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HISTORY OF THE WIRE COMMUNICATIONS: BEGINNINGS OF THE TELEGRAPH AND THE TELEPHONE. |
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History of the telephone central . The idea to establish
a telephone central , by which a telephone could connect with any
other telephone , seems to have been suggested by Edwin T. Holmes,
who was be the first person to have a home telephone , and directed
a central of this class in 1877 in connection with his alarm system
against burglars in Boston. The transmission of the word, in the
beginning, was so incomplete, that the subscriber had to refer to
the telephone central operator the message that was to be repeated
to the other subscriber. |
Tending a great phone cable for 1200 people of the American Telephone & Telegraph Co. toward 1890. |
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Each one of these operators was provided with a receiver and a transmitter, kept in position in their heads by means of a sheet or cap, being therefore the operator's hands free. The front of the board was perforated by a great number of small holes called "jacks" and next to each hole a tiny electrical lamp was placed. Each one of these holes represented the end of a telephone line. Between the operator and the vertical face of the bard there was a narrow shelf, from where jutted out hundreds of terminals with the brass extremity. These ones were called "pins", and were linked to the flexible cord ends, of advisable length. |
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When a subscriber took his receiver off the hook, one of the tiny lamps of the operator's board shined, and the closest operator took one of the pins and she inserted it in the jack adjacent to the ignited lamp. Then the lamp extinguished, but at the same time another one in the bank next to the flexible wire ignited . In that moment the operator closed a commutator located in the bank or shelf that connected her telephone with the subscriber'one and said: "Central" When receiving the desired number , the operator took another pin, connected it under the bank to the first one, inserted it in the jack that belonged to the requested number and pushed a button that made sound the bell of the telephone of the person to whom the subscriber was calling. As soon as the person answered the call, picking up the receiver , the first adjacent lamp to the flexible wire turned off , indicating the operator that it had been done the requested connection. As the telephone of the operator was disconnected of the line after receiving the wished number, she was freed to establish another connections. When the subscriber in a line hanged up the receiver in the hook, the flexible wire adjacent lamp of the corresponding subscriber turned on , the operator pulled the pin, turning off the lamp, and the pin was placed again in the shelf. In an active telephone central the lamps of the board were continuously turning on and off , accompanied by the calls, "central! ", and the tictac of the pins. To the visitor the front of the board looked extremely simple; but in the reverse it had a more complicated construction, as it is seen in the image in this page. Another type of telephone central , whose use increased soon as the automatism technologies progressively replaced the operators, was that in which the connections were made by an automatic machine, that was directed by the person who made the call. Instead of waiting for the operator to ask the telephone number , the subscriber, automatically connected his telephone with that one of any other subscriber turning a numbered sphere with the successive numbers of the wished telephone. The automatic machine (a typical model can be seen in this page) connected both telephones, and the subscriber who called could then make directly ring the bell of the telephone of the other subscriber , representing this device the automatic telephone central in its first stages of development . How had been perfected progressively
the lines of great length. |
Reverse of a telephone switchboard in use towards the 1920's : Here the terminals of the different telephone lines concurred, so that all of them were within reach of the operator who was seated at the other side of the board.
The machine that replaced the ears and human arms of the operator. Photo of one of the first electrical telephone centrals in use at the beginning of the XX century . The subscriber indicated in his telephone dial wheel the number of the telephone that he needed to contact, and electrical currents permitted in the telephone central soon to put in movement a mechanism that made all the work to establish the communication in both lines. |
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DISPOSITION OF
A TELEPHONE SWITCHBOARD
Antigua foto de una gran central telefónica de la ciudad de Nueva York , donde se pueden ver a las operadoras atendiendo a los abonados telefónicos . A professor of the University of Columbia, the Dr Michael I. Pupin (1854 - 1935), recognized that this disturbance was due to the uneven transmission of the different voice frequencies, and made a mathematical investigation of the existing conditions in order to find a remedy. The study of the mathematics is for many people less attractive than many other matters, because it seems to them that it leads to less practitioners results. The error of this opinion is clear in this example, as in many other, because the mathematical investigation of Dr Pupin discovered the way to remedy the imperfect transmission in the long lines .He advised to the telephone companies to put coils in determined places of their lines and he predicted them an improvement in the transmission of the language .The lines between New York and Chicago thus were equipped with such "inductance coils", and with noticeable success .Later they were equipped with coils the western lines of Chicago to Denver, so it was then possible to speak clearly between New York and Denver, at a distance of 3,520 kilometers. Other coils of analogous function were soon installed in the underground telephone cables that linked Boston, New York, Philadelphia and Washington. London and Paris had later been connected by a telephone cable provided with these coils through the English Channel, and other continental lines, being born in Paris, finally linked London to Berlin, Vienna and Rome. Although the invention of the Dr
Pupin allowed to increase the distance at which it could be transmitted
the human voice, the transcontinental communications had been doubtlessly
impossible without the additional installation of "repeaters"
that worked in conjunction with the inductance coils. The telegraph
companies used repeaters many years before but the type used by
them was of too slow action to reproduce the human voice. After
having tested many artifices, that resulted defective, it was found
finally a repeater or satisfactory amplifier based on the principle
of the lamp of three electrodes (denominated triode) that consisted
of a crystal tube, where the most perfect air emptiness possible
was produced, containing an incandescent filament, a grill or grid
of threads and a thin plate (anode and cathode), placed next to
others in the mentioned order. If both ends of a telephone line
were connected respectively to the grid and to the filament, the
telephone current sent to an prolongation of this line connected
to the plate and the filament was many times greater than in the
first line, and it varied in intensity in an exact reproduction
of the first weaker current. The installation of these amplifiers in several points between New York and San Francisco made possible in those years to speak clearly between these two cities. With the completion of the submarine telephone cable between Cayo Hueso and Cuba it was achieved the telephone conection between Cuba and the Catalina island . The voice in this case was transmitted under the Ocean, from Cuba to the United States, crossing the continent to California and soon it gave the final jump by radiotelephony to Catalina , in a total distance of 8,752 kilometers. The constant growth of traffic in
the long distance telephone lines in the United States in 1930 had
led to the need to find some methods of multiple telephony that
would allow to transmit several telephone messages simuntaneously
by a pair of wires. El first step in this direction was given with
the introduction of the circuit called ghost by means of which up
to three telephone messages by two pairs of wires could be transmitted
simultaneously. Another later development of transmission was a method of telephony called "messenger", that allowed to transmit several telephone messages by a pair of wires, in any direction, at the same time. In this system, each person, when speaking in the transmitter, made continuously vary the intensity of an alternating current of very high frecuency maintained in the line. This alternating current was produced by a lamp of three electrodes, similar in the construction to the telephone amplifier. One said that the person who spoke in the transmitter "modulated" this current of high frequency that served as a messenger (or carrier) of the vibrations of the voice, but returned so quickly to its state that it did not produce any sound on the receiving telephone. The vibrations of the voice reproduced, nevertheless, in the diaphragm of the receiver, and the transmission of the language was as clear as in the ordinary line. The most important element in the
multiple messenger system was an artifice in the receiving station
called "filter". The filters (equivalent to an actual
splitter of frequencies) was connected to each end of the line between
each pair of telephones. Its mission consisted of not letting circulate
through them but a selected carrier current , so that when several
carrier currents of different frequencies circulated in the telephone
line at the same time , each filter in the receiving station allowed
to pass only one of those carrier currents (that means that the
filter separated the frequencies) to the receiver. This way, the
different telephone conversations transmitted by a pair o wires
simultaneously were classified in the receiving station and transmitted
to the corresponding subscribers. The same system had been applied
to the multiple telegraphy so that 20 telegraphic dispatches by
a pair of wires could be sent at the same time.
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