Chapter XIX

HOW ELECTRIC CURRENTS ARE DISTRIBUTED FOR USE.

=138. Conductors and Insulators.= To carry the powerful current from the generating station to distant places where it is to give heat, power, or light, or even to carry the small current of a single cell from one room to another, _conductors_ must be used. To keep the current from passing into the earth before it reaches its destination _insulators_ must be used. The form of conductors and insulators used will depend upon the current and many other conditions. It should be remembered that the current has to be carried to the lamp or motor, through which it passes, and then back again to the dynamo, to form a complete circuit. A break anywhere in the circuit stops the current. Insulators are as important as conductors.

=139. Mains, Service Wires, etc.= From the switchboard the current flows out through the streets in large conductors, or _mains_, the supply being kept up by the dynamos, just as water-pressure is kept up by the constant working of pumps. Branches, called _service wires_, are led off from the mains to supply houses or factories, one wire leading the current into the house from one main, and a similar one leading it out of the house again to the other main.

In large buildings, pairs of wires, called _risers_, branch out from the service wires and carry the current up through the building. These have still other branches--_floor mains_, _etc._, that pass through halls, etc., smaller branches finally reaching the lamps. The sizes of all of these wires depend upon how much current has to pass through them. The mains in large cities are usually placed underground. In some places they are carried on poles.

=140. Electric Conduits= are underground passages for electric wires, cables, etc. There are several ways of insulating the conductors. Sometimes they are placed in earthenware or iron tubes, or in wood that has been treated to make it water-proof. At short distances are placed man-holes, where the different lengths are joined, and where branches are attached.

Fig. 154 shows creosoted wooden pipes; Fig. 155 shows another form of wooden pipe. Fig. 156 shows a coupling-box used to join Edison tubes. The three wires, used in the three-wire system, are insulated from each other, the whole being surrounded by an iron pipe of convenient length for handling. Fig. 157 shows sections of man-holes and various devices used in conduit work.

=141. Miscellaneous Appliances.= When the current enters a house for incandescent lighting purposes, for example, quite a number of things are necessary. To measure the current a meter is usually placed in the cellar. In new houses the insulated conductors are usually run through some sort of tube which acts as a double protection, all being hidden from view. Fig. 158 shows a short length of iron tube with a lining of insulating material. Wires are often run through tubes made of rubber and various other insulating materials.

Where the current is to be put into houses after the plastering has been done, the wires are usually run through _mouldings_ or supported by _cleats_. Fig. 159 shows a cross-section of moulding. The insulated wires are placed in the slots, which are then covered.

Fig. 160 shows a form of porcelain cleat. These are fastened to ceilings or walls, and firmly hold the insulated wires in place. Fig. 161 shows a wood cleat. Fig. 162 shows small porcelain _insulators_. These may be screwed to walls, etc., the wire being then fastened to them. Fig. 163 shows how telegraph wires are supported and insulated. Fig. 164 shows how wires may be carried by tree and insulated from them.

=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.

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.

=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.

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.

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.

Fig. 179 shows lamps arranged side by side, or "in parallel," between the two main wires. The current divides, a part going through each lamp that operates. The total resistance of the circuit is not as large as in the series arrangement, as the current has many small paths in going from one main wire to the other. Fig. 179 also shows the ordinary _two-wire system_ for incandescent lighting, the two main wires having usually a difference of potential equal to 50 or 110 volts. These comparatively small pressures require fairly large conductors.

_The Three-Wire System_, Fig. 180, uses the current from two dynamos, arranged with three main wires. While the total voltage is 220, one of the wires being neutral, 110 volts can be had for ordinary lamps. This voltage saves in the cost of conductors.

_The Alternating System_, Fig. 181, uses transformers. The high potential of the current allows small main wires, from which branches can be run to the primary coil of the transformer. The secondary coil sends out an induced current of 50 or 110 volts, while that in the primary may be 1,000 to 10,000 volts.