs. 171 to 175 show various ways of making small conductors. They are made very flexible, for some purposes, by twisting many small copper wires together, the whole being then covered with insulating material. [Illustration: Fig. 177.] Figs. 176, 177, show sections of submarine cables. Such cables consist of copper conductors insulated with pure gutta-percha. These are then surrounded by hempen yarn or other elastic material, and around the whole are placed galvanized iron armor wires for protection. Each core, or conductor, contains a conductor consisting of a single copper wire or a strand of three or more twisted copper wires. =144. Lamp Circuits.= As has been noted before, in order to have the electric current do its work, we must have a complete circuit. The current must be brought back to the dynamo, much of it, of course, having been used to produce light, heat, power, etc. For lighting purposes this is accomplished in two principal ways. [Illustration: Fig. 178.] Fig. 178 shows a number of lamps so arranged, "in series," that the same current passes through them all, one after the other. The total resistance of the circuit is large, as all of the lamp resistances are added together. [Illustration: Fig. 179.]