1902 Encyclopedia > Telephone > The Working of Telephone Circuits

(Part 5)


The Working of Telephone Circuits

The method first employed for working a telephone line was extremely simple. A single line of wire, like an ordinary telegraph line, had a Bell telephone included in it at each end and the ends were put to earth. Words spoken to the telephone at one end could be heard by holding the telephone to the ear at the other. To obviate the inconvenience of placing the telephone to the month and the ear alternately, two telephones were commonly used at each end, joined either parallel to each other or in series. The contrivance generally adopted for calling attention is the call bell, rung either by a small magneto-electric machine or by a battery. The telephone was switched out of circuit when not in use and the bell put in its place, an ordinary key being used for putting the battery in circuit to make the signal. This arrangement is still employed, a hook being attached to the switch lever so that the mere hanging up of the telephone puts the bell in circuit. In some cases, when the bell is rung by a magneto machine, the coil of the machine is automatically cut out of circuit when it is not in action, but the turning of the handle moves a centrifugal arrangement by which it is thrown in.

At first it was usual to employ the same instrument both as transmitter and as receiver, and to join it in the direct circuit. But is was soon found that the microphone transmitter could only be used to advantage in this way when the total resistance of the circuit, exclusive of the microphone, was small compared with the resistance of the microphone,—that is, on very short lines worked with low resistance telephones. The transmitter on long and high resistance lines worked better by joining indirectly in a local circuit, in the manner shown in fig. 13, the microphone, a battery, and the primary of an induction coil, and putting the line in circuit with the secondary of the induction coil, which acted as the transmitter. The resistance of the microphone can thus be made a large fraction of the total resistance of the circuit in which it is placed ; hence, by using considerable currents, small variations in its resistance can be made to induce somewhat powereful currents in the line wire. 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 indicate, 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.

Translation from one line to another, or form one section to another of the same line, is effected by putting the primary of an induction coil in the place of the receiving telephone, the secondary being in circuit with the second line or section. This plan is useful where the same message is to be sent to different places at once (distributed), and is sometimes used for translating from a double wire to a single wire system. Probably a better plan is to work a microphone by the membrane of the receiving telephone, and retransmit the message, taking new energy from second battery. [Footnote 133-1] When the induction coil arrangement is used for translating from a double to a single wire circuits, 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. Everything else being similar, the resistances of the coils should be in nearly the same ratio as the resistances of the lines in which they are placed.

In a large town it is neither practicable nor desirable to connect each subscriber directly with all the other subscribers, hence a system of "exchanges" has been adopted. An exchange is a central station to which wire are brought from the different subcribers, any tow of whom can be put in telephone communication with each other when the proper pairs of wires are joined together in the exchange. The arrangement is illustrated in fig. 15, where C represents an exchange from which wires radiate to the points a, b, c, d… Suppose a wishes to speak to d; he communicates his wish to an attendant at C, who first calls d, and then connects 6 to 1, making the circuit continuous from a to d. The arrangements at the exchange for facilitating connexions very considerably, but are similar in principle to the switch boards used in telegraphy. Each of the wires is first brought to an indicator and then to a set of terminals arranged in an orderly manner or a board the number of the terminal for any one wire being the same as the number under the shutter of the indicator in that circuit. In many cases the terminals take the forms of spring clips, which connect the line to earth, and under which a thin piece of metal, covered with insulating material on one side and called a "jack," can be readily inserted for connecting that circuit with any other. A piece of flexible wire cord, carrying a jack at each end, forms a ready and common medium of connexion ; but in many cases the switch board is arranged with cross strips of metal so that by inserting a jack into the terminals of the two wires they can be both connected to the same strip of metal and therefore together. In large exchanges one switch board of moderate size is not sufficient, and so number are fitted, being connected together by several conductors, in order that no interruption may ensue in consequence of these being all occupied. A line on one board is connected with one on another by joining the terminal of the first to one of the conductors connecting the two boards by a jack-cord, and then by another jack-cord connecting that conductor to the terminal of the other line. Thus different switch boards may be looked upon as separate exchange, 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 subscriber 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. The attendant at the central exchange puts the puts the subscriber in communication with the district he requires, and the attendant there calls the other subscriber and joins the two subscribers’ lines together. In some cases neighbouring district exchange have, besides a common means of communication through the central exchange, an independent connexion. These arrangements are diagrammatically illustrated in fig. 16, where 1, 2, 3, 4, 5, 6 represent district exchanges and C the central exchange ; district 3 and 4 and 4 and 5 are supposed to have independent connexions.

An arrangement was proposed about two years ago by Mr D. Sinclair of the Glasgow telephone exchange for allowing small district exchanges to be worked by the attendants at the central exchanges. [Footnote 134-1] The two exchanges are connected by a trunk line and from the district exchange wires are led to the different subscribers. These wires are in the normal state of matters connected with contact plates, over which an arm joined to the trunk wire can be made to travel. Suppose the central exchange wishes to speak to any one of the subscribers, the arm is made to travel round, by currents sent from the exchange through an electromagnetic step by step arrangement, until it comes in contact with the proper plate, after which the subscriber is called in the ordinary way. When one subscriber belonging to the district exchange wishes to speak to another in the same district, he rings the bell in the ordinary way, and this operation disconnects all other subscribers and puts him in connexion through the trunk line with the central exchange. The attendant there ascertains to whom it is that he wishes to speak, and by moving round the contact arms puts the two subscribers’ line in contact.

The indicator, or annunciator as it is sometimes called, is shown in fig.17. It consists of an electromagnet M, which on a current being sent through it pulls down the armature a, relatives the catch c, and allows the shutter d to fall down, exposing a plate p, on the front of which the number of the subscriber is printed. When the exchange is called, the shutter d is dropped, the attendant connects the line leading to the exchange table with the terminal corresponding to the indicator, and finds who is wanted ; then he calls that subscriber, make the through connexion, and puts up the shutter. When the subscribers have finished, both call the exchange or, as it is commonly put, "ring off" ; this drops both shutters and serves as the signal that they have finished speaking.

The principle of transmitting sound by the radiophone will be understood from fig. 18. M represents a mirror, from which beam of light is reflected through the lens l to a second mirror m, and m forms a diaphragm against the back of which the sound vibration sent through the tube t are made to impinge. The beam of light, after being reflected from m, passes through the lower lens, l, and thence as a nearly parallel beam to the parabolic reflector R. A photophonic receiver P, supposed in this case to be a spiral of selenium wire wound on the surface of a cylinder, is placed at the focus of the reflector so that the beam of light from m is concentrated on it. In circuit with the receiver P a battery B and a telephone T are included and through the circuit a feeble electric current flows continuously. The photophonic receiver should be placed so as to receive as little light as possible from any other source than the mirror m. Words spoken through the tube t make the mirror m vibrate, so that the beam of light reflected from it becomes more or less spread. The lens l is then unable to bring the beam into parallelism, and the intensity of the reflexions from R to P is varied, therefore also the current through the coil of the telephone, which in consequence gives out a sound. The amount of spreading of the beam, being proportional to the intensity of the vibrations of m, and this again proportional to the intensity of the sounds, the sounds heard in the telephone and similar to those produced at the end of t. Theoretically the receiver may be at any distance from the transmitter, but considerable difficulty arises if the distance is great.

One of the simplest forms of the phonograph is shown in fig. 19. It consists of a rigid spindle S screwed for about one-third of its length, and fitted to work smoothly but tightly in the frame f, f, which is securely attached to a sole plate P. On the spindle a drum D is fixed, the axis of which conincides accurately with that of the spindle. On the surface of the drum a screw is same pitch is that on the spindle. A fly-wheel W is fixed to one end of the spindle, and is provided with a handle H, by which the spindle and drum can be conveniently turned. One of the bearings has either a screw thread cut along it, or is fitted with one or more studs which work easily, but without shake, in the screw thread. When the spindle is turned, it receives a transverse motion, and a point fixed relatively to the sole plate P and touching the drum traces out a spiral on its surface, exactly coinciding with the screw thread cut on it. A mouthpiece M, like that of a telephone transmitter, provided with a diaphragm of parchment or similar substance, is mounted on a lever, which is pivoted at h and provided with a set screw b. 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 outward. To use the instrument, the drum D is covered with a sheet of somewhat stiff tinfoil, and the mouthpiece is adjusted as shown in the figure, with the needle point over the hollow part of the tinfoil, and fixe by the set screw to make a slight indentation in it. The drum is then turned and words spoken in a somewhat loud and clear tone in front of the mouthpiece. The vibrations of the diaphragm cause the needle point to make indentations more or less deep, according to the intensity of the sound, in the surface of the tinfoil. If the mouthpiece is then raised, the drum turned back to its original position, the mouthpiece lowered so that the point rests on the groove which it previously made, and drum again turned, the diaphragm, acted on by the needle point passing over the indentation, will give out the same words which were spoken to it. (T. GR.)


133-1 See Thomson and Houston, Tel. Journ., 15th August 1878.

134-1 See Proc. Phil. of Glasgow, vol. xvii p. 39.

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