II. TELEPHONIC INSTRUMENTS (cont.)
The lines used for telephone purpose are, generally speaking so far as erection, mode of insulation, and so on are concerned, much the same as those used or ordinary telegraphs. In towns where a very large number of wires radiate from one centre or exchange as it is called, where thick wires are unsightly, and where it is often necessary to provide for long spans, a comparatively thin wire of strong material is employed. For this reason various bronzes, such as silicon, aluminium, &c., have come to be extensively used for making wires for telephone lines. They are made from about the twentieth to the thirtieth of an inch in diameter, and are found to wear well in the somewhat mixed atmosphere of town ; and owing to their lightness and considerable tensile strength it is comparatively easy to erect them and keep them in order. The main objection to them is the high electrical resistance they oppose to the current. The lines on a town exchange system are not, however, as a rule, so long as to make this objection of great importance. But long lines, such as those extending between towns some miles apart, should be made of pure copper wire hard drawn. It has lately been found possible to draw copper so hard as to be almost equal to bronze in strength, and yet to retain about three times the electric conductivity of that substance. Copper and bronze wires possess great advantages for telephonic purposes over the iron wires employed in telegraph lines, in that they offer a much lower effective resistance to the rapidly undulating and intermittent currents produced by telephonic transmitters. The electric resistance opposed by a wire to the passage of such a current is always greater than that opposed to a steady current, and this difference is much more marked when the wire is of magnetic material like iron. This in creased resistance rises in proportion to the rapidity of the undulations of the current ; consequently high notes are more resisted than low notes. Besides this variable resistance, telephony has to contend with "self-induction" (see ELECTRICITY, vol. viii. p.76 sq.) of the current on itself, and this is by no means unimportant, especially on long circuits. [Footnote 132-1] The marked difference between iron and copper for long circuits is plainly shown by the fact that Rysselberg and others have spoken clearly to a distance of over 1000 miles through a copper wire insulated on poles, whereas Preece could not work a similar line of iron wire between London and Manchester.
The electrostatic capacity of the lien (see TELEGRAPH, p. 115 above) is also diminished by the use of thin wires of highly conducting material. They should all if possible be erected on poles at a considerable height above the earth. It is not practicable to work an ordinary underground line through more than 20 miles, and cable telephony through distances of over 100 miles may in the present state of science be put down as an impossibility.
Another element of great importance in connexion with telephone lines, which in most cases does not require to be attended to in ordinary telegraph circuits, is the induction from one line to another (see ELECTRICITY, vol. viii. p. 76 sq.) When two lines having, as in ordinary telegraphy, an earth connexion at each end run for any great distance, say a mile or more, parallel to each other on the same supports, a conversation which is being carried on through one of them can be overhead by means of the telephones on the other. This is due to the fact that, when a current is suddenly set up in one closed circuit, it induced an instantaneous current in any other closed circuit which is near to it. This induced current not only destroys the privacy of the circuit in question but also lowers its efficiency. The mischief of is even greater when telegraphs and telephone lines run along the same route supported on the same poles, because the strong intermittent currents sent through telegraph wires, and the irregular manner in which the intermit-tences follow each other, induce a series of such powerful secondary currents in the telephone lines that that noise heard in the telephone is often sufficient, when the lines is a mile or two long, to phone is often sufficient, when the line is a mile or two long, to drown all speech. In the case of parallel telephone lines the best, if not the only, cure is to use return wires, and arranged them so that the currents induced in the outgoing wire shall be neutralized by the corresponding current induced in the incoming wire. For mixed telegraph and telephone circuits various methods have been preposed ; but the most generally approved plan is to have return wires. For circuits worked wholly on the return principles the main thing to be attended to is the symmetrical arrangement of the wires, so that the outgoing and incoming wires may be subjected to the same influence. This is nearly provided for by running them in such a way that they may be all supposed to lie on the surface of a cylinder in lines parallel to its axis, the two wires at the opposite ends of a diameter being always used for the same circuit. When more than four wires at the opposite ends of a diameter being always used for the same circuit. When more than four wires form the group complete compensation is not obtained in this way, because the current is always stronger near the transmitting end of the line than near the receiving end, on account of the very sensible effect of the capacity and the leakage of the line. It is therefore best to arrange the wires in groups of fourthat is, in pairs cirucuitsand run them so as to form spiral lines round an axial line equidistant from each of the four wires. Any pair of wires forming a circuit which runs parallel to other wires can be arranged so as to be very nearly free from induction by interchanging position relatively to the other wires at short distances along the line. Care must, however, be taken, when more than one group of four or when than one pair are run, that the compensation produced by the twisted arrangement of one set, or of the interchanges of the wires in the different pairs, is not spoiled by the twisting or interchanging of another set or pair. Telephone lines running parallel to telegraph lines should be formed into one or more groups, each being run on the twist plan so as to eliminate as completely as possible the effect of the telegraph signals; the small residual effect of the telephone signals is of comparatively little importance is such a case. A twisted cable of telephone wire may, when each circuit is formed by diametrically opposite wires, be placed in the same tube with similar cables employed for telegraph purposes. The central wire of the cable may be used either as a telegraph line or as a telephone line having an earth return. Another method is to use powerful telephone transmitters and insensitive receivers ; that is to say, make the telephone currents so powerful that the telegraphic induced currents will be small in comparison, and use receivers so insensitive as to suit such currents. One of the main obstacles in the way of this method at present is the difficulty of getting strong telephonic currents, for even the best transmitters are not yet sufficiently powerful, and there is, besides a decided tendency towards a loss of quality in the sound when the transmitter is made powerful. A third method is to render the telegraphic of the intermittences. This is quite possible because the number of currents sent per second, even on fast working circuits, is not such as to produce a high musical note. If, then, the currents be made in some way to rise slowly to their full strength and fall again slowly to zero the diaphragm of the receiving instrument, instead of showing the sudden rise and sudden fall as present would move so slowly backwards and forwards that the ear would not be disturbed by the sound. Perhaps the simplest way to accomplish this is to place an electromagnet in the circuit of the telegraph line at the sending station, for the self-induction of the magnet coil prevents the current assuming its strength suddenly. But on telegraph circuit where speed is of great importance this method cannot be followed owing to the retardation of the telegraph signals and the consequent loss of spend thereby occasioned.
An ingenious application of the method of compensation just indicated has been made by Rysselberg, who has used not only wires carried on the same poles as the telegraph but even the telegraph lines themselves for telephone purposes. The arrangement of his system is shown in fig. 14., where L and L1 represent two telegraph lines. Between these, at each end, are inserted two condensers C1, C2 and a Telephone T, together with transmitters, &c., so that, supposing the telegraph instrument removed, the two wires would be an ordinary telephone circuit worked through condensers. The telegraphs apparatus consists of an ordinary receiver R, sending battery B, and key K, together with a condenser C, instead between the earth and the lines terminal of the key, and two electromagnetic inductors E, E´. When the key is depressed the current is retarded by the electromagnet E and the condenser C, which has to be charged, giving in fact additional electrostatic capacity at the sending end of the line. The current is still further retarded by the electromagnet E´ ; hence the condenser C1 becomes charged so gradually that very little disturbance is noticeable in the telephone T. The condensers C1, C2, prevent leakage from one line to the other, but have sufficient capacity to allow the telephone to act as if it were in a metallic circuit.
132-1 See papers by Prof. Hughes, Proc. Soc. Tel. Eng., vol. xv. p. 6 and Proc. Roy. Soc., vol. xl. P. 468, with remarks on them by Prof. H. F. Weber, Tel. Journ., vol. xviii. p. 321 and vol. xix. p. 30 ; by Oliver Heaviside, Phil. Mag., vol. xxii.p. 118 ; by Rayleigh, Phil. Mag., vol. xxi. p. 381 and vol. xxii. p. 469. See also Prof. Chrystal on the "Differential Telephone," in Trans. Roy. Soc Edinb., vol. xxxi. pp. 609-636.
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