The reader, on comparing these signals with the Morse code on page [560], will have no difficulty in discovering their relation to it.
TELEGRAPHIC LINES.
It now remains to give some account of the line, that is, the conductor by which the sending and receiving instruments are united, and along which the currents flow. Overhead lines are nearly always constructed with iron wires, which are usually ⅙ in. in diameter, and are coated with some substance to protect them from oxidation. Zinc is often used for this purpose, the wire being drawn through melted zinc, by which it becomes covered with a film of this metal—a process known as “galvanizing” iron. Another mode is to cover the wires with tar, or to varnish them from time to time with boiled linseed oil, and this must be done in populous places, where the gases in the air are liable to act upon the zinc. Sometimes underground wires are used, and these are often made of copper, covered with gutta-percha, and are laid in wooden troughs, or in iron pipes. They are protected by having tape or other material, saturated with tar or bitumen, wound round them. The poles employed to suspend the overhead wires are generally made of larch or fir, of such a length that when securely fixed in the ground they rise 12 ft. to 25 ft. above it, and at the top have a diameter of about 5 in. About thirty poles are required for each mile, and every tenth pole forms a “stretching-post,” being made stronger than the others and provided with some appliance by which the wires can be tightened when required. The wires are attached to the posts by insulating supports; but at every pole there is always some “leakage,” the amount of which depends on the form, material, and condition of the insulators. Glass is quite unsuitable, because its surface strongly attracts moisture, which thus forms a conducting film. All things considered, porcelain is found to be the best insulating material for this purpose, since moisture is not readily deposited on its surface, and even rain runs off without wetting it; and it is durable, strong, and clean. Fig. [293] shows a telegraph post, with brown salt-glazed stoneware insulators, shaped like hour-glasses, with a perspective view and section of one of them. Another form of insulator, shown in Fig. [294], has a stalk or hook of porcelain, with a notch, into which the wire is simply lifted, and is protected above by a porcelain bell. This form, or some modification of it, is that most generally used.
Fig. 293.—Telegraph Post and Insulators.
Fig. 294.
It need hardly be remarked that only a single wire is required with most of the modern instruments for communication between any two places. Each of the many wires often seen attached to the telegraph posts along a road or railway represents a distinct line of communication—that is, one wire may connect the two termini, another may join an intermediate station and a terminus, a third may belong to two intermediate stations, and so on. We have already alluded to the discovery by Steinheil of the apparent conducting power of the earth; and if we must continue to think of complete circuits, we must regard the earth as replacing for telegraphic purposes the second or return wire, which was at first supposed essential. For instance, when a battery current had to be sent from Station A, Fig. [295], which we may suppose to be London, to Station B, which we may call Slough, it was at first thought requisite to provide a wire for the return of the current after it had traversed the coils at the receiving station. But now the connections are made as shown in Fig. [296], where the return wire is dispensed with, except a small portion at each end, which is connected with a large plate of copper buried in the earth; the arrows show the direction of the current, according to the commonly received notion. By this plan the current is increased in intensity, for the “earth circuit” appears to offer less resistance than the copper wire. The view, however, which regards the earth not as a conductor in the same sense as the wire, but as the great reservoir or storehouse of electricity, accords better with known facts.
Fig. 295.—Wire Circuit.