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.
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.
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.
Fig. 177.