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.
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.
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.
