The Line.—From fig. 3 some idea may be gained as to the mode of carrying a wire or series of wires over posts, these posts being carried along the sides of a road or railway. In towns, wires are carried " over-house," or by underground pipes, the wires in the latter case being insulated by means of a gutta-percha or other suitable covering. The subterranean method is being applied to extended lines, especially in Germany, and is found to answer as well as the over-head system, while it avoids many of the casualties to which the latter is liable. In pole and over-house lines, the wires are kept from each other and from contact with the earth by insulators of various kinds. White porcelain and brown stoneware are the chief materials used. The former, when of good quality, well glazed and well burned, is perhaps the most perfect of all insu lating materials, and does not deteriorate with age. The fewer the poles are in num ber on which the wires are suspended, the better is the insulation and the less the cost, but the liability to accident is probably greater. The number of poles used varies from 16 to 30 per mile, and is governed by the number of wires carried, the configura tion of the track, and other considerations. On road lines, the number of poles is generally larger than in the case of telegraphs carried alongside railways, the greater levelness and straightness of the latter reducing the number of supports required. The wire chiefly used for inland telegraph purposes is of iron, galvanized, and of No. 8 (I in.) gauge. The conductivity of a wire increases in the ratio of the square of its diameter (the resistance decreasing in the inverse ratio), and the advantage of using a thicker wire on the longer lines is thus seen. No. 4 wire is, for this reason, used on some of the longer lines.
The Earth—Earth Ourrents.—Mention has been made of the "earth," in the preced ing description. This is the technical expression used in relation to the fact discovered by Steinheil in 1838, that the earth itself serves the purpose of completing the circuit, and renders the employment of a second or return wire unnecessary. The "earth" may consist of a buried plate of metal connected with the battery or line-wire, and of suffi cient surface to afford the necessary diffusion. The gas or water pipes of a town form excellent " earths," care being taken that the connection is made with the pipe itself, and not with a branch, where a badly made joint might spoil the connection. Where dissimilar " earths" are in use, as for instance, a copper plate at one end, and an iron pipe at the other, a quasi battery is created, and vexatious currents pass along the line. Hence the "earths" on a circuit should be made alike. The earth, being regarded as a large reservoir of electricity, offers no sensible resistance to the passage of the cur rent, in the same way as the ocean would receive or supply at any point an indefinite quantity of water. While this quality of the earth is one of the most valuable aids to telegraphy (reducing so materially the cost of wire erection), it presents at times those embarrassing interruptions known as earth current& These currents, at all times unwelcome visitors to a telegraph office, are very variable, changing rapidly at times from positive to negative, altering their direction with the hour of the day, and leaving one circuit to appear on another in a manner not explainable. The lines most liable to such disturbances are those running n.e. and s.w.; that is, connecting places separated in a straight line in those directions, and without reference to the actual direction of the wires. The easiest remedy for earth currents, when they are of sufficient strength to affect the lines, is to dispense with the earth connection, and revert to the original plan of using two wires. Thus between places where there are two wires, both may be dis connected from " earth," and used as a complete metallic circuit. Another remedy has
been found in extending the circuit by joining to it a further wire, the terminal point of which lies beyond the direction or line in which the earth current is flowing. We must refer to the larger treatises on telegraphy for information regarding lateral induction, velocity of electric discharge, the tests for resistance, insulation, etc., and also for notices of some of the less prominent pieces of apparatus now found in the instrument room of the electrician.
The Relay.—We now proceed to notice several methods by which the transmission of signals is facilitated or accelerated. First among those may be placed the relay. Siemen's polarized relay is now in very extensive use in this country. In the previous description of the Morse, we have assumed the instrument to be worked directly by the current sent along the line. On long circuits, however, direct working could only be accomplished by great battery power, as, owing to inevitable loss by leakage, a current loses greatly before it reaches its destination. It is found to be a much better arrange ment to have the instrument worked by a "local current," derived from a local battery at the receiving station. The mode by which this is accomplished will be seen from the diagrams. In the figures, N S is a hard steel permanent magnet, whose a. end S has a slit, in which the soft iron armature, a, is pivoted. To this armature a thin aluminium tongue, b, is attached, which, by making contact at the point c (fig. 4), completes the local circuit. To the n. end, N, of the permanent magnet the soft iron cores of the electromagnet are fixed, as shown in the sketch. When the armature is equidistant from the poles of the electromagnet, it is equally attracted by both; but if it be brought nearer to one than to the other, it will be held there, because it is under the influence of a more powerfully attracting force. Since the relative distance between the armature and the two pole-pieces may be increased at will, the attraction between either pole and the armature may be regulated with any degree of 'nicety. The electrical contacts for the local circuit are seen in fig. 4, which is shown with the local circuit completed. When two stations far apart are to be connected by telegraph, it is usual to trans mit the signal to a half-way station, and thence to re-transmit it to its destination. The retransmission is not effected by manipulative skill, but by mechanical contri vance, so that, while the half-way station may read the message sent, no time is lost in the transmission. This is effected by making the intermediate instrument act as a relay in transmitting a message to the next station. The system, to be fully explained, would require more detail than we can here give to it. We shall only show how it is effected, leaving out of account how all the stations can communicate as in one circuit. The current C,, (fig. 6), from the sending-station enters the coil M, and goes thence to earth P, and returns as shown by arrow O. The instrument may record or not as required, but its doing so in no way interferes with translation. The copper pole C, of the battery CC, is connected with the lever //' of the register, and the zinc pole is to earth. When the lever is drawn down by the current C,, it strikes against the point at the top of the pillar p, that checks its motion. The pillar p is joined to line L2, running to the further station, and when the lever falls, a second circuit—viz., that of the battery CZ—is closed, in which C, the lever, the pillar the further station, the earth, P, and Z are all included. Thus, as //' prints at the inter mediate station, it at the same time sends a new printing-current to the next. When it ceases to print, so does the instrument at the distant station.