Thomas Lomar Gray

“ It appears to be pretty well established that carbon in one form or another is the best material for one or both of the contacts of a microphone transmitter. When both the contacts are of carbon and the surfaces have consider able area, say from a quarter to half an inch in diameter, the sounds are loud, but have a tendency to harshness. When, as in the Blake transmitter, one of the contacts is a piece of polished gas carbon and the other a small sphere of platinum about the twentieth of an inch in diameter, the articulation is clear, but less loud. ”

“ The current, therefore, rises almost to its full value for each signal when the time of contact is as small as the five-hundredth part of a second, or, on the Wheatstone instrument, when the speed is about 500 words per minute. There is, however, a very sensible increase in the effective resistance of the circuit, especially when iron wire is used, when signals are sent as rapidly as 100 per second, so that higher battery power is required for fast than for slow rates of speed. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 their 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 more 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. ”

“ 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 type-wheels are placed side by side, but can be turned independently of each other. Beneath them a platen is carried on one end of a lever, whose other end is attached to the armature of an electro magnet. Between the platen and the wheels a ribbon of paper broad enough to cover the edges of both wheels is passed. The instrument is worked by three lines of wire, one for driving each type-wheel and one for printing and feeding the paper forward. The movement of the type-wheel is accomplished by an escapement acted on by closing and opening the circuit of an electromagnet. ”

“ Thus, during the time the receiving instrument at one station is in use, its type- wheel is kept in perfect agreement with the sending wheel at the other station ; and, if the electric action keeps time, a wrong letter cannot be printed unless the rate of the clockwork is at fault by some such amount as one or two per cent. If the two wheels are allowed to run a long time without the electric maintenance of agreement, they will be found more or less at variance, as the pieces of clockwork, however good, cannot be perfect. ”

“ 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. ”

“ 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 resistance at a fault can sometimes be greatly diminished by repeated application alternately of the positive and negative poles of a powerful battery to the cable, but this should never be resorted to if it can possibly be avoided. The direct deflexion method of taking wire resistance is most suitable for these tests. The resist ance seems to diminish gradually after the battery is applied until it reaches a minimum value, after which it again increases. This maximum deflexion should be taken as indicating most nearly the true wire resistance up to the fracture. ”

“ The several methods that have been proposed for the transmission of a number of messages in one direction on the same wire are reducible to two classes. In one the time which a revolving con tact slider takes to make one revolution is divided into as many intervals as there are sets of sending and receiving instruments on the line, and by means of it the current is closed through the different sets of apparatus in succession. ”

“ 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. ”

“ The requisite energy is derived from the battery. If there are other resistances in the circuit it is, in some cases, better to join it as a shunt to the primary circuit of the induction coil. It may even prove advantageous to insert resistances in the circuit, increase the battery power, and join the microphone as here indicated, because in this way powerful currents can be obtained in the line without the harshness which is apt to be produced by the variations of a strong current passing through the microphone. ”

“ The total resistance of the cable being known, it is easy to determine the position of the fault. When the fault has a high resistance it is necessary to make a cor rection for the want of perfect insulation in the sound part of the cable. When both ends of the cable are not available, measure the potential at the testing end and the resistance between that end and the earth, and simultaneously measure, by means of a slide re sistance and zero galvanometer or by means of a quadrant electro meter, the potential at the distant end. ”

“ TELEPHONE
TELEPHONY is the art of reproducing sounds at a distance from their source. The term was first used by Philip Reis of Friedrichsdorf, in a lecture delivered before the Physical Society of Frankfort in 1861. [1] But, although this lecture and Reis's subsequent work received considerable notice, little progress was made until the subject was taken up between 1874 and 1876 by Alexander Graham Bell, a native of Edinburgh, then resident in Boston, Mass. Bell, like Reis, employed electricity for the reproduction of sounds ”

“ The capacity of a uniform cable is inversely as its length ; hence, if C be the total capacity of the perfect cable and C^ the capacity of one section, the length of that section is ZCj/C. When as is almost always the case the cable is not quite uniform in electrical quality and in temperature, a table or a curve showing the wire resistance, the insulation, and the capacity up to any point from either end should be kept for reference. Com- It is not at all uncommon in cables for one side of a fracture to plete be partially insulated through the conductor not breaking exactly fracture . ”

“ 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 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. ”

“ Dolbear s con denser tele phone. A telephone transmitter and a receiver on a novel plan were patented in July 1877 by Edison, shortly after the introduction of Bell s instruments. The receiver was based on the change of friction produced by the passage of an electric current through the point of contact of certain substances in relative motion. In one form a drum, mounted on an axis and covered by a band of paper soaked in a solution of caustic potash, is turned under a spring the end of which is in contact through a platinum point with the paper. ”

“ This induced current not only destroys the privacy of the circuit in question but also lowers its efficiency. The mischief is even greater when telegraph 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 the noise heard in the tele phone is often sufficient, when the line is a mile or two long, to Methods drown all speech. ”

“ 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. ”

“ Thus different switch boards may be looked upon as separate exchanges, connected together by a number of trunk wires after the manner described below. In a large system it is much more convenient and economical to have exchanges in the various districts, and connect these with a central exchange by a sufficient number of trunk lines. A sub scriber in one district wishing to speak to a subscriber in another calls the exchange in his own district and is put in communication by the attendant stationed there with the central exchange. ”

“ It is convenient for this purpose to dispense with the charge in the electrometer jar and needle and connect the needle to the pair of quadrants which are joined to the cable. The deflexion is then proportional to the square of the potential and is always to one side of zero, so that the whole range of the scale is available for the de flexion. The tests for wire-resistance and capacity are practically the same as those already described. They are in ordinary circumstances of much less importance than the insulation tests. ”

“ 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 pro duced by telephonic transmitters. ”

“ When the instrument requires to be highly sensitive, as for testing purposes, it is shielded from the action of the ship s and the earth s magnetism by enclosing it in a massive iron case. For signalling pur poses the controlling magnet is arranged to produce at the needle a field so strong that the effect of variations of external mag netism is inappreciable. ”

“ This system is more expensive than the open circuit system, as the battery is always at work ; but it offers some ad vantages on circuits where there are a number of intermediate stations, as the circuit is under a constant electromotive force and has the same resistance no matter which station is sending or receiving. The arrangement at a wayside station is shown at W. When the circuit is long and contains a large number of stations, the sending battery is sometimes divided among them in order to give greater uniformity of current along the line. ”

“ A mouthpiece M, like that of a telephone transmitter, provided with a diaphragm of parch ment or similar substance, is mounted on a lever, which is pivoted at k and provided with a set screw 5. A blunt needle point is either fixed to the centre of the diaphragm or carried by a light spring in such a way as to press on the centre of the diaphragm with the needle point projecting outwards. ”

“ In the case of the dash the ink-wheel is brought into contact with the paper by the first current as before and is pulled back by the reverse current after three times the interval. The armature acts on an inking disk on the principle described above, save only that the disk is supplied with ink from a groove in a second wheel, on which it rolls : the grooved wheel is kept turning with one edge in contact with ink in an ink-well. ”

“ Probably a better plan is to work a microphone by the membrane of the receiving telephone, and re transmit the message, taking new energy from a second battery. 1 When the induction coil arrangement is used for translating from a double to a single wire circuit, or vice versa, it is necessary to make the induction coil suit the circuits, so that either coil may be used as primary, according to the end from which the message is sent. ”

“ In towns where wires. 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 a town ”

“ For cables a fairly reliable test can be obtained from the capacity even when the insulation at the fault is somewhat imperfect, if it be sufficient to hold the greater part of the charge for a few seconds, since the amount of loss in any short interval can be estimated by a separate test. ”