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Encyclopedia Britannica - Main :: PAI-PAS |
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PART I .LAND AND SUBMARINE TELEGRAPHY Historical Sketch.Although the history of practical electric telegraphy does not date much further back than the middle of the igth century, the idea of using electricity for telegraphic purposes is much older. It was suggested again and again as each new discovery in electricity and magnetism seemed to render it more feasible. Thus the discovery of Stephen Gray and of Granville Wheeler that the electrical influence of a charged Leyden jar may be conveyed to a distance by means of an insulated wire gave rise to various proposals, of which perhaps the earliest was that in an anonymous letter' to the Scots Magazine (vol. xv. p. 73, 1753), in which the use of as many insulated conductors as there are letters in the alphabet was suggested. Each wire was to be used for the transmission of one letter only, and the message was to be sent by charging the proper wires in succession, and received by observing the' From correspondence found among Sir David Brewster 's papers after his death it seems highly prcbable that the writer of this letter, which was signed " C. M.," was Charles Morrison, a surgeon and a native of Greenock, but at that time resident in Renfrew
movements of small pieces of paper marked with the letters of the alphabet and placed under the ends of the wires. A very interesting modification was also proposed in the same letter, viz. to attach to the end of each wire a small light ball which when charged would be attracted towards an adjacent bell and strike it. Some twenty years later G. L. Le Sage proposed a similar method, in which each conductor was to be attached to a pith ball electroscope. An important advance on this was proposed in 1797 by Lomond,' who used only one line of wire and an alphabet of motions. Besides these we have in the same period the spark telegraph of Reiser, of Don Silva, and of Cavallo, the pith ball telegraph of Francis Ronalds (a model of which is in the collection of telegraph apparatus in the Victoria and Albert Museum), and several others.Next came the discovery of Galvani and of Volta, and as a consequence a fresh set of proposals, in which voltaic electricity was to be used. The discovery by Nicholson and Carlisle of the decomposition of water, and the subsequent researches of Sir H. Davy on the decomposition of the solutions of salts by the voltaic current were turned to account in the water voltameter telegraph of Summering and the modification of it pro-posed by Schweigger, and in a similar method proposed by Coxe, in which a solution of salts was substituted for water. Then came the discovery by G. C. Romagnosi and by H. C. Oersted, of the action of the galvanic current on a magnet. The application of this to telegraphic purposes was suggested by Laplace and taken up by Ampere, and afterwards by Triboaillet and by Schilling, whose work forms the foundation of much of modern telegraphy. Faraday's discovery of the induced current produced by passing a magnet through a helix of wire forming part of a closed circuit was laid hold of in the telegraph of Gauss and Weber, and this application was at the request of Gauss taken up by Steinheil, who brought it to considerable perfection. Steinheil communicated to the Gottingen Academy of Sciences in September 1838 an account of his telegraph, which had been constructed about the middle of the preceding year. The currents were produced by a magneto-electric machine resembling that of Clarke. The receiving apparatus consisted of a multiplier, in the centre of which were pivoted one or two magnetic needles, which either indicated the message by the movement
paper .Steinheil appears to have been anticipated in the matter of a recording telegraph by Morse of America, who in 1835 constructed a rude working model of an instrument; this within a few years was so perfected that with some modification in detail it has been largely used ever since (see below). In 1836 Cooke, to whom the idea appears to have been suggested by Schilling's method, invented a .telegraph in which an alphabet was worked out by the single and combined movement
' See Arthur Young, Travels in France, p. 3. were used for a considerable time on some of the railway lines rn England. Another series of instruments, introduced by Cooke and Wheatstone in 1840, and generally known as " Wheat-stone's step-by-step letter-showing " or " ABC instruments," were worked out with great ingenuity of detail by Wheatstone in Great Britain and by Breguet and others in France. The Wheatstone instrument in the form devised by Stroh is still largely used in the British Postal Telegraph Department. Wheatstone also described and to some extent worked out an interesting modification of his step-by-step instrument, the object of which was to produce a letter-printing telegraph. But it never came into use; some years later, however, an instrument embodying the same principle, although differing greatly in mechanical detail, was brought into use by Royal E. House
Construction of Telegraph Circuits.The first requisite for electro-telegraphic communication between two localities is an insulated conductor extending from one to the other. This, with proper apparatus for originating electric currents at one end and for discovering the effects produced by them at the other end, constitutes an electric telegraph. Faraday's term " electrode," literally " a way (ii os) for electricity to travel along," might be well applied to designate the insulated conductor along which the electric messenger is despatched. It is, however, more commonly and familiarly called " the wire " or " the line." The apparatus for generating the electric action at one end is commonly called the transmitting apparatus or initrument, or the sending apparatus or instrument, or some-times simply the transmitter or sender. The apparatus used at the other end of the line to render the effects of this action perceptible to the eye or ear, is called the receiving apparatus or instrument. In the aerial or overground system of land telegraphs the use of copper wire has become very general. The advantage of the high conducting power which copper possesses Over-is of especial value in moist climates (like that of ground the United Kingdom), since the effect of leakage over lines. the surface of the damp
In the British Postal Telegraph system five sizes of iron wire are in general use, weighing respectively 200, 400, 450, 600 and 8uo lb per statute mile, and having electrical resistances (at 6o F.) of 26-64, 13.32, 11.84, 8-88 and 6.66 standard ohms per statute mile respectively. The sizes of copper wire employed have weights of too, 150, 200 and 400 lb' per statute mile, and have electrical resistances (at 6o F.) of 8.782, 5.855, 4.391 and 2.195 standard ohms respectively. Copper wire weighing 60o and 800 lb per mile has also been used to some extent. The copper is " hard drawn
The lines are carried on poles, at a sufficient height above the ground, by means of insulators. These vary in form, but essentially they consist of a stem of porcelain, coarse earthen-ware, glass or other non-conducting substance, protected by an overhanging roof or screen. The form in general use on the British postal lines is the " Cordeaux screw," but the " Varley double cup " is still employed, especially by the railway companies. The latter form consists (fig. 1) of two distinct cups (c, C), which are moulded and fired separately, and afterwards cemented together. The double cup gives great security against loss of insulation due to cracks extending through the insulator, and also gives a high surface insulation. An iron bolt (b) cemented into the centre of the inner cup is used for fixing the insulator to the pole or bracket. This form of insulator is still largely used and is a very serviceable pattern, though possessing the defect that the porcelain cup is not removable from the iron bolt on which it is mounted. The Cordeaux insulator (fig. 2) is made in one piece. A coarse screw-thread is formed in the upper part of the inner cup, and this screws on to the end of the iron bolt by which it is supported. Between a shoulder, a, in the iron bolt and a shoulder in the porcelain cup, c, is placed an indiarubber ring, which forms a yielding washer and enables the cup to be screwed firmly to the bolt, while preventing the abrasion of the porcelain against the iron. The advantage of the arrangement is that the cup can at any time be readily removed from the bolt. At the termination of a line a large insulator (fig. 3), mounted on a strong steel bolt having a broad base flange, is employed. Connexion is made into the office (or to the under-ground system, as is often the case) from the aerial wire by means of a copper conductor, insulated with gutta-percha, which passes through a " leading in ' cup, whereby leakage is prevented between the wire and the pole. The insulators are planted on creosoted oak arms, 22 in. sq. and varying in length from 24 to 48 ins., the 24 and 33 in. arms taking two, and the 48 in. four, insulators. The unequal lengths of the 24 and 33 in. arms are adopted for the purpose of allowing one wire to fall clear of that beneath it, in the case of an insulator breaking or the securing binder giving way. The poles are of red fir, creosoted, this method of preservation being the only one now used for this purpose in the United Kingdom. The number of poles varies from about 15 to 22 per m. of line; they are planted to a depth of from 2 to 4 ft. in the ground. For protection from lightning each pole has an " earth wire " running from the top, down to the base. Gutta-percha-covered copper wires were formerly largely used for the purpose of underground lines, the copper conductor weighing 40 lb per statute mile, and the gutta-percha covering 50 lb (90 lb total). The introduction of paper cables, i.e. copper wires insulated with carefully dried paper of a special quality, has practically entirely super- seded the use of wires insulated with gutta-percha. The paper cables consist of a number of wires, each enveloped in a loose covering of well-dried paper, and loosely laid up together with a slight spiral " lay " in a bundle, the whole being enclosed in a stout lead pipe. It is essential that the paper covering be loose, so as to ensure that each wire is enclosed in a coating not of paper only, but also of air; the wires in fact are really insulated from each other by the dry air, the loose paper acting merely as a separator to prevent them from coming into con-tact. The great advantage of this air insulation is that the electrostatic capacity of the wires is low (about one-third of that which would be obtained with gutta-percha insulation), which is of the utmost importance for high-speed working or for long-distance telephonic communication. As many as 1200 wires are sometimes enclosed in one lead pipe. Between London and Birmingham a paper cable 116 m long and consisting of 72 copper conductors, each weighing 15o lb per statute mile, was laid in 1900. The conductors are enclosed in a lead pipe, 24 in. in outside diameter and ; in. thick, which itself is enclosed in cast iron spigot-ended pipes, 3 in. in internal diameter, and buried 2 ft. below the surface of the roadway. At intervals of 2 M. " test pillars " are placed for the purpose of enabling possible faults to be accurately located. Each conductor has a resistance (at 60 F.) of 5.74 ohms per statute mile, and an average electrostatic capacity per mile between adjacent wires of oo6 microfarad, or between wire and earth of o.1 microfarad; the insulation resistance of each wire is about 5000 megohms per mile. The under-ground system of paper cables has been very largely extended. Cables between London, Glasgow, Edinburgh, Liverpool Leeds, Bristol, Exeter and other important towns have been laid, and eventually telegraphic communication between every important town in the United Kingdom will be rendered safe from interruptions caused by gales or snowstorms. The one disadvantage of paper cables is the fact that any injury to the lead covering which allows moisture to penetrate causes telegraphic interruption to the whole of the enclosed wires, whereas if the wires are each individually coated with gutta-percha, the presence of moisture can only affect those wires whose covering is defective There is no reason for doubting, however, that, provided the lead covering remains intact, the paper insulation is imperishable; this is not the case with gutta-percha-covered wires. In order to maintain a system of telegraph lines in good working condition, daily tests are essential. In the British Postal Telegraph Department all the most important Testing; wires are tested every morning between 7.3o and 7.45 A.M., in sections of about 200 miles. The method adopted consists in looping the wires in pairs between two testing offices, A and B (fig. 4); a current is sent from a battery, E, through one coil of a galvanometer, g, through a high resistance, r, through one of the wires, 1, and thence back from office B (at which the wires are looped), through wire 2, through another high resistance, r', through a second coil on the galvanometer, g, and thence to earth. If the looped lines are both in good condition and free from leakage, the current sent out on line i will be exactly equal to the current received back on line 2; and as these currents will have equal but opposite effects on the galvanometer needle, no deflection of the latter will be produced. If, however, there is leakage, the current received on the galvanometer will be less than the current sent out, and the result will be a deflection of the needle proportional to the amount of leakage. The galvanometer being so adjusted that a current of definite strength through one of the coils gives a definite deflection of the needle, the amount of leakage expressed in terms of the insulation resistance of the wires is given by the formula Total insulation resistance of looped lines=I-R(1)/d z); in which R is the total resistance of the looped wires, including the resistance of the two coils of the galvanometer, of the battery, and of the two resistance coils r and r' (inserted for the purpose of causing the leakage on the lines to have a maximum effect on the galvanometer deflections). In practice the resistances r, r' are Under-ground lines. J of 10,000 ohms each. The deflection observed on the galvanometer when the line's are leaky is d, while D is the deflection obtained through one coil of the galvanometer with all the other resistances in circuit; anti assuming that no leakage exists on the lines, this deflection is calculated from the " constant " of the instrument, i.e., from the known deflection obtained with a definite current. For the purpose of avoiding calculation, tables are provided showing the values of the total insulation according to the formula, corresponding to various values of d. If the insulation per mile, i.e.; the total insulation multiplied by the mileage of the wire loop, is found to be less-than 200,000 ohms, the wire is considered to be faulty. The climatic conditions in the British Islands are such that it is not possible to maintain, in unfavourable weather, a higher standard than that named, which is the insulation obtained when all the insulators are in perfect condition and only the normal leakage, due to moisture, is present. There are three kinds of primary batteries in general use in the British Postal Telegraph Department, viz., the Daniell, Batteries. the bichromate, and the Leclanche. The Daniell type consists of a teak trough divided into five cells by slate partitions coated with marine glue. Each cell contains a zinc plate, immersed in a solution of zinc sulphate, and also a porous chamber containing crystals of copper sulphate and a copper plate. The electromotive force of each cell is 1.07 volts and the resistance 3 ohms. The Fuller bichromate battery consists of an outer jar containing a solution of bichromate of potash and sulphuric acid, in which a plate of hard carbon is immersed; in the jar there is also a porous pot containing dilute sulphuric acid and a small quantity (2 oz.) of mercury, in which stands a stout zinc rod. The electromotive force of each cell is 2I4 volts, and the resistance 4 ohms. The Leclanche is of the ordinary type, and each cell has an electromotive force of 1.64 volts and a resistance of 3 to 5 ohms (according to the size of the complete cell, of which there are three sizes in use). Dry cells, i.e. cells containing no free liquid, but a chemical paste, are also largely employed; they have the advantage of great portability. Primary batteries have, in the case of all large offices, been displaced by accumulators. The force of the set of accumu-Accumu- lator cells provided is such as to give sufficient power /attars. for the longest circuit to be worked, the shorter circuits being brought up approximately to a level, as regards resistance, by the insertion of resistance coils in the circuit of the transmitting apparatus of each shorter line. A spare set of accumulators is provided for every group of instruments in case of the failure of the working set. For working " double current," two sets of accumulators are provided, one set to send the positive and the other set the negative currents; that is to say, when, for example, a double current Morse key is pressed down it sends, say, a positive current from one set, but when it is allowed to rise to its normal position then a negative current is transmitted from the second set of accumulators. It is not possible to work double current from one set alone, as in this case, if one key of a group of instruments is up and another is down, the battery would be short-circuited and no current would flow to line. The size of the accumulators employed varies from a cell capable of an output of 8 ampere-hours, to a size giving 7 50 ampere-hours. Submarine Cables.A submarine cable (figs. 5-7), as usually manufactured, consists of a core a in the centre of which is a strand of copper wires varying in weight for different cables between 70 and 65o lb to the nautical mile. The stranded form was suggested by W. Thomson (Lord Kelvin) at a meeting of the Philosophical Society of Glasgow in 1854, because its greater flexibility renders it less likely to damage the insulating envelope during the manipulation of the cable. The central conductor is covered with several continuous coatings of gutta-percha, the total weight of which varies between 70 and 65o lb to the mile. Theoretically for a given outside diameter of core the greatest speed of signalling through a cable is obtained when the diameter of the conductor is 606 (I/\i) the diameter of the core, but this ratio makes the thickness of the gutta-percha covering insufficient for mechanical strength. Owing to the high price of gutta-percha the tendency, of recent
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