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PART I

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Originally appearing in Volume V26, Page 513 of the 1911 Encyclopedia Britannica.
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PART I .—See also:LAND AND SUBMARINE TELEGRAPHY See also:Historical See also:Sketch.—Although the See also:history of See also:practical electric telegraphy does not date much further back than the See also:middle of the igth See also:century, the See also:idea of using See also:electricity for telegraphic purposes is much older. It was suggested again and again as each new See also:discovery in electricity and See also:magnetism seemed to render it more feasible. Thus the discovery of See also:Stephen See also:Gray and of See also:Granville See also:Wheeler that the See also:electrical See also:influence of a charged See also:Leyden See also:jar may be conveyed to a distance by means of an insulated See also:wire gave rise to various proposals, of which perhaps the earliest was that in an See also:anonymous See also:letter' to the Scots See also:Magazine (vol. xv. p. 73, 1753), in which the use of as many insulated conductors as there are letters in the See also:alphabet was suggested. Each wire was to be used for the transmission of one letter only, and the See also:message was to be sent by charging the proper wires in See also:succession, and received by observing the ' From See also:correspondence found among See also:Sir See also:David See also:Brewster's papers after his See also:death it seems highly prcbable that the writer of this letter, which was signed " C. M.," was See also:Charles See also:Morrison, a surgeon and a native of See also:Greenock, but at that See also:time See also:resident in See also:Renfrew. movements of small pieces of See also: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 See also:light See also:ball which when charged would be attracted towards an adjacent See also:bell and strike it. Some twenty years later G. L. Le See also:Sage proposed a similar method, in which each conductor was to be attached to a See also:pith ball See also:electroscope. An important advance on this was proposed in 1797 by See also:Lomond,' who used only one See also:line of wire and an alphabet of motions.

Besides these we have in the same See also:

period the spark See also:telegraph of Reiser, of See also:Don See also:Silva, and of See also:Cavallo, the pith ball telegraph of See also:Francis Ronalds (a See also:model of which is in the collection of telegraph apparatus in the See also:Victoria and See also:Albert Museum), and several others. Next came the discovery of See also:Galvani and of See also:Volta, and as a consequence a fresh set of proposals, in which voltaic electricity was to be used. The discovery by See also:Nicholson and See also:Carlisle of the decomposition of See also:water, and the subsequent researches of Sir H. See also:Davy on the decomposition of the solutions of salts by the voltaic current were turned to See also:account in the water voltameter telegraph of Summering and the modification of it See also:pro-posed by Schweigger, and in a similar method proposed by See also:Coxe, in which a See also:solution of salts was substituted for water. Then came the discovery by G. C. Romagnosi and by H. C. Oersted, of the See also:action of the galvanic current on a magnet. The application of this to telegraphic purposes was suggested by See also:Laplace and taken up by See also:Ampere, and afterwards by Triboaillet and by Schilling, whose See also:work forms the See also:foundation of much of See also:modern telegraphy. See also:Faraday's discovery of the induced current produced by passing a magnet through a See also:helix of wire forming part of a closed See also:circuit was laid hold of in the telegraph of See also:Gauss and See also:Weber, and this application was at the See also:request of Gauss taken up by Steinheil, who brought it to considerable perfection. Steinheil communicated to the See also:Gottingen See also:Academy of Sciences in See also:September 1838 an account of his telegraph, which had been constructed about the middle of the preceding See also:year.

The currents were produced by a magneto-electric See also:

machine resembling that of See also: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 See also:movement of an See also:index or by striking two bells of different See also:tone, or recorded it by making See also:ink dots on a ribbon of paper. Steinheil appears to have been anticipated in the See also:matter of a recording telegraph by See also:Morse of See also:America, who in 1835 constructed a See also:rude working model of an See also: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 See also: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 of three needles. Subsequently, in See also:conjunction with See also:Wheatstone, he introduced another See also:form, in which five See also:vertical index needles, each worked by a See also:separate multiplier, were made to point out the letters on a See also:dial. Two needles (for some letters, one only) were acted upon at the same time, and the letter at the point of intersection of the direction of the indexes was read. This telegraph required six wires, and was shortly afterwards displaced by the single-See also:needle See also:system, still to a large extent used on railway and other less important circuits. The single-needle instrument is a vertical needle galvanoscope worked by a See also:battery and See also:reversing handle, or two " tapper " keys, the motions to right and See also:left of one end of the index corresponding to the dashes and dots of the Morse alphabet. To increase the See also:speed of working, two single-needle See also:instruments were sometimes used (See also:double-needle telegraph). This system required two line wires, and, although a remarkably serviceable apparatus and in use for many years, is no longer employed. Similar instruments to the single and double needle apparatus of Cooke and Wheatstone were about the same time invented by the Rev. H.

Highton and his See also:

brother See also:Edward Highton, and ' See See also:Arthur See also:Young, Travels in See also:France, p. 3. were used for a considerable time on some of the railway lines rn See also:England. Another See also:series of instruments, introduced by Cooke and Wheatstone in 1840, and generally known as " See also:Wheat-See also:stone's step-by-step letter-showing " or " See also:ABC instruments," were worked out with See also:great ingenuity of detail by Wheatstone in Great See also:Britain and by Breguet and others in France. The Wheatstone instrument in the form devised by Stroh is still largely used in the See also:British Postal Telegraph See also:Department. Wheatstone also described and to some extent worked out an interesting modification of his step-by-step instrument, the See also:object of which was to produce a letter-See also:printing telegraph. But it never came into use; some years later•, however, an instrument embodying the same principle, although differing greatly in See also:mechanical detail, was brought into use by Royal E. See also:House, of See also:Vermont, U.S., and was very successfully worked on some of the See also:American telegraph lines till 186o, after which it was gradually displaced by other forms. Various modifications of the instrument are still employed for stock telegraph purposes. 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 See also: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 See also:

eye or See also:ear, is called the receiving apparatus or instrument. In the aerial or overground system of land telegraphs the use of See also:copper wire has become very See also:general. The See also:advantage of the high conducting See also:power which copper possesses Over-is of especial value in moist climates (like that of ground the See also:United See also:Kingdom), since the effect of leakage over lines. the See also:surface of the See also:damp insulators is much less See also:notice-able when the conducting power of the wire is high than when it is See also:low, especially when the line is a See also:long one. Copper is not yet universally employed, See also:price being the governing See also:factor in its employment; moreover, the conducting quality of the See also:iron used for telegraphic purposes has of See also:late years been very greatly improved. 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 See also:statute mile, and having electrical resistances (at 6o° F.) of 26-64, 13.32, 11.84, 8-88 and 6.66 See also: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 See also:drawn," and has a breaking See also:strain as high as 28 tons per sq. in.; the test strain required for the iron wire is about 222 tons. The particular sizes and descriptions of wires used are dependent upon the See also:character of the " circuits " the longer and more important circuits requiring the heavier wire. 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 See also:stem of See also:porcelain, coarse earthen-See also:ware, See also:glass or other non-conducting substance, protected by an overhanging roof or See also:screen.

The form in general use on the British postal lines is the " Cordeaux See also:

screw," but the " See also:Varley double See also: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 See also:security against loss of insulation due to cracks extending through the insulator, and also gives a high surface insulation. An iron See also:bolt (b) cemented into the centre of the inner cup is used for fixing the insulator to the See also:pole or See also:bracket. This form of insulator is still largely used and is a very serviceable See also: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-See also: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 See also:shoulder, a, in the iron bolt and a shoulder in the porcelain cup, c, is placed an indiarubber See also:ring, which forms a yielding washer and enables the cup to be screwed firmly to the bolt, while preventing the See also: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 See also:

steel bolt having a broad See also:base flange, is employed. Connexion is made into the See also:office (or to the under-ground system, as is often the See also:case) from the aerial wire by means of a copper conductor, insulated with See also: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 See also: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 See also: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 See also:depth of from 2 to 4 ft. in the ground. For See also:protection from See also:lightning each pole has an " See also:earth wire " See also:running from the See also: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 See also:total). The introduction of paper cables, i.e. copper wires insulated with carefully dried paper of a See also: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 See also:spiral " See also:lay " in a bundle, the whole being enclosed in a stout See also:lead See also: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 See also: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 See also: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 See also:

London and See also:Birmingham a paper See also: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 See also:diameter and ; in. thick, which itself is enclosed in See also:cast iron spigot-ended pipes, 3 in. in See also: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 See also:average electrostatic capacity per mile between adjacent wires of o•o6 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, See also:Glasgow, See also:Edinburgh, See also:Liverpool See also:Leeds, See also:Bristol, See also:Exeter and other important towns have been laid, and eventually telegraphic communication between every important See also:town in the United Kingdom will be rendered safe from interruptions caused by See also: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 See also: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 See also:order to maintain a system of telegraph lines in See also:good working See also:condition, daily tests are essential. In the British Postal Telegraph Department all the most important Testing; wires are tested every See also:morning between 7.3o and 7.45 A.M., in sections of about 200 See also: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 See also:

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 See also: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 See also: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; See also:anti assuming that no leakage exists on the lines, this deflection is calculated from the " See also: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 See also:loop, is found to be less-than 200,000 ohms, the wire is considered to be faulty. The See also:climatic conditions in the British Islands are such that it is not possible to maintain, in unfavourable See also: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 See also:present. There are three kinds of See also:primary batteries in general use in the British Postal Telegraph Department, viz., the See also:Daniell, Batteries. the bichromate, and the Leclanche. The Daniell type consists of a See also:teak trough divided into five cells by See also:slate partitions coated with marine See also:glue.

Each See also:

cell contains a See also:zinc See also: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 See also:Fuller bichromate battery consists of an See also:outer jar containing a solution of bichromate of potash and sulphuric See also:acid, in which a plate of hard See also:carbon is immersed; in the jar there is also a porous pot containing dilute sulphuric acid and a small quantity (2 oz.) of See also:mercury, in which stands a stout zinc See also:rod. The electromotive force of each cell is 2•I4 volts, and the resistance 4 ohms. The Leclanche is of the See also:ordinary type, and each cell has an electromotive force of 1.64 volts and a resistance of 3 to 5 ohms (according to the See also:size of the See also:complete cell, of which there are three sizes in use). Dry cells, i.e. cells containing no free liquid, but a chemical See also: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 See also: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 See also:positive and the other set the negative currents; that is to say, when, for example, a double current Morse See also: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 See also: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-See also:hours, to a size giving 7 50 ampere-hours.

Submarine Cables.—A submarine cable (See also:

figs. 5-7), as usually manufactured, consists of a core a in the centre of which is a strand of copper wires varying in See also:weight for different cables between 70 and 65o lb to the nautical mile. The stranded form was suggested by W. See also:Thomson (See also:Lord See also:Kelvin) at a See also: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.

End of Article: PART I

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