But for the reason above stated the voltaic current is by no means a fleet messenger COMparefl with other agents which are at the command of the electrician. Consequently it is not the electric stream generated in this powerful battery which was designed to be actually scut across the Atlantic on the performance of telegraph service. This primary power is only used to call up and stimulate the energy of a more speedy traveller. The voltaic current, generated in the battery, is transmitted to induction coils, arranged in pairs, each coil being arranged as In Itulunkorfrs apparatus, a figure of which is given under 31 AOSETts ELECTRICITY, col. 125. It is the secondary current induced by this apparatus which It was proposed should perform the work of rushing *creole the Atlantic. This independent secondary current was there.ore the trtiosat;ssion currrsl, and the coil in which it was produced was properly the col. The coils were used in paira, because each one inductively increases the power of its neighbour, and in return has its own energy inductively increased as welL The great besting power of the battery-current is rendered harmleem by the adze and extent of the primary coil through which it is pealed. If at any time, by accident, the cm rent should find a short course for itself in consequence of the silk covering of the wire being injured, the accident is Immediately indicated by the rapid rise of the temperature of the coiL The transmission-current necessarily gets considerably weakened when It has passed through a distance of 1800 or 1900 miles. Con sequently this weakened current was not to be immediately employed to print or record the signals transmitted. The weakened trauamission. current was merely caused to opeu and close the outlet of a fresh battery destined to do the printing or recording labour. The strand of the Atlantic cable was continued into a coil of fine wire, wound about • bar of soft iron. When the transmission-current flowed through the coil, the bar became a temporary magnet, which had the direction of its polarity determined by the nature of the current (positive or negative) sent through the coiL The pole which is north when the transtnimion-current is positive, becomes south when the transmission current is negative. Near to the temporary magnet a permanent magnet was so placed that it could traverse backwards and forwards upon a pivot as it was actuated by the temporary magnet. The north pole of the permanent magnet was attracted by the south pole of the temporary one, and eke tread; so that as the polarity of the temporary magnet was reversed, the permanent magnet was made to traverse. When it traversed one way, it opened the outlet of the local battery by effecting a contact, and caused it to print; when it traversed the other way it shut off the current of the local battery, so that it ceased to print.
It was the peculiar advantage of this relay-instrument (as it is called) that the temporary magnet had no other work to do than to turn the permanent magnet upon its almost frictionless pivot. It had no spring to overcome, such as is more commonly employed in this class of inetniments. The arrangement was so sensitive that the apparatus could be put in action by a fragment of zinc and a sixpence pressed against the tongue. These relays might indeed be ordinarily heard clicking backwards and forwards, and working automatically when the largo induction-coils were in operation within a few feet of them, actually doing a little business on their own account, although not iu communication with any current, and transmitting the same signals and messages 88 those which were being forwarded through the agency of the induction-coils. As the poles of the induction-coil magnets were reversed, the poles of the relay-magnets were actuated different ways. Mr. Whitehouse made the instruments even more delicate by apply ing a second permanent magnet, so that it could be made by a screw adjustment to increase or diminish the attraction acting on the working magnet, either way. When the printing battery was brought into operation by the relay, it delivered its message by the agency of one of Professor Morse's printing instruments already described.
Professor W. Thomson, in commenting on the above arrangement (` Encyc. Brit.,' art. ' Electric Telegraph prefers the voltaic battery to any other source of electricity "for all great telegraphic work ;" and lie expresses his conviction—somewhat boldly, we think,—that "if no induction coils and no battery power, either positive or exceeding 20 cella of Daniell's negative, had ever been applied to the cable since the landing of its ends, imperfect as it then was, it would be now in full work day and night, with no prospect or probability of failure." Secondly, as to the line.—lo most parts of England the wires, as from the commencement of the system in this country, are supported on poles at a height of several feet from the ground ; but in a few cases, such as along the mail-coach road from London to Dover, a sub terranean arrangement has been adopted: the wires being encased in a wooden trough, and deposited a foot or two beneath the surface of the ground. This arrangement is also adopted in the streets of London,
and of other large towns. It has also been adopted by the English and Irish Magnetic Company, on a great extent of their lines; and it is that adopted in Prussia. The wires from the central station in London are insulated by being wrapped with cotton thread, and coated with a mixture of tar, resin, and grease, or gutta percha is used in preference. Nino such wires are packed in a half-inch leaden pipe, and four or five such pipes are packed in an iron pipe about 3 inches in diameter, which pipes are laid under the foot-pavements, and are thus conducted to the terminal stations of various railways. Testing-posts are placed at intervals along the streets, by which any failure can be detected and the locality of the defective wire ascertained at least within the dis tance of two posts. The London District Telegraph Company do not bury their N%irea, but are now engaged in weaving a metallic web over the tops of our houses.
The exposed conducting wires which run along tho side of a railway are of galvanised iron, about the sixth of an inch iu diameter. The higher price of copper prevents it from being employed, although this metal is ft much better conductor of electricity than iron. 1Then a great length of wire is to be stretched between two distant points, without immediate support, steel wire is sometimes used. The gal vanised iron wire, in the neighbourhood of large manufacturing towns, is liable to be attacked by the sulphur acids of the amok°, and the tine being converted into a soluble sulphate is washed off by the rain, and the iron wire becomes quickly corroded. We have already spoken of the eartheuware or glass insulators attached to the 'elide, for supporting the wire. An insulator should not only be a non conductor, but it should throw of the rain quickly and completely, otherwise the moisture will form a conductor to the earth ; indeed, the dripping of wet from one lice to another below it, will sometimes turn away the current from its work. Wet and foggy weather always has the effect of diminishing the current and requiring greater battery power; unless, indeed, the plan should be adopted of switching the line, that is, instead of sending a message along a direct line, where the wires are enveloped in rain or damp fog, or are otherwise "sick" as it is called, the line is switched on to another and more cir cuitous route, where the wires are in good working order. Spiders threads covered with dew sometimes divert the current, as does also atmospheric electricity. Indeed, a thunder-storm was formerly a source of danger to the telegraph clerks. Professor Loomis says that the telegraphic wires are very sensitive to an approaching storm, and often become highly charged, even when the storm is so distant that the thunder cannot be heard nor the lightning seen. Under such cir cumstances, if one stand in the room of a telegraphic station, and place one hand upon a telegraphic wire, and rest the other on the wire which communicates with the earth, a sharp shock is felt in the arms, and sometimes across the breast. This shock is very painful; although when the two wires are brought within striking distance of each other, only a faint spark is to be seen. But when the thunder-cloud is near, such experiments are dangerous. In such case, a thunder-cloud pass ing over the wires may charge them to such an extent that the electricity may fuse the thin wire of the 'electro-magnet, and render the magnet itself unserviceable. On some occasions an explosion takes place in the telegraph-room sufficient to fuse thick wires and to expose the clerks to considerable danger. A weak charge of atmospheric electricity has the same effect on the wires as the cur rent of a voltaic battery ; it makes a point in the telegraphic register. If, however, a storm pass over the wires, these points become numerous ; and as they show themselves between the points of a telegraphic message, they make the writing indistinct, and often illegible, so that on such occasions the clerks usually suspend their labours. Various contrivances have been made for drawing off electricity from the wires, without disturbing the current, advan tage being taken of the tension of the former, which gives it a striking distance not possessed by the latter. In addition to atmospheric electricity the line is liable to be disturbed by what are called " earth currents." If both ends of a long wire be connected with the earth, currents will pass through the line apparently in consequence of variations in the electrical condition of the earth in different places. 3tr. Varley has observed that these currents are continually flowing about the earth in one direction or another throughout the day, and reach a maximum about 2.40 P.M. During magnetic storms or the aurora borealis,currenta are sufficiently strong to interrupt the working of the lines : they flow sometimes in one direction, sometimes in another, and often change their direction in a few seconds.