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More than a thousand million telephone calls were made during 1936 in the London area alone. The modern automatic exchanges which made this possible are marvels of ingenuity


A SECTION OF LONDON’S FIRST AUTOMATIC EXCHANGE—at Holborn. The apparatus is pare of the Tandem exchange which transmits calls from one manual exchange to another, thus effecting a considerable saving of time.








A SECTION OF LONDON’S FIRST AUTOMATIC EXCHANGE - at Holborn. The apparatus is part of the Tandem exchange which transmits calls from one manual exchange to another, thus effecting a considerable saving of time.










THE telephone is such a commonplace feature of many homes that its countless users take it for granted. The operations performed by the subscriber who uses the telephone are extremely simple, and have become a matter of routine, particularly where the automatic telephone is concerned.


It is difficult to imagine a greater contrast than that between the simplicity of the domestic telephone and the extraordinary complexity of the exchange with which it is connected. The modern telephone exchange represents the triumph of a highly specialized branch of electrical engineering, and the progress made in the still more specialized science of automatic telephony in the past few years has been amazing.


In 1927 there were fewer than 20,000 automatic telephones in Great Britain. At the end of 1936 there were nearly 600,000, served by some 1,600 automatic exchanges. In the area known as the London Telecommunications Region the number of calls originated during 1936 was 1,007,000,000, of which 77 per cent were completed at the first attempt.


These figures give an excellent idea of the magnitude and the reliability of the modern telephone service. The fact that the average time between the origination of the call and the establishment of a connexion was thirty-seven seconds is a tribute to the efficiency of the system.


The adjective “automatic” is much misused in these days, but the designation “automatic telephone” is no misnomer. The subscriber lifts his receiver, waits for a certain characteristic tone to reach him, and dials the number he requires. Apparatus at his exchange responds to the simple mechanical operations which he has carried out.


THE COMPLEXITY OF THE AUTOMATIC TELEPHONE SYSTEM is shown by the apparatus at a modern telephone exchangeThe apparatus finds a free line for him, connects him with the distant exchange which he is seeking, and sends impulses to that exchange which cause the distant subscriber’s bell to ring. The distant subscriber lifts his receiver, and finds himself in communication with the person who originated the call.


The whole operation has been performed without human interference or assistance, by a collection of apparatus which, to the uninitiated, seems almost to be capable of thinking. To the electrical engineer the automatic telephone represents merely another of the multifarious uses to which electricity may be put, but few people not professionally connected with electrical engineering have a clear idea of the working of the system.





THE COMPLEXITY OF THE AUTOMATIC TELEPHONE SYSTEM is shown by a relatively small bank of apparatus at a modern exchange. The illustration shows “directors” being inspected at the Pollards exchange, Norbury, London. Directors translate the three letters dialled by a subscriber and transmit an appropriate series of impulses along the first free line to the desired exchange.





The following is a brief outline of the sequence of events which follow the dialling of a number, simplified by the omission of the three-letter “code” signifying the distant exchange. In the imaginary instance under consideration, a subscriber to an automatic telephone system wishes to establish communication with another subscriber whose number is 2343.


As he lifts his receiver, a piece of apparatus known as a “line switch” searches for a disengaged unit of the automatic apparatus and connects it to the calling line. As the caller dials “2”, HIS dial sends two impulses along the line, raising the arm of a selector switch to the second level of a bank of contacts, which has ten levels in all (see diagram on this page).


The contact arm of this “first selector”, having found its correct level - “Level 2” - now sweeps round in a semicircle, until it finds a contact not in use. As soon as this contact has been found, the calling subscriber has access to the “2000 class” - any number between 2000 and 2999. Should a free contact not be available - ten possible contacts are provided - the selector arm proceeds to a further position and the subscriber hears the “engaged” tone.


TELEPHONE SELECTORS IN ACTION for a call to a subscriber



SELECTORS IN ACTION for a call to a subscriber whose telephone number is 2343. On the left is the “thousands” selector; the central unit selects “hundreds” and the “tens and units” selector is on the right.




If, however, a contact is made, the process is then taken up by a second selector. The subscriber has, by now, dialled “3”; the three impulses sent along the line by this operation are disregarded by the first selector, which has finished its work, but are taken up by the second selector. Its “wiper”, or contact arm, is raised to the third level, which it proceeds to sweep until it finds a disengaged connexion. The subscriber is now connected to the third group of the hundreds - any number between 2300 and 2399 is now available to him. His last two figures are ignored by the first two selectors, which remain fixed in position, and a final selector comes into operation. The subscriber dials “4”, and the wiper of this selector rises to the fourth level - the forties. He dials “3”, and it travels to the third contact along the fourth level, this connecting the calling subscriber with the line to which has been assigned the number 2343. A ringing machine comes into operation and passes impulses along this line, causing the bell at 2343 to ring at regular intervals until the receiver is removed.


LINKS BETWEEN TWO TELEPHONE SUBSCRIBERS, shown in simplified formIf the owner of number 2343 happens to be engaged on another call at the time, the calling subscriber hears the characteristic “engaged” tone. If 2343 is free, the lifting of his receiver flashes back an impulse, through the whole series of selectors, which stops the ringing tone and registers the call on the calling subscriber’s meter at the exchange. If 2343 is engaged, or does not answer, this signal is not sent. As soon as the calling subscriber has replaced his receiver, the whole train of selectors is released and is immediately available for use by other subscribers.


In such areas as London, where several automatic exchanges are necessary, the system becomes far more intricate. The three-letters code preceding the number necessitates the use of another piece of apparatus known as a “director”. The director automatically translates the three-letter code, selects a free line to the exchange which is being called, and then passes on the electrical impulses which represent the numbers dialled, in the sequence already described.





LINKS BETWEEN TWO SUBSCRIBERS, shown in simplified form. The line switch selects the first disengaged unit of the automatic apparatus, and the three selectors find the number by stages, as shown in the diagram at the top of this page.





The person originating the call lifts his receiver and dials three letters and four figures. The automatic apparatus immediately connects him with a “line switch”, which searches for disengaged automatic apparatus and connects it to the line of the calling subscriber.


The calling line is thus given access to the director, which receives the impulses from the subscriber’s dial and steers the call through to the required exchange. This is done by means of a first selector, which secures a route to an appropriate junction selector. The junction selector finds a disengaged junction line - that is, a line connecting the calling exchange with the distant exchange. At the distant exchange the four-figure sequence, described above, is gone through by way of the thousands selector, the hundreds selector and the final selector for tens and units.


The feature of the entire system which necessitates the greatest ingenuity is the relatively small number of lines between exchanges. One suburban exchange may have 9,000 private subscribers connected to its switchboards; but there will certainly not be 9,000 lines connecting it with another exchange in the vicinity. The number of junction lines will be just sufficient to carry the maximum volume of traffic that is generally handled between the two exchanges.


Hence the automatic apparatus must make use of a large amount of apparatus that is not exclusive to each particular subscriber. A City subscriber, when he dials Croydon 2343, is not always connected to that number by the same line. The automatic apparatus searches round the ends of the various lines which are available between the City and Croydon exchanges, and the City subscriber is given the first free line.


Electric-Magnetic Relay


Possibly the first line encountered by the mechanical operator will be free; possibly it may have to explore five or six lines before a disengaged one is found. The subscriber, however, knows nothing of this, for the whole operation is carried out almost instantaneously, while he is dialling the three-letter code representing the name of the exchange with which he is about to communicate.


The interior of the modern automatic exchange is filled with a maze of intricate apparatus, constantly tested. In orderly racks are mounted all the pieces of apparatus through which the subscriber’s dialled call passes - line switches, directors and selectors. As the visitor walks between the racks of apparatus, constant series of clicks testify to the activity of subscribers. He can watch a particular line switch until it is appropriated for a call; it will suddenly spring into life, search for a disengaged line, make contact with it and remain until connexion has been established.


The selectors and the other units are screened by neat dust-covers; everything is spotlessly clean, and the atmosphere of the exchange is artificially kept at a constant level of dryness, for moisture and dust are the worst enemies of the extremely delicate apparatus in use. The functioning of the entire automatic telephone system depends upon one particular principle - that of the electro-magnetic relay. This extraordinarily flexible and delicate piece of apparatus may be described as a means of converting electrical energy - even of the most minute amplitudes - into mechanical movements, which may be used to operate other electrical circuits. It depends for its functioning on the simple principle of the electro-magnet, and on the fact that a piece of iron may suddenly be converted into a magnet by an impulse transmitted from a great distance.


FAULT-FINDING at an automatic telephone exchangeLine switches, directors, selectors, the subscriber’s dial, even the bell which tells him that someone is calling him - all depend upon this principle. When a caller dials the figure 2, and two impulses speed along the wires to the exchange, it is a relay which raises the wiper arm of the selector to the second bank of contacts. The first impulse, flowing through the coil of a relay, converts the iron core temporarily into an electro-magnet, and a small armature is attracted towards the core. The second impulse gives the armature a second movement.





FAULT-FINDING at an automatic exchange is largely carried out by a piece of apparatus known as a routiner. When once the fault has been localized, however, adjustments must be made by hand. An engineer engaged on this work carries an inspection lamp on his forehead so that his hands may be free.





The relay, moreover, sets a second series of operations in motion. Its armature is merely a means to an end, and when it moves it completes a second electrical circuit - or perhaps a whole series of electrical circuits - which causes other things to happen. There is no end to the list of applications of the modern sensitive relay - and the modern telephone exchange must necessarily house some scores of thousands of relays. The relays which operate selectors and other apparatus have to be adjusted to exceedingly fine limits. Even a minute speck of dust might cause trouble. The working parts are small and light, and many of them have to operate extremely quickly. The precautions against failure at an automatic exchange are amazingly thorough, and a special machine known as an “automatic routiner” takes each individual selector in turns and tests it thoroughly. If the selector is in perfect order, the routiner passes on to the next. Its passage is marked by a small travelling light on an indicator board.


If, however, a fault is detected by the routiner, the light stops in its passage. An easily visible alarm light is illuminated, an alarm bell rings, and the engineer knows at once where the trouble is located. At important exchanges this routine testing is an almost constant process.


Although the amount of current required to operate each individual relay is minute, the total requirements of an exchange are considerable. All current is supplied by banks of storage batteries of extremely large capacity. Some of these batteries weigh as much as 60 tons. The battery room of a modern telephone exchange bears little resemblance to the more familiar battery charging station.


The cells stand in orderly rows and are mounted on porcelain insulators; the bus-bars connecting the various banks of cells to the switchgear are mounted high up - well out of reach, although the voltages used are not sufficiently high to be dangerous to the staff.


“Floating Battery” Principle


The batteries are charged by motor generators driven from the supply mains, and many automatic devices are used to prevent possible breakdowns. While one set of batteries is in use, another is being charged. Spares are always available; and in some busy exchanges the “floating battery” principle is used. This means that batteries are on charge even while they are in use. The motor generators never supply current directly to the telephone apparatus; such procedure would mean that a temporary stoppage of the motor, or a breakdown of the supply mains, would cause the complete failure of the telephone service. With the large batteries used, power is always available for a considerable period nowadays without the necessity of using the outside supply at all. Many other machines are used in an exchange. Electric motors with specially shaped armatures generate the special currents for ringing subscribers’ bells and for producing the various tones denoting “engaged”, “number unobtainable”, “dialling tone” and the like.


The smaller apparatus in use includes several thousands of fuses for the protection of circuits against such contingencies as the falling of a high-tension cable across an overhead telephone line. Lightning also has to be guarded against - and the tracing and replacement of a blown fuse is a simpler matter than the repairing of a few thousand wires.


The outside cables are generally brought in under the floor of an exchange, whence they go to distribution boards so arranged that the ends of any particular subscriber’s wires can be traced easily and quickly. From these huge distribution boards an incredibly complicated network of wire leads to the various sections of the automatic apparatus. Every wire, however, may be easily traced by the engineers, who are thoroughly familiar with the layout of each section of the apparatus.


A TYPICAL BATTERY ROOM supplying the current required by an automatic telephone exchange



A TYPICAL BATTERY ROOM, supplying all the current required by an automatic telephone exchange. Banks of twenty-five cells provide current at a potential of 50 volts ; alternative banks are provided, one being kept on charge while the other is in use. Overhead bus-bars carry the current to the switchgear.





Even at an automatic exchange there must be a staff of operators to deal with toll calls and “assistance” calls - the latter coming from local subscribers who are in difficulty and fall back on the well-known expedient of dialling 0. If a person originating a call is unable to get into communication with the desired number, the operator at the exchange may ask him to dial the number again. While this is being done, an engineer at the exchange can watch the call going through, and in the process is able to trace a fault in the subscriber’s dialling apparatus, or a mistake in the method of dialling. Alternatively, the operator may dial the desired number from the exchange, making contact with the subscriber when connexion has been established.


Toll calls going out from local telephone users are handled by the operators, who have to be familiar with the correct “routing” of such calls. A call from a London toll exchange to Rottingdean, Sussex, for instance, may have to go through Brighton. The operator in London makes connexion with a disengaged junction line to Brighton, and is automatically informed by a signal when the Brighton operator is connected. She will then say “Rottingdean”, and the Brighton operator will make the necessary contact with the line to the Rottingdean exchange. A further signal informs the London operator that the Rottingdean operator is “through”, and she will then give the number required.


This sequence of operations will naturally vary over different routes. The whole system is bound up with the fact that there is an inevitable “narrowing down” between exchanges - that is, the number of local lines to each exchange from its subscribers is enormously in excess of the number of lines available between one exchange and another. It is precisely this fact that necessitates the complexity of automatic apparatus, which always has to undertake the task of finding a disengaged line in the shortest possible time.


Congestion Meter


A typical local exchange has also to deal with calls made from local call-boxes. Further mechanism is necessary to check the payments made in such instances. The operator is informed by tone signals as the coins are placed in the slot, and she has control over the call. Automatic call-boxes involve still further complications, such as the mechanism which insures that Button B will not return a caller’s coins when once, he has made contact with the person he is calling.


An interesting piece of apparatus installed in certain exchanges is known as a Congestion Meter. Experience enables engineers to decide upon the maximum number of calls with which the apparatus can deal every minute. The meter is set to this figure, and a constantly moving needle on its scale indicates the number of calls being handled. Should this increase, for any particular reason, to a figure higher than that decided upon as a safety limit, the meter will automatically light a warning lamp and sound an alarm bell, enabling the engineers to take action.


Subscribers’ meters may be seen at an automatic exchange, neatly arranged in racks. They work on a principle roughly analogous to that of a mileage recorder on a bicycle; each fresh impulse received along the appropriate line will “trip” the apparatus and raise the recorded figure by one.


TA FEW OF THE WIRES at a testing desk used at the Clerkenwell telephone exchangehe telephone service of such a country as Great Britain is under a central control which deals with every one of its multitudinous branches, and requires the services of a vast staff of engineers and technicians to maintain smooth and reliable working. That such working is possible at all is a tribute to the inventive genius of the engineers, to the high standard of accuracy maintained by the firms which build the apparatus, and to the skill of the staff who look after it.




A FEW OF THE WIRES at a testing desk, used at the Clerkenwell exchange. The intricacies of the telephone system of a large city are indescribable, but specially trained engineers know the purpose of every piece of apparatus—and almost of every single wire—and extensive breakdowns are extremely rare.



[From part 17, published 22 June 1937]



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Modern Telephone Exchanges