Twentieth Century Inventions - Charles Gibson

Telephonic Inventions


Poulsen's Telegraphone

In the opening years of the present century, the now well-known Danish inventor, Waldemar Poulsen, brought before the public a most ingenious device, which was called sometimes a 'telegraphone,' at other times a 'micro-phonograph,' or a 'telephonograph,' or, again, a 'magnetophonograph.' All these names are descriptive of the apparatus, the function of which is to record and reproduce a spoken message by electrical means.

The earliest form of the instrument was very like an ordinary phonograph in appearance. There was a cylindrical drum of brass about 11 inches in length by 5i inches in diameter. Instead of a wax cylinder there was a spiral of 225 turns of steel piano wire, of a diameter of about one millimetre. Instead of the travelling tympanum and stylus of the phonograph, there was a small electromagnet, which travelled along in contact with the wire.

In the ordinary phonograph, the original sound causes the tympanum to vibrate and its stylus makes corresponding indentations on the revolving surface of wax. In the ordinary telephone we have an electric current under control of a vibrating diaphragm, causing the electro-magnet in the receiver to vary its magnetization, and thus set up vibrations in the diaphragm of the receiver. In the telegraphone, we have a telephone transmitter, the controlled current from which is led to an electro-magnet, but instead of this magnet controlling a vibratory disc, it magnetizes the steel wire on the cylinder, as it passes beneath the poles of the magnet. We may picture spots, as it were, of magnetization being stored along the whole length of the long steel wire.

Again, in the ordinary phonograph, we have the reproduction of the sound produced by the indentations on the wax cylinder reacting upon the stylus and the tympanum, and in this way producing sound vibrations. In the telegraphone, we cause the spots of magnetization to react upon the little electro-magnet by drawing the wire over its poles. The varying magnetic field sets up induced electric currents which control an ordinary telephone receiver, and reproduce the recorded speech. In this early form of telegraphone, the steel wire could contain little more than one hundred words.

In another type of the instrument, Poulsen placed the brass cylinder, with its surrounding spiral of wire, in a vertical position, and within a stirrup-shaped frame. In this instrument, the cylinder and the recording wire remained stationary, while the little electro-magnet was rotated around the cylinder, describing a spiral, so that its travel corresponded exactly with the spiral of the stationary wire.

The reproduction of speech by the telegraphone is much more perfect than is possible with the ordinary phonograph. In the latter the voice vibrations set in motion a diaphragm or tympanum which is not perfectly free, for the stylus which is attached to it rests upon the wax cylinder. There is no such dampening effect in the telegraphone, in which the telephone transmitter has a perfectly free diaphragm.

The layman might suppose that the record on the wax cylinder would be a more permanent record than the variations of magnetization in the steel wire, but it is quite the other way about. The telegraphone records may be reproduced 10,000 times without any decrease in intensity or deformation being noted, and is practically permanent, if so desired. But few records will be required to be permanent, and it will be convenient to use the wire again for further records. It is only the magnetization which we wish to obliterate, and this is accomplished very easily by exposing the wire to a constant magnetic field. The recording magnet may be connected to a battery, or a permanent magnet may be used in its place, and when the wire is passed rapidly between the poles of the magnet all traces of the variations of magnetization disappear.

In another type of the instrument a flat steel band or ribbon is used in place of a wire. This steel ribbon is passed from one reel to another, and on its way it is exposed to the influence of the recording magnet. The steel ribbon measures about 3 millimeters in width, and is 0.05 millimeters in thickness. It may be made any desired length, and a continuous conversation extending over an hour could be recorded on the one ribbon. Of course, a long wire may be used instead of the flat ribbon if desired. The speed of the ribbon or wire is about seven feet per second, or somewhat quicker than the pace of a smart walk.

When the instrument is used for dictation purposes, an obliterating magnet may be placed a few inches from the recording magnet, so that the ribbon or wire passes the obliterating magnet immediately before receiving the recorder. This is very convenient, for if the speaker desires to correct anything he has dictated, he may reverse the travel of the ribbon and obliterate as much as desired, and then once more take up his dictation.

The instrument, when intended for dictation purposes, is mounted in t small case. On the top of this case are two revolving reels carrying the steel ribbon or wire. These reels are driven by a small motor, which is placed within the case. On the outside, and placed centrally between the two revolving reels, is a movable arm fitted with the recording and obliterating magnets. The ribbon or wire passes through sapphire bushings on this central arm, and then passes over the magnets. The case is provided with a switch which serves to change the various circuits of the recording and speaking battery.

When the typist is taking down a letter from this mechanical dictator, she may have occasion to stop the machine, but that there may be no difficulty in picking up the dictation again, the machine, when stopped, reverses automatically and runs back a few feet of ribbon. In this way, the end of the last sentence is repeated before proceeding with the next. The machine not only performs this reversal automatically, but it cuts out the obliterating magnet so that the record will not be destroyed. This arrangement is particularly convenient to the typist, for the machine is at a distance from her, being placed on the dictator's desk, probably in another room. The typist has only the telephone receiver and the starting and stopping switches beside her. She receives a signal when the telegraphone is free to dictate to her. She then has the machine under her own control.

The inventor has provided means of taking a record suitable for transmission by post. The instrument, devised for this purpose, makes the magnetic record on a flat steel disc, both sides of which may be used. Or if preferred, a polished cylinder of magnetically hard steel may be used. Suitably tempered Bohler steel has been found to give the best results. The thin steel discs are more convenient for postage, and run no risk of injury in transit. These discs measure 13 centimeters in diameter, and are 0.5 millimeters in thickness.

The two reels, between which the steel ribbon or wire is stretched, may be placed some distance apart, and in this way it is possible to tap its record by quite a number of separate electro-magnets, and thus reproduce the sound in as many separate telephone circuits. Arranged in this manner it might be used for purposes similar to those for which the electrophone has been used.

The most interesting use to which the telegraphone has been put is to record conversation coming over an ordinary telephone line. By means of a switch the telegraphone may be connected to a telephone line, so that if a subscriber calling up finds that the person to whom he desires to speak is absent, he may leave the verbal message in the telegraphone attached to the distant end of the line, instead of dictating it to a third party who might not understand it.

It is possible to arrange that the telegraphone left alone in an office will accept any conversation sent over the line. In such circumstances it is necessary for the instrument to intimate to the distant subscriber that the principal is not in, but that he expects to be back at, say, four o'clock. This leaves the calling subscriber free to ring up later or to dictate his message to the telegraphone, if that is more convenient. In this case the telegraphone is arranged to operate its own switches by automatic means.

Poulsen has suggested that his telegraphone might act as a telephone relay; but one of the chief requirements of such relays is in connection with submarine telephone cables, and, of course, to be of real service on a long cable it would be necessary to have the relays inserted in the cable, which is impossible. We shall see at a later point in the present chapter how practical relays have been inserted in submarine cables. Meantime it will be of interest to consider a German invention which records and reproduces sound with the aid of photography.

Ruhmer's Photographophone

The invention of the photographophone is based upon a discovery made in the end of 1900. In December of that year, a demonstration was given by W. Duddell, before the Institute of Electrical Engineers, showing that when the current of an are varied there was a corresponding contraction or expansion of the vapour column between the carbons. Duddell showed that when the variations were sufficiently rapid, not only was sound produced, but by controlling the variations of the are by musical notes, these could be reproduced by the arc. Even speech could be reproduced, although at that time the words were not very clear.

The arrangement of the apparatus was to place upon the lecture table an open are lamp, to which was connected a telephone circuit leading from some other part of the building. At the distant end of the telephone circuit there was an ordinary telephone transmitter connected to a storage battery of two cells. This circuit formed the primary of an induction coil or 'transformer,' the secondary circuit of which contained a condenser and the distant arc lamp. The arc lamp was, of course, energised by the ordinary lighting current from the mains. The variations of the battery current in the primary circuit, due to the influence of the vibrating telephone transmitter, induced variations in the arc.

When a tuning fork was held in front of the distant telephone transmitter, the arc in the lecture-room was heard to sing out the notes very clearly. If a person whistled a tune into the telephone transmitter, the arc gave a very faithful reproduction. A song was reproduced, although the words were not very distinct. This arrangement, which was described as a 'musical arc,' a 'whistling arc,' and more recently as a 'speaking arc,' suggested, to Ernst Ruhmer, of Berlin, in 1901, the idea of his photographophone.

Ruhmer's idea was to photograph the varying light from the speaking arc, and then use the record to reproduce the sound when desired. The making of the record means in reality the taking of a kinematograph picture, not of the arc itself, but of the series of variations of light produced by the arc. The kinematograph film is placed in a light-tight box and is arranged in such a fashion that it can be run from one reel to another at a regular pace by means of a small motor. The rate of travel of the film is about 21 meters per second, which is very much the same rate as that of the wire in the telegraphone. The light of the speaking arc is focused upon the film, which is kept moving within the box. The film is then developed and fixed in the ordinary way, and we have a kinematograph record of the varying amounts of light. These variations may be seen distinctly on the film, forming a series of shaded bands.

In order to reproduce the sound, it is necessary that the photographic film should control a telephone receiver. The connecting link between these two very different things is a selenium cell, such as has been described already in connection with Korn's electrical transmission of photographs. In the photographophone the kinematograph film is interposed between a source of light and a selenium cell, so that a flickering light will fall upon the selenium. The flickerings will, of course, correspond with the variable transparency of the film-record, and as the electrical resistance of the selenium cell will vary in sympathy with the variations of light which fall upon it, the electric current passing through the cell will vary also in the same manner. This variable battery current passes through a pair of telephone receivers placed in circuit with the selenium cell. In this manner the undulations of light produce corresponding undulations of sound.

The distinctness of the reproduced sound is remarkable, and the loudness may be increased by a more powerful source of light to act upon the selenium cell. It is possible to reproduce the sound with a clearness equal to that of ordinary telephone transmission.

The inventor points out that his system of reproducing sound has an ad vantage over other systems, in that he call produce so easily any number of positive copies from his original negative film. A positive or negative film act equally well in the instrument.

Ruhmer suggests that his photographophone might be used in conjunction with the ordinary picture-kinematograp to reproduce actions and sounds simultaneously. For such a purpose he would use a loud-speaking telephone, such as is described below.

Loud-Speaking Telephone

In the end of 1912, a public demonstration of a loud-speaking telephone was given in America. These telephones were fitted up in the large hall at the Boston Electrical Show, and were used to make announcements to the public, and also to distribute phonographic music. One little incident showed a practical reproduction. The difficulty has been that the diaphragm of the transmitter has a certain tone which is fundamental to it, and when the weight of the moving part attached is increased, this fundamental tone is increased also. Indeed, it is necessary to keep the moving mass small to prevent this fundamental tone interfering with the reproduction of the words transmitted. The same difficulty exists in the receiver.

It was found that it would be of very great advantage to use a phosphor-bronze diaphragm in the receiver in place of the usual iron diaphragm. It is obvious that the phosphor-bronze diaphragm cannot be operated directly by the electro-magnet of the receiver. The inventor attached a lever to the diaphragm, as is done in the phonograph between the tympanum and the stylus. The lever, which is attached to the diaphragm of the loud-speaking telephone, has at its free end an iron armature, which is controlled by the electromagnet of the receiver. In the normal condition of the receiver the diaphragm is free from all tension.

Our present purpose is not concerned with the future of loud-speaking telephones, but use of the invention. A child having been separated from its parents in the crowd, was taken to a position of safety, and an announcement of this fact was made immediately throughout the great building, so that no time was lost in bringing the parents and the child together again. It would be most convenient to be able to give instructions to a huge crowd, and there would seem to be quite a wide field of usefulness in enabling a speaker to address an immense audience.

In the Boston demonstration, there were ninety loud-speaking telephones placed throughout the great building, and these were all operated by one telephone transmitter placed in a sound-proof glass-enclosed cabinet, which for demonstration purposes was in the basement of the Grand Hall.

The problem of loud-speaking telephones is to obtain sufficient volume of sound, without detracting from the clearness. In earlier inventions it was usually the case that a really loud-speaking telephone was obtained at the expense of the clearness of articulation, or, on the other hand, that the volume of sound was sacrificed in order to have clear one wonders if it may be that some day our great men will address audiences of a hundred thousand people gathered together in one place.

Submarine Telephone Cables

As already mentioned, the Danish inventor, Valdemar Poulsen, suggested that his telegraphone might be used as a telephone relay. The most urgent requirement for a telephone relay, however, is in a submarine telephone cable, which debars the use of anything mechanical.

As submarine telegraph cables were in use at the time of the invention of the telephone, it was natural to try to speak from Europe to America. But when telephone instruments were attached to the two ends of a trans-Atlantic cable, it was found quite impossible to transmit speech. The trouble was that the electrostatic capacity of the cable, whether it be buried in the sea or in the ground, acts as a leyden jar. The capacity of every ten yards of the cable is about equivalent to the capacity of an ordinary leyden jar, so that the capacity of an Atlantic cable will not be far short of half a million leyden jars. This means that the telephone current had to charge the equivalent of these before it could operate an instrument at the distant receiving station.

Not only does the electrostatic capacity of the cable cause a decrease in the amplitude of the current vibrations, but with the telephone it affects the higher-pitch notes more than the lower ones, causing an inequality in the waveform of the current. The higher-pitch notes are not retarded so much as the lower ones, and this adds to the confusion. Indeed, it was found impossible to carry on conversation through more than twenty miles of submarine cable.

In 1900, Professor Pupin, in the United States, showed that speech was greatly improved by the introduction of coils of wire at intervals in the cable. This invention was founded upon the earlier scientific work of Oliver Heaviside, who showed that if the inductance of a conductor were increased, its capacity could be neutralised.

The term 'inductance' may be defined as the property of a circuit by virtue of which the passage of an electric current, in producing a magnetic field, is necessarily accompanied by an absorption of electric energy. This property makes it more difficult to start or stop a current in the circuit. Electrical inductance is analogous to inertia in ordinary matter, and this might seem to mean that the less inductance the better for the telephone circuit, but it is not so. Heaviside showed that telephone cables wanted greater inductance to neutralise the capacity effect. If we think of capacity as being analogous to a vacuum into which electricity will rush, we can see that inductance will hinder this onrush and neutralise it.

No attempts were made to put these ideas into practice, until Pupin had published his experiments with what he called 'loading coils.' The inductance of a circuit is greatly increased by winding the wire in a spiral, but it would not be convenient to make the conductor of a cable in the form of a continuous spiral. But Pupin showed that if loading coils were introduced at certain intervals, relative to the wave-length, the effect was the same as though the inductance were equally spread throughout the cable. A telephone cable with inductance coils inserted in it every mile or so, is known as a loaded cable, and it is due to this invention that we have greatly improved telephonic communication where long cables are necessary or convenient.

After the publication of Pupin's invention, these loading coils were applied to long-distance overhead telephone lines, and the improvement was very marked. The special interest, however, is in making submarine telephony possible over greater distances. The loading coils consist of rings built up of fine iron wire to act as a core for a coil of silk-wound copper wire.

The engineers hesitated to undertake the laying of a submarine cable having heavy protuberances in it every mile or so. When it was decided to lay a loaded cable in Denmark, the cable was loaded continuously throughout its length, by winding over it many layers of fine iron wire, well insulated from the copper conductor. This plan was adopted in order to simplify the laying of the cable, but the electrical results fell far short of what had been obtained by Pupin's arrangement.

When the British Post Office, in 1910, desired to lay a new telephone cable between England and France, they decided to try a Pupin-loaded cable. This cable required great skill in laying it at the bottom of the English Channel. Special precautions had to be taken in handling the loaded parts of the cable, which occurred about every mile. This cable has proved very successful, but we still await some further advance to enable us to speak by submarine telephone from Europe to America. It seems probable that wireless telephony will make this unnecessary.

Automatic Telephone Exchanges

To any one who has watched the operators in a large Telephone Exchange, it might seem an impossible thing to invent an automaton capable of making all the necessary connections between several thousands of subscribers. But with the opening years of the twentieth century an entirely automatic exchange became practicable.

The pioneer inventors have been Americans, and one of the most ingenious systems is the invention of Almon B. Strowger. His system was adopted in Chicago, and by 1912 there were as many as 30,000 subscribers to the automatic exchanges in that city, while the grand total under the control of Strowger devices was at that time 300,000 subscribers.

In Great Britain we have been making our Manual Exchanges more easily worked by shifting some of the responsibilities on to automatic inventions. For instance, the lifting of the subscriber's telephone receiver lights up a signal lamp indicating to the operator that the subscriber desires to speak. When the operator has put this subscriber through to the other subscriber's instrument, the operator does not require to ring up that subscriber, as the telephone bell rings automatically until the subscriber lifts the receiver to reply. Then, again, when the conversation is finished it is no longer necessary to call off, for when the two subscribers hang up their telephone receivers a lamp glows in front of the operator who made the connection, and the connecting plug may be removed from the jack. In some cases there has been added an automatic cut-out to save the operator the duty of disconnecting at the end of the conversation. It is becoming common practice to register the number of calls automatically by a special meter on the subscriber's line. But the operator still remains to receive instructions and to make the actual connections.

British telephone engineers seem to have been attracted more by semi-automatic exchanges, which are more automatic than is described in the foregoing paragraph, but our present interest is not in the relative advantages between semi-automatic and entirely automatic systems. The latter are of more interest from the invention point of view.

In the Automatic Exchanges there are, of course, no operators at all; there are only mechanical selectors and connectors, which are under the control of the individual subscribers. Instead of a subscriber giving instructions to an operator, the subscriber himself moves a lever on a dial attached to his telephone, and the automatons at the Exchange do the rest.

In the 'two-wire system,' which is perhaps the most easily understood, the subscriber' s dial is marked off in numerals from 1 to 50, and corresponding to each of these numbers is a small hole into which a pin, carried by the indicating lever, may be inserted. The moving of the dial indicator causes a commutator, behind the dial, to rotate one complete revolution for each movement from figure to figure. Each revolution of this commutator sends an electric impulse along the line to the Exchange apparatus. The lever is moved round the dial to the numeral desired, and momentarily locked in that position by inserting the pin into the hole corresponding to the numeral. When released, the lever springs back to zero, and in doing so it rotates the commutator a definite number of times, according to the distance through which the indicating lever had been moved. No matter how slow the subscriber may have been in moving the lever round to the desired number, the actual operation of the commutator is done smartly by the springing back of the lever.

In the earlier dials of the 'three-wire system' there were only ten numerals, 1 to 9 and O. When the subscriber desired to call, say, 5204, he merely pulled the dial round to 5 and let it spring back, then to 2, again to 0, and finally to 4, allowing it to spring back at each move. As already explained, the dial under consideration has 50 numerals. In addition to these, every fifth number is marked off with a letter; A being opposite to 1, B opposite 5, C opposite 10, D opposite 15, and so on. These numerals take the place of the thousands and hundreds in the subscriber's numbers. For instance, instead of a subscriber's number being 1136, it is AA36, and another instead of being 4218 is DB18, and so on. In calling DB18, the subscriber would first move the indicator of his dial to the letter D, insert the pin by depressing the lever, and then allow the lever to spring back. He would then repeat these operations, but taking the indicator to the letter B this time, and then on the next move to the figure 18. At each release of the lever, a definite number of electric impulses reaches the Exchange, and the mechanism by which those impulses make the necessary connections is most ingenious.

First of all the line wire reaches what is called a 'finder.' This is a switch to which 50 incoming line wires are attached. This finder is analogous to an operator ready to respond to any one of 50 subscribers to whom that operator is to attend. The function of the finder is to connect the line of any one of these subscribers to a 'selector,' and this is accomplished in the following manner.

In the finder switch the 50 incoming lines are fixed to 50 contact pieces in an are of a circle. A lever or contact arm can sweep over these, touching each one in succession, as it describes the arc. There is, attached to the finder switch, a vibrating relay, the action of which is similar to that of the gong_ stick in a trembler bell. This vibrating relay furnishes an intermittent current to a motor_ magnet which operates a ratchet wheel and moves the contact arm over the 50 contact pieces, which represent the ends of the subscribers' telephone lines. Whenever any one of the 50 subscribers lifts his telephone receiver off his instrument, the vibrating relay on the finder switch is set in motion, and the contact arm will touch each contact in succession until it reaches the contact to which the calling subscriber's line is attached. Here it finds no earth connection, owing to the subscriber's telephone having been lifted, and the motor-magnet is cut out, leaving the contact arm at rest on the desired telephone wire. In other words, the finder switch has connected itself automatically to the line of the subscriber who is calling, and it is ready to conduct any signals from his dial to the 'first selector' switch in the Exchange.

Of course, this finder switch might be dispensed with if each subscriber's line had a special selector to itself, but this would be analogous to every subscriber to a Manual Exchange having a special operator to attend to that one line.

So far the subscriber has merely lifted his telephone, preliminary to initiating a call, but, without any guidance or even a thought on his part, the finder switch has connected his particular line to a selector. The subscriber now proceeds to signal the telephone number of the subscriber to whom he desires to be connected. This he does, by moving the indicator of his dial, in the manner already described.

But it will simplify matters to form a mental picture of the distribution of all the subscribers' lines in the connecting switches, and it will serve our purpose to picture an Exchange having 5000 subscribers. We have dealt already with a group of fifty subscribers, whose lines are all connected to one finder switch. We may add to our present picture one hundred such finder switches, having in all 5000 lines under control. We have seen that the function of these finders is to connect the calling subscriber to a selector, from which further connections are to be made. Those further operations will be understood more easily if we leave off at this point, and consider the other end of the arrangement, the lines as they leave the Exchange to carry the telephone current to the subscriber being called.

In the first place we picture one 'connector,' from which fifty lines go out to as many individual subscribers. This connector has the fifty lines connected to contact pieces in the form of an arc; indeed, it is just like a finder switch used conversely. In the finder switch the contact arm conducted the incoming current from one of the fifty lines to the selector, whereas in the connector the contact arm conducts the outgoing current from a selector to one of the fifty outgoing lines. As each connector controls fifty subscribers' lines there will be one hundred such connectors to operate the 5000 lines.

These hundred connectors are divided into ten sections of ten connectors each. Tracing the connections leading to the outgoing lines, we find that the ten contact arms of the ten connectors are connected by as many wires to a selector, which, of course, has ten corresponding contact pieces in its arc. As each of the ten sections of connectors has one of these selectors, there will be ten such selectors in all. The ten contact arms of these ten selectors are each connected by a wire to another selector, known as the 'first selector.' This first selector is the one which we have considered already in connection with the finder switch, and to which the finder connects the calling subscriber.

Looking at the whole arrangement now from the caller's end, we picture his line entering one of the finder switches to which it is permanently connected. Then we find the contact arm of this switch linking the subscriber's line to the first selector. The subscriber now signals the first letter, say of the number desired. The impulses sent along the line by this signal cause the motor-magnet on the first selector to move its contact arm to the fourth contact piece, which leads to 'D' switch. The subscriber is now connected through to this particular 'second selector,' and his second signal, say 'B,' operates the motor-magnet in this second selector, moving its contact arm to the second contact piece, which leads to the 'B' connector. The subscriber's line is now connected to the contact arm of the connector in which the line he desires is permanently situated. His next signal, say '18,' operates the motor-magnet of the connector, moving its contact arm around the arc to the eighteenth contact piece, whereupon the two telephone lines are connected together. In this manner the calling subscriber has been 'put through' automatically to the subscriber to whom he desires to speak.

In the foregoing description we have traced all the steps by which any one of the fifty subscribers' lines situated in one finder switch can be connected at will to any one of the total 5000 subscribers' lines. If this were all, it would mean that only one of each fifty subscribers could speak at the same time. So it becomes necessary to provide six finder switches for each group of fifty subscribers, and the arrangement is that any six of the fifty can get through at one time. This permits twelve percent of the subscribers to speak simultaneously, and that is a larger percentage than is possible with the purely Manual Exchange, in which case an operator has only the means of putting ten percent of her subscribers through at one time.

But this multiplication of finder switches means a corresponding multiplication of first selectors; one for each finder. And instead of there being only ten second selectors connected to each first selector, there are forty. The contact pieces for connecting the first selectors to the second selectors are increased accordingly, and the contact arms automatically hunt out a non-busy line. All this arrangement is necessary in order to ensure that a calling subscriber may find a disengaged path all the way through to the subscriber whom he is calling.

It will be observed that the multiplicity of connecting wires in an Automatic Exchange differs from the multiplicity of wires in a Manual Exchange. In the latter it is a case of multiplying the connections of each individual subscriber, so that the whole of the subscriber's lines are repeated and repeated right throughout the multiple-board till each set of operators has a complete set of all the subscribers' lines connected to the Exchange. In the Automatic Exchange, it will be observed that it is the connecting wires that are multiplied, each subscriber's line being taken to one connector only.

Even with all this provision of different paths by which the calling subscriber may be put through to another subscriber, it may happen that the particular subscriber being-called is already engaged. In this case a relay, situated at the connector, will switch on an intermittent current, and the calling subscriber, hearing this engaged signal in his receiver, replaces it, to call up again a little later. It is quite impossible for him to connect his instrument to any other telephone which happens to be engaged.

When a subscriber's manipulation of his signalling dial has put him through to the instrument of the subscriber to whom he desires to speak, he pulls the indicator a short distance and allows it to spring back once more. The consequent revolutions of the commutator attached to his dial energies another relay, which switches on the ringing current to the line to which he has already gained a through connection. The distant subscriber's bell continues to ring until the receiver is lifted in the act of responding to the call. When, at the end of the conversation, the two subscribers hang up their receivers, a releasing relay is operated on each of the pieces of connecting mechanism, and the respective contact arms are moved back to zero, and are then ready to receive impulses from any other subscriber.

It might be supposed that any fault occurring in a finder, a selector, or a connector, would necessitate a considerable delay until repairs were made. But none of these switches have their connecting wires soldered to them; they are fitted with plugs and jacks. If a fault should occur, a mechanic merely pulls out the defective switch and inserts a complete duplicate in its place, leaving the defective one free to be repaired at leisure. In this way two mechanics are able to undertake to keep a large automatic exchange in good working order.

In connection with the Manual Exchanges in this country, we have automatic pay-boxes at public call offices, but the operator at the Exchange instructs us when she is ready to attend to the money-box. She instructs us how many pennies we are to place singly in the box, and we have to turn a handle round for each penny separately. By this simple means we switch on a signalling current to the operator, who, when satisfied with our honesty, connects our line to the line of the subscriber to whom we desire to speak.

In America they have most ingenious money-boxes, which are entirely automatic. The subscribers to the Automatic Exchanges may have these money-boxes attached to their telephone instruments, and pay for each call as they go along, instead of being taxed according to an automatic meter. Suppose that the subscriber to whom he desires to speak is situated outside of the central area, and that there is a charge of ten cents for the first three minutes, and only five cents for each succeeding three minutes. This might seem to be beyond the capabilities of an automaton, but not only will this money-box undertake these duties, it will accept any number of nickels the subscriber may care to place in its hopper, and at the end of the conversation it will return to him any money not required for the length of conversation which has taken place.

The money-box holds the nickels in trust for the calling subscriber until the distant subscriber responds by lifting the receiver from his telephone. Thereupon the hopper device automatically drops ten cents from the pile of nickels into the bottom of the box, and the subscribers can converse together. At the end of the first three minutes, a disc at the central office makes contact and causes other five nickels to drop into the bottom of the box, leaving the line connected through for other three minutes. At the end of the next three minutes the same operation takes place, and will be repeated so long as the subscribers keep their telephones off the hooks, and so long as the money lasts.

If it happens that the telephones are replaced before the money is exhausted, the money-box will automatically open and deposit the remaining coins on a tray beside the subscriber. If, on the other hand, the subscriber has not put sufficient nickels in the hopper, the automaton does not disconnect him, without first of all giving him an opportunity of adding the necessary coins. If it did, he would have to initiate a new call at the expense of ten cents, in place of five, for the next three minutes. Suppose lie has anticipated only a conversation of three minutes, but at the end of that time he is still engaged in conversation, and the money in the hopper is exhausted. In this case his line is left through, but he hears his friend asking where he has gone, as he cannot hear him. The subscriber realises that his time is up, and that he must deposit more nickels in the hopper. As soon as he does so, the hopper counts out five cents for the box and then allows him to continue his conversation for other three minutes.

The trunk charges to certain districts may be at reduced rates after a certain hour in the evening; but the automaton can deal with this case also. The subscriber finds that in these circumstances his money-box does not call for a second payment until the end of a six minutes' conversation, and so on. To accomplish this, all that is necessary is that the mechanic at the Exchange turns a certain switch at a certain hour, and even this could be done automatically by clockwork if desired.