For a long time inventors have been striving to do away with all such defects by abolishing the exchange operators, and substituting mechanism to work the exchanges automatically, and during the last few years the system of the Automatic Electric Company, of Chicago, has been brought to great perfection. This system is in extensive use in the United States, and is employed in two or three exchanges in this country. Unfortunately the mechanism of this system is extremely complicated, so that it is impossible to describe it fully in a book of this kind; but some idea of the method of working may be given without entering into technical details.
Each subscriber’s telephone instrument is fitted with a dial which turns round on a pivot at its centre. This dial has a series of holes round its circumference, numbered consecutively from 1 to 9, and 0. Suppose now a subscriber wishes to speak to a friend whose telephone number is 2583. He removes the receiver from its hook, places his finger in the hole marked 2, and turns the dial round in a clockwise direction until his finger comes in contact with a stop. He then removes his finger, and the dial automatically returns to its original position. He then places his finger in the hole marked 5, and again turns the dial as far as the stop, and when the dial has returned to the normal position he repeats the process with his finger placed successively in the holes marked 8 and 3. He now places the receiver to his ear, and by the time he has done this the automatic mechanism at the exchange has made the necessary connexions, and has rung the bell of subscriber number 2583. On completing the conversation each subscriber returns his receiver to its hook, and the exchange mechanism returns to its normal position.
The turning of the dial by the finger coils up a spring, and this spring, acting along with a speed governor, makes the dial return to its first position at a certain definite speed as soon as the finger is removed. During this retrograde movement a switch automatically sends out into the line a certain number of impulses, the number being determined by the hole in which the finger is placed. In the case supposed, groups of two, five, eight, and three impulses respectively would be sent out, each group separated from the next by an interval during which the subscriber is turning the dial.
Now let us see what takes place at the exchange. The subscriber’s instrument is connected to a mechanical arrangement known as a “line switch.” This switch is brought into play by the act of removing the receiver from its hook, and it then automatically connects the subscriber’s line to what is called a “first selector” switch. The group of two impulses sent out by the first turning of the dial raises this first selector two steps, and it then sweeps along a row of contacts connected to “trunks” going to the 2000 section. Passing by occupied trunks, it finds an idle one, and so connects the line to an idle “second selector.” This selector is operated by the second group of impulses, five in number, and after being raised five steps it acts like the first selector, and finds an idle trunk leading to the 2500 section. This places the caller’s line in connexion with still another switch called a “connector,” and this switch, operated by the remaining groups of eight and three impulses, finds the required tens section, and selects the third member of that section. If the number 2583 is disengaged, the connector switch now sends current from the central battery to this instrument, thus ringing its bell, and it also supplies speaking current to the two lines during the conversation, restores the exchange mechanism to its original condition as soon as the conversation is ended and the subscribers have hung up their receivers, and registers the call on the calling subscriber’s meter. If the connector finds the number engaged, it sends out an intermittent buzzing sound, to inform the caller of the fact. All these operations take time to describe, even in outline, but in practice they are carried out with the utmost rapidity, each step in the connecting-up process taking only a small fraction of a second.
For ordinary local calls the automatic system requires no operators at all, but for the convenience of users there are usually two clerks at the exchange, one to give any information required by subscribers, and the other to record complaints regarding faulty working. For trunk calls, the subscriber places his finger in the hole marked 0, and gives the dial one turn. This connects him to an operator at the trunk switchboard, who makes the required connexion and then calls him up in the usual way.
It might be thought that the complex mechanism of an automatic exchange would constantly be getting out of order, but it is found to work with great smoothness. Each automatic switchboard has a skilled electrician in attendance, and he is informed instantly of any faulty working by means of supervisory lamps and other signals. Even without these signals the attendant would be quickly aware of any breakdown, for his ear becomes so accustomed to the sounds made by the apparatus during the connecting-up, that any abnormal sound due to faulty connecting attracts his attention at once. However detected, the faults are put right immediately, and it often happens that a defective line is noted and repaired before the subscriber knows that anything is wrong.
On account of its high speed in making connexions and disconnexions, its absolute accuracy, and its privacy, the automatic telephone system has proved most popular wherever it has been given a fair trial. Its advantages are most obvious in large city exchanges where the traffic during business hours is tremendously heavy, and it is probable that before very long the automatic system will have replaced manual methods for all such exchanges.
The telephone system is more highly developed in the United States than in this country, and some of the exchanges have been made to do a great deal more than simply transmit messages. For instance, in Chicago there is a system by which a subscriber, on connecting himself to a special circuit, is automatically informed of the correct time, by means of phonographs, between the hours of 8 a.m. and 10 p.m. New York goes further than this however, and has a regular system of news circulation by telephone. According to Electricity, the daily programme is as follows: “8 a.m., exact astronomical time; 8 to 9 a.m., weather reports, London Stock Exchange news, special news item; 9 to 9.30 a.m., sales, amusements, business events; 9.45 to 10 a.m., personal news, small notices; 10 to 10.30 a.m., New York Stock Exchange and market news; 11.30 a.m. to 12 noon, local news, miscellaneous; 12 noon, exact astronomical time, latest telegrams, military and parliamentary news; 2 to 2.15 p.m., European cables; 1.15 to 2.30 p.m., Washington news; 2.30 to 2.45 p.m., fashions, ladies’ news; 2.45 to 3.15 p.m., sporting and theatrical news; 3.15 to 3.30 p.m., closing news from Wall Street; 3.30 to 5 p.m., musical news, recitals, etc.; 5 to 6 p.m., feuilleton sketches, literary news; 8 to 10.30 p.m., selected evening performance—music, opera, recitations.” Considering the elaborate nature of this scheme one might imagine that the subscription would be high, but as a matter of fact it is only six shillings per month.
The telephone has proved of great value in mine rescue work, in providing means of communication between the rescue party and those in the rear. This end is achieved by means of a portable telephone, but as the members of a rescue party often wear oxygen helmets, the ordinary telephone mouthpiece is of no use. To overcome this difficulty the transmitter is fastened round the throat. The vibrations of the vocal cords pass through the wall of the throat, and thus operate the transmitter. The receiver is fixed over one ear by means of suitable head-gear, and the connecting wire is laid by the advancing rescuers. A case containing some hundreds of feet of wire is strapped round the waist, and as the wearer walks forward this wire pays itself out automatically.
By the time that the telephone came to be a really practical instrument, capable of communicating over long distances on land, the Atlantic telegraph cable was in operation, and an attempt was made to telephone from one continent to the other by means of it, but without success. In speaking of submarine telegraphy in [Chapter XVII]. we saw that the cable acts like a Leyden jar, and it was this fact that made it impossible to telephone through more than about 20 miles of cable, so that transatlantic telephony was quite out of the question. It was evident that little progress could be made in this direction unless some means could be devised for neutralizing this capacity effect, as it is called, of the cable, and finally it was discovered that this could be done by inserting at intervals along the cable a number of coils of wire. These coils are known as “loading coils,” and a cable provided with them is called a “loaded cable.” Such cables have been laid across various narrow seas, such as between England and France, and England and Ireland, and these have proved very successful for telephonic communication. The problem of transatlantic telephony however still remains to be solved. Experiments have been made in submarine telephony over a bare iron cable, instead of the usual insulated cable. Conversations have been carried on in this way without difficulty between Seattle, Washington, U.S.A., and Vashon Island, a total distance of about 11 miles, and it is possible that uninsulated cables may play an extremely important part in the development of submarine telephony.