# Encyclopædia Britannica, Ninth Edition/Telegraph

TELEGRAPH

TELEGRAPH (from τῆλε and γράφω) signifies an instrument to write at a distance. The term is specifically applied to apparatus for communicating intelligence to a distance in unwritten signs addressed to the eye or ear, and has only recently had application to those wonderful combinations of inanimate matter which literally write at a distance the intelligence committed to them. The chief object of the present article is to explain the principles and practice of the electric telegraph, and we shall allude to other telegraphic systems only to illustrate the general principles of signalling.

Signalling generally.A word expressing an idea may, according to a pre-arranged plan of signalling, be communicated by voice, by trumpet calls, by gun fire, by gesture or dumb signs, by lamp signals, by flags, by semaphore, or by electric telegraph. The simplest system of word-signalling hitherto practised is that of the nautical flag telegraph, in which each hoist represents a word by a combination of four flags in four distinct positions (see Signals, Naval). If n denote the number of flags, supposed all different, out of which the four to be sent up may be selected, the number of different ideas which can be expressed by a single hoist is n(n-1)(n-2)(n-3) since there are n varieties out of which the flag for each of the four positions may be independently chosen. To commit to memory so great a number of combinations, which amount to 358,800 if n=26, would be a vain effort; the operators on each side must therefore have constant recourse to a dictionary, or code, as it is called. For the sake of convenient reference each flag is called by the name of a letter of the alphabet, and all that the operator has to bear in mind is the letter by which each flag is designated. Sometimes the words to be expressed are spelled out by means of the flags as in ordinary language; but, as in most words there are more than four letters, as scarcely any two consecutive words are spelled with four or less than four letters, and as more than four flags at a time cannot be conveniently used, the system of alphabetic signalling frequently requires the use of two hoists for a word, and scarcely ever has the advantage of expressing two words by one hoist. It is therefore much more tedious than code signalling in the nautical telegraph.

In point of simplicity spoken words may be considered as almost on a par with the nautical telegraph, since each word is in reality spoken and heard almost as a single utterance. Next in order comes the system of spelling out words letter by letter, in which—instead of, as in the nautical telegraph, 358,800 single symbols to express the same number of ideas— 26 distinct symbols are used to express by their combinations any number whatever of distinct ideas. Next again to this may be ranked the system by which several distinct successive signals are used to express a letter, and letters thus communicated by compound signals are combined into words according to the ordinary method of language. It is to this last class that nearly all practical systems of electro-telegraphic signalling belong. But some of the earliest and latest proposals for electric telegraphs are founded on the idea of making a single signal represent a single letter of the alphabet; as instances we may name those early forms in which separate conductors were used for the different letters; a method suggested by Professor W. Thomson[1] in 1858 in which different strengths of current were to be employed to indicate the letters; and the various forms of printing telegraph now in use.

I. Historical Sketch of Early Telegraphs.

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 of three needles. Subsequently, in conjunction with Wheatstone, he introduced another form, in which five vertical index needles, each worked by a separate multiplier, were made to point out the letters on a dial. Two needles 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-needle 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 battery and reversing key, the motions to right and left of one end of the index corresponding to the dashes and dots of the Morse alphabet. To increase the speed of working, two single-needle instruments were sometimes used (double-needle telegraph). This system required two lines of wire, and, along with all multiple-wire systems, soon passed out of use. Similar instruments to the single and double needle ones of Cooke and Wheatstone were about the same time invented by the Rev. H. Highton and his brother Edward Highton, and were used for a considerable time on some of the railway lines in England. Another series of instruments, introduced by Cooke and Wheatstone in 1840, and generally known as “Wheatstone's step-by-step letter-showing” or “ABC instruments,” were worked out with great ingenuity of detail by Wheatstone in Great Britain and by Bréguet and others in France. They are still largely used for private wires, but are being rapidly displaced by the telephone.[5] 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 of Vermont, U.S., and was very successfully worked on some of the American telegraph lines till 1860, after which it was gradually displaced by the Phelps combination telegraph. The House instrument is not now in use, but various modifications of it are still employed for private lines and for stock telegraphs, such as Calahan's and the universal stock telegraphs, Phelps's stock printer, Gray's automatic printer for private lines, Siemens's and Phelps's automatic type printers, &c. (see infra, pp. 120-121).

II.General Description of Electric Telegraphs for Land and Sea.

Essential apparatus.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 for electricity to travel along, might be well applied to designate the insulated conductor along which the electric messenger is despatched. It is, how ever, 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 instrument, or the sending apparatus or instrument, or sometimes simply the transmitter or sender. The apparatus used at the other end of the line to render the effects of this action perceptible to any of the senses—eye, ear, or taste, all of which have been used in actual telegraphic signalling —is called the receiving apparatus or instrument.

Overground lines.In the aerial or overground system of land telegraphs the main line consists generally of a “galvanized” iron wire from one-sixth to a quarter of an inch in diameter, stretched through the air from pole to pole, at a sufficient height above the ground for security. The supports or insulators, as they are called, by which it is attached to the poles are of very different form and arrangement in different telegraphs, but consist essentially of a stem of glass, porcelain, coarse earthenware, or other non-conducting substance, protected by an overhanging screen or roof. One end of the stem is firmly attached to the pole, and the other bears the wire. The best idea of a single telegraphic insulator may be got from a common umbrella, with its stem of insulating substance attached upright to the top of a pole and bearing the wire supported in a notch on the top outside. The umbrella may be either of the same substance as the stem—all glass or all glazed earthenware, for instance—or of a stronger material, such as iron, with an insulating stem fitted to it to support it below. Very good insulators may be made of continuous glass; but well-glazed porcelain is more generally used, or rather earthenware, which is cheaper, less brittle, and less hygroscopic, and insulates well as long as the glazing is sufficient to prevent the porous substance within from absorbing moisture.

One of the best forms Varley's double cup insulator —is shown in fig. 1. It consists 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 (6) cemented into the centre of the inner cup is used for fixing the insulator to the pole or bracket.
Fig. 1.—Varley's Double Cup Insulator, one-fourth full size.
Underground lines. In the underground system the main line generally consists of a copper wire, or a thin strand of copper wires, covered with a continuous coating of gutta percha, india-rubber, or some equivalent insulating substance, served with tarred tape and enclosed in earthenware, iron, or lead pipes laid below the surface of the ground. This system is largely used for street and tunnel work, and to a considerable extent, especially in Germany, for ordinary lines. Each tube generally contains a number of wires, which are either laid up into a cable and covered with a serving of tarred tape or hemp before being drawn into the tube, or—as is more commonly the case in the United Kingdom—simply laid together in a parallel group and tied at intervals with binders, which are removed as the wires are drawn into the tube. On some long underground lines in Germany the insulated wires are laid up into a cable, served with jute or hemp, and sheathed with a continuous covering of iron wires, precisely similar to the submarine cables described below. The cable is laid in a deep trench and coated with bitumen. This form of cable is easily laid, and if properly manufactured is likely to be very durable.

Submarine cables.Submarine Cables.—A submarine cable (figs. 2-4), as usually manufactured at present, 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 400 lb to the mile.

Fig. 2.

 Fig. 3. Fig. 4.

Figs. 2-4—Sections of three types of submarine cables, full size. Fig. 2.—Type of shore end. Fig. 3.—Intermediate type. Fig. 4.—Deep sea type.

The stranded form was suggested by Prof. W. Thomson at a meeting of the Philosophical Society of Glasgow in 1854, because its greater flexibility renders it less likely to damage the insulating envelope the manipulation of the cable. The central conductor is covered with several continuous coatings of gutta percha, the total weight of which also varies between 70 and 400 lb to the mile. With a light core the weight of the gutta percha generally exceeds that of the copper, while in some heavy cores the copper is heavier. The different coatings of gutta percha and of the conductor are usually separated by a thin coating of Chatterton's compound (a mixture of gutta percha, resin, and Stockholm tar), in order to make them adhere firmly together. This practice has recently been departed from by Messrs Siemens Brothers, who have succeeded by an improved process of manufacture in getting perfect adhesion without the use of the compound. The core is served with a thick coating of wet jute, yarn, or hemp (h), forming a soft bed for the sheath, which consists of soft iron, or of homogeneous iron, wires of the best quality. The sheathing wires are usually covered with one or two servings of tarred canvas tape (t), or of tarred hemp, laid on alternately with coatings of a mixture of asphaltum and tar. The weight of the iron sheath varies greatly according to the depth of the water, the nature of the sea bottom, the prevalence of currents, and so on. Fig. 2 shows the intermediate type again sheathed with a heavy armour to resist wear in the shallow water near shore. In many cases a still heavier type is used for the first mile or two from shore, and several intermediate types are often introduced, tapering gradually to the thin deep-water type. Captain S. Trot and Mr F. A. Hamilton have proposed[6] to abandon the iron sheath and substitute a strong double serving of hemp, laid on in such a way as to prevent twisting when the cable is under tension. This suggestion, which is a revival with some modifications of an old idea, is, however, still in the experimental stage.

We will now describe very briefly a few of the most important processes in the manufacture and submergence of submarine cables.

Their manufacture.In manufacturing a cable (fig. 5) the copper strand is passed through a vessel A containing melted Chatterton's compound, then through the cylinder C, in which a quantity of gutta percha, purified by repeated washing in hot water, by mastication, and by filtering through wire-gauze filters, is kept warm by a steam-jacket.

Fig. 5.

As the wire is pulled through, a coating of gutta percha, the thickness of which is regulated by the die D, is pressed out of the cylinder by applying the requisite pressure to the piston P. The newly coated wire is passed through a long trough T, containing cold water, until it is sufficiently cold to allow it to be safely wound on a bob bin B. This operation completed, the wire is wound from the bobbin B on to another, and at the same time carefully examined for air-holes or other flaws, all of which are eliminated. The coated wire is treated in the same way as the copper strand, the die D, or another of the same size, being placed at the back of the cylinder and a larger one substituted at the front. A second coating is then laid on, and after it passes through a similar process of examination a third coating is applied, and so on until the requisite number is completed. The finished core changes rapidly in its electric qualities at first, and is generally kept for a stated interval of time before being subjected to the specified tests. It is then placed in a tank of water and kept at a certain fixed temperature, usually 75 Fahr., until it assumes approximately a constant electrical state. Its conductor and dielectric resistance and its electrostatic capacity are then measured. These tests are generally repeated at another temperature, say 50 Fahr., for the purpose of obtaining at the same time greater certainty of the soundness of the core and the rate of variation of the conductor and dielectric resistances with temperature. Should these tests prove satisfactory the core is served with jute yarn, coiled in water tight tanks, and surrounded with salt water. The insulation is again tested, and if no fault is discovered the served core is passed through the sheathing machine, and the iron sheath and the outer covering are laid on. As the cable is sheathed it is stored in large water-tight tanks and kept at a nearly uniform temperature by means of water.

Submersion.The cable is now transferred to a cable ship, provided with water-tight tanks similar to those used in the factory for storing it. The tanks are nearly cylindrical in form and have a truncated cone fixed in the centre, as shown at C, fig. 6. The cable is carefully coiled into the tanks in horizontal flakes, each of which is begun at the outside of the tank and coiled towards the centre.

Fig. 6.—Diagram of cable tank and paying-out apparatus of submarine cable.

The different coils are prevented from adhering by a coating of whitewash, and the end of each nautical mile is carefully marked for future reference. After the cable has been again subjected to the proper electrical tests and found to be in perfect condition, the ship is taken to the place where the shore end is to be landed. A sufficient length of cable to reach the shore or the cable-house is paid overboard and coiled on a raft or rafts, or on the deck of a steam-launch, in order to be connected with the shore. The end is taken into the testing room in the cable -house and the conductor connected with the testing instruments, and, should the electrical tests continue satisfactory, the ship is put on the proper course and steams slowly ahead, paying out the cable over her stern. The cable must not be over strained in the process of submersion, and must be paid out at the proper rate to give the requisite slack. This involves the introduction of machinery for measuring and controlling the speed at which it leaves the ship and for measuring the pull on the cable. The essential parts of this apparatus are shown in fig. 6. The lower end e of the cable in the tank T is taken to the testing room, so that continuous tests for electrical condition can be made. The upper end is passed over a guiding quadrant Q to a set of wheels or fixed quadrants 1, 2, 3,... then to the paying-out drum P, from it to the dynamometer D, and finally to the stern pulley, over which it passes into the sea. The wheels 1, 2, 3,... are so arranged that 2, 4, 6,... can be raised or lowered so as to give the cable less or more bend as it passes between them, while 1, 3, 5,... are furnished with brakes. The whole system provides the means of giving sufficient back-pull to the cable to make it grip the drum P, round which it passes several times to prevent slipping. On the same shaft with P is fixed a brake-wheel furnished with a powerful brake B, by the proper manipulation of which the speed of paying out is regulated, the pull on the cable being at the same time observed by means of D. The shaft of P can be readily put in gear with a powerful engine for the purpose of hauling back the cable should it be found necessary to do so. The length paid out and the rate of paying out are obtained approximately from the number of turns made by the drum P and its rate of turning. This is checked by the mile marks, the known position of the joints, &c., as they pass. The speed of the ship can be roughly estimated from the speed of the engines; it is more accurately obtained by one or other of the various forms of log, or it may be measured by paying out continuously a steel wire over a measuring wheel. The average speed is obtained very accurately from solar and stellar observations for the position of the ship. The difference between the speed of the ship and the rate of paying out gives the amount of slack. The amount of slack varies in different cases between three and ten per cent., but some is always allowed, so that the cable may easily adapt itself to inequalities of the bottom and may be more readily lifted for repairs. But the mere paying out of sufficient slack is not a guarantee that the cable will always lie closely along the bottom or be free from spans. Whilst it is being paid out the portion between the surface of the water and the bottom of the sea lies along a straight line, the component of the weight at right angles to its length being supported by the frictional resistance to sinking in the water. If, then, the speed of the ship be ${\displaystyle v}$, the rate of paying out ${\displaystyle u}$, the angle of immersion ${\displaystyle i}$, the depth of the water ${\displaystyle h}$, the weight per unit length of the cable ${\displaystyle w}$, the pull on the cable at the surface P, and A, B constants, we have—

${\displaystyle \mathrm {P} =h\{w-{\frac {\mathrm {A} }{\sin i}}f(u-v\cos i)\}............(\alpha )}$

and ${\displaystyle w\cos i=\mathrm {B} f(v\sin i).................(\beta )}$

where ${\displaystyle f}$ stands for “function” The factors ${\displaystyle \mathrm {A} f(u-v\cos i)}$ and ${\displaystyle \mathrm {B} f(v\sin i)}$ give the frictional resistance to sinking, per unit length of the cable, in the direction of the length and transverse to the length respectively. [7] It is evident from equation (${\displaystyle \beta }$) that the angle of immersion depends solely on the speed of the ship; hence in laying a cable on an irregular bottom it is of great importance that the speed should be sufficiently low. This may be illustrated very simply as follows: —suppose ${\displaystyle aa}$ (fig. 7) to be the surface of the sea, ${\displaystyle bc}$be the bottom, and ${\displaystyle cc}$ the straight line made by the cable; then, if a hill H, which is at any part steeper than the inclination of the cable, is passed over, the cable touches it at some point ${\displaystyle t}$t before it touches the part immediately below ${\displaystyle t}$t, and if the friction between the cable and the ground is sufficient the cable will either break or be left in a long span ready to break at some future time. It is important to observe that the risk is in no way obviated by the increasing slack paid out, except in so far as the amount of sliding which the strength of the cable is able to produce at the points of contact with the ground may be thereby increased. The speed of the ship must therefore be so regulated that the angle of immersion is as great as the inclination of the steepest slope passed over. Under ordinary circumstances the angle of immersion ${\displaystyle i}$ varies between six and nine degrees.[8]

Fig. 7.

Qualities of a Telegraph Line.—The efficiency of the telegraph depends on three qualities of the main line (1) its conducting quality, (2) its insulation, and (3) its electrostatic capacity.

Conducting qualities.1. The conducting quality of a wire or other elongated portion of matter is measured by the quantity of electricity which it allows to flow through it when a stated ”electromotive force,“ or “difference of electric potentials,” is maintained between its two ends. It may be most naturally, and is in point of fact generally, expressed in terms of resistance to transmission, regarded as a quality inverse to that of conducting power, and expressed numerically by the reciprocal of the measure of the conducting power. An independent explanation and definition of the electrical resistance of a conductor may be given as follows: the electrical resistance of a conductor is measured by the amount of electromotive force, or difference of potentials, which must be maintained between its ends to produce a stated strength of electric current through it.

Insulation.2. The true measure of the insulation of a body is the resistance to conduction offered by its supports. The reciprocal of this, or the conducting power of the supports, measures the defectiveness of the insulation. Since no substance yet known is absolutely a non-conductor of electricity, perfect insulation is impossible. If, however, the supports on which a telegraph wire rests present, on each part and on the whole, so great a resistance to electric conduction as to allow only a small portion of the electricity sent in, in the actual working, at one end to escape by lateral conduction, instead of passing through the line and producing effect at the other end, the insulation is as good as need be for the mode of working adopted. With the good insulation attained in a submarine line, round every part of which the gutta percha is free from flaws, no telegraphic operation completed within a second of time can be sensibly influenced by lateral conduction. A charge communicated to a wire thus insulated under water, at the temperature of the sea-bottom, is so well held that, after thirty minutes, not so much as half of it is found to have escaped. From this, according to the familiar “compound interest” problem, it appears that the loss must be at the rate of less than five per cent, per two minutes.

Electrostatic capacity.3. In 1849 Werner Siemens proved that “when a current is sent through a submerged cable a quantity of electricity is retained in charge along the whole surface, being distributed in proportion to the tension of each point,”; that is to say, to the difference of potentials between the conductor at any point and the earth beside it. In 1854 Faraday showed the effect of this “electrostatic charge” on signals sent through great lengths of submerged wire, bringing to light many remarkable phenomena and pointing out the “inductive” embarrassment to be expected in working long submarine telegraphs. In letters[9] to Professor Stokes in November and December of the same year, Prof. W. Thomson gave the mathematical theory of these phenomena, with formulæ and diagrams of curves, containing the elements of synthetical investigation for every possible case of practical operations. Some of the results of this theory are given at the end of the present article. The conductor of a submarine cable has a very large electrostatic capacity in comparison with that of a land telegraph wire in consequence of the induction, as of a Leyden phial, which takes place across its gutta percha coat, between it and its moist outer surface, which may be regarded as perfectly connected with the earth, that is to say, at the same potential as the earth. The mathematical expressions for the absolute electrostatic capacity C, per unit of length, in the two cases are as follows.

Submarine line.Let D = diameter of the inner conductor, supposed to be that of a circular cross section, or of a circle inappreciably less than one circumscribed about the strand which constitutes a modern submarine conductor; ${\displaystyle \mathrm {D} '}$ = outer diameter of the insulating coat; I = specific inductive capacity of the gutta percha or other substance constituting the insulating coat. Then

${\displaystyle \mathrm {C} ={\frac {\mathrm {I} }{2\log _{e}\mathrm {D} '/\mathrm {D} }}}$

Air line.In the case of a single wire of circular section, diameter D, undisturbed by the presence of others, and supported at a constant height h above the earth by poles so far apart as not to influence its capacity sensibly—

${\displaystyle \mathrm {C} ={\frac {\mathrm {I} }{2\log _{e}\mathrm {D} '/\mathrm {D} }}}$

Example 1. In a submarine cable in which D'= 1 centimètre; D =0·4 centimètre; and I = 3·2—

${\displaystyle \mathrm {C} ={\frac {\mathrm {I} }{2\log _{e}\mathrm {D} '/\mathrm {D} }}={\frac {3.2\times .4343}{2\times .3979}}=1.75}$

Example 2. In a land line in which D = 0·6 centimètre and h =600 centimètres—

${\displaystyle \mathrm {C} ={\frac {\mathrm {I} }{2\log _{e}4h/\mathrm {D} }}={\frac {.4343}{7.204}}={\frac {1}{16.6}}}$

The capacity, therefore, is in this case less than one-twenty-ninth of that of the submarine cable of example 1 for the same length.

Telegraph Testing.

Standards of Measurement. Standards of Measurement.—A brief consideration of the standards according to which the electrical qualities referred to in the last section are measured, and the measurements to be described in this section are made, will render the statements of those qualities and quantities more definite. A complete and universally comparable system of standards for physical measurements can be obtained by adopting arbitrarily as fundamental units those of length, mass, and time, and expressing in terms of these in a properly defined manner the units of all the other quantities. The units now adopted all over the world for electrical measurements take the centimètre as the unit of length, the gramme as the unit of mass, and the mean solar second as the unit of time. There are two systems in use, the electrostatic and the electromagnetic. In the former the mutual forces exerted by two bodies, each charged with static electricity, are taken as the starting-point, and in the latter the mutual forces exerted between a current of electricity and a magnet. The units according to these two systems are definitely related; but as we deal in the present article with the electromagnetic system we give the following brief account of it only.

Units. The dyne or unit force is that force which, acting on a gramme of matter, free to move, imparts to it a velocity of 1 centimètre per second. Unit quantity of magnetism or unit magnetic pole is that quantity of magnetism which, when placed at a distance of 1 centimètre from an equal and similar quantity of magnetism or a magnetic pole, repels it with unit force. Unit magnetic field is a field which, when a unit quantity of magnetism or a unit magnetic pole is placed in it, is acted on by unit force. Unit current is a current which, when made to flow round a circle of unit radius, produces a magnetic field of 2π units' intensity at the centre of the circle, or acts on a unit quantity of magnetism placed at the centre of the circle with 2π units of force. Unit quantity of electricity is the quantity conveyed by the unit current in one second. Unit difference of potential is the difference of potential between the ends of a conductor of unit length when it is placed with its length at right angles to the direction of force in a unit magnetic field and kept moving with a velocity of 1 centimètre per second in the direction at right angles to its own length and to the direction of the magnetic force. Unit electromotive force is produced in a closed circuit if any unit of its length is held in the manner, and moved in the direction and with the velocity, described in the last section. Unit resistance is the resistance which, when acted on by unit electromotive force, transmits unit current. Unit capacity is the capacity of a body which requires unit quantity of electricity to raise its potential by unity. The units above specified are generally referred to as the absolute C.G.S. electromagnetic units of the different quantities. In practice their magnitudes were found inconvenient, and certain multiples and submultiples of them have been adopted as the practical units of measurement: thus the ohm is equal to 109 C.G.S. units of resistance; the volt is equal to 108 C.G.S. units of electromotive force; the ampere is equal to 10-1 C.G.S. units of current; the coulomb is equal to 10-1 C.G.S. units of quantity; the farad is the capacity which is charged to a volt by a coulomb, and is equal to 10-9 C.G.S. units of capacity; the microfarad is the millionth part of the farad, and is equal to 10-15 C.G.S. units of capacity.

We are here chiefly concerned with the units of electromotive force, resistance, and capacity. No universally recognized standard of electromotive force has yet been established, but the want has been to a great extent supplied by the potential galvanometers, electrostatic voltmeters, standard cells, and other instruments devised by Sir W. Thomson and others. The work of Lord Rayleigh, Dr. Fleming, and other experimenters on the Clark and Daniell standard cells has shown conclusively that an electromotive force can be reproduced with certainty within one -tenth per cent. of accuracy by means of either of these cells. Specimens of the standard unit of resistance, or ohm, made of an alloy of platinum and silver, or of platinum and iridium, have been constructed, and can be relied on, if properly taken care of, to remain very nearly accurate from year to year. Similar specimens of the standard unit of capacity or microfarad which remain very nearly constant have been successfully produced. For a fuller treatment of this subject and of the methods of determining the different units, see Electricity, vol. viii. p. 40 sq. [10]

Telegraph line testing consists mostly of comparisons of the resistance of the conductor and the insulator with sets of standard resistances, and of comparisons of the inductive capacity of the line or cable with standard condensers of known capacity. When, as is sometimes the case, the strength of the current flowing through the line or through a particular instrument is to be determined, it is measured by an electrodynamometer, or by a current galvanometer, properly constructed for indicating currents in absolute measure. In the absence of such an instrument it may be obtained accurately by the use of a standard galvanometer in a known or determined magnetic field, or, taking advantage of Faraday's discovery of the electro-chemical equivalents, by measuring the amount of silver or of copper deposited by the current when it is made to pass through an electrolytic cell; or the electromotive force per unit resistance of the circuit may be determined by the use of standard resistances and a standard cell. Space does not allow us to do more than simply refer to these methods, the first two at least of which involve accurate and somewhat difficult experimental work. [11]

Measurement of wire resistance by Wheatstone's bridge.Measurement of Wire Resistance.—(1) By Wheatstone's bridge. [12]Let l (fig. 8) be the line with its distant end connected with the earth, a and b known resistances, x a resistance which can be varied, G a galvanometer, K a single lever key, K1 a reversing key, and B a battery.

Put the zinc pole of the battery to the line and adjust the resistance x until the galvanometer G shows no deflexion when K1 is depressed. We then have, assuming no electromotive force in the line, l = ax1/b. Next put the copper pole to the line and repeat the test, and suppose in this case l = ax2/b; if these two values of l nearly agree the true value may be taken as 2ax1x2/b(x1+x2. The effect of an electromotive force in the line itself is nearly eliminated by reversing the battery.

1. From correspondence found among Sir David Brewster's papers after his death it seems highly probable 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.
2. See Arthur Young, Travels in France, p. 3.]
3. The reader interested in the early history of the electric telegraph may consult Edward Highton, The Electric Telegraph, London, 1852; Moigno, Traité de Télégraphie Électrique, Paris, 1849; and Sabine, History of the Electric Telegraph, London, 1869.
4. For the different forms, see Prescott's Electricity and the Electric Telegraph, pp. 562-602.
5. Journ. Soc. Telegr. Eng., vol. xii. p. 495.
6. See Sir W. Thomson, Mathematical and Physical Papers, vol. ii. p. 165.
7. For details of cable manufacture and laying consult Douglas's Telegraph Construction, London, 1877, and Captain V. Hoskiær's Laying and Repairing of Electric Telegraph Cables, London, 1878.
8. Published in Proc. Roy. Soc. for 1855.
9. For the development of this important part of electrical science, see Weber, "Messungen galvanischer Leitungswiderstände nach einein absoluten Maasse," in Poggendorff's Annalen, March 1851; Thomson, "Mechanical Theory of Electrolysis," "Application of the Principle of Mechanical Effect to the Measurement of Electromotive Force, and of Galvanic Resistances in Absolute Units," and "Transient Electric Currents," in Phil. Mag., 1851 and 1852; Weber, Electrodynamische Maassbestimmungen, insbesondere Zurückführung der Stromintensitatsmessungen aufmechanischen Maass, Leipsic, 1856; Thomson, "On the Electric Conductivity of Commercial Copper," "Synthetical and Analytical Attempts" on the same subject, and "Measurement of the Electrostatic Force between the Poles of a Daniell's Battery, and Measurement of the Electrostatic Force required to produce a Spark in Air," Proc. Roy. Soc., 1857 and 1860; reprint of Reports of Brit. Assoc. Committee on Electr. Stand., &c., edited by Prof. F. Jenkin; Thomson, Electrical Units of Measurement, a lecture delivered at Institution of Civil Engineers, 1883; Reports of the International Conference for the Determination of the Electrical Units, held at Paris in 1882 and 1884; A. Gray, Absolute Measurements in Electricity and Magnetism, London, 1884.
10. See A. Gray, Absolute Measurements in Electricity and Magnetism, pp. 27, 74; also T. Gray, Phil. Mag., November 1886. The following quotation from the art. Telegraph in the 8th ed. of the Ency. Brit, shows how comparatively recent is the introduction of anything like absolute measurement in telegraph testing: —"The ordinary test for insulation consists in applying a galvanic battery, with one pole to earth and the other through a galvanometer coil, to the line of telegraph of which the remote end is kept insulated. If the insulation of the whole line were perfect, the galvanometer needle would stand at zero; but, when looked for with a battery of suitable power and a galvanometer of suitable sensibility, indications of a current are always found, unless it is a very short length of very perfectly insulated line that is tested. The absolute measure of the strength of this current divided by the absolute measure of the electromotive force of the battery gives an absolute measure for the insulation of the cable. No telegraphic testing ought in future to be accepted in any department of telegraphic business which has not this definite character, although it is only within the last year that convenient instruments for working in absolute measure have been introduced at all, and the whole system of absolute measurement is still almost unknown to practical electricians." It was put in practice systematically for the first time in 1859, in experiments by Prof. F. Jenkin.
11. For this theory, see Electricity, vol. viii. p. 44.