At the intervals of about 60 m. substantial brick buildings were
erected for use as repeater stations to house the equipment,
apparatus, power plant and test boards that make up a modern repeater
station. General plans were completed for extending this cable from
Pittsburgh to Chicago.
The installation of these toll cables resulted in economies due not only to the reduced annual charges on additional circuits as required and less expense for routine maintenance, but also to the fact that the losses resulting from storm damage to open wire were avoided, as were also losses in revenue and reaction on the service during storm periods. Much is thus done to stabilize the toll plant and reduce expenditures as well as further to improve the service. By means of the improvements which had been made it became possible to carry on satisfactory talks over wires in cables more than 2,000 m. in length (where commercial conditions justify such cables) and this was accomplished with practically no more copper in each circuit than had been used in the earliest forms of cable which, as lately as 1882, caused serious interference with transmission when employed in lengths of only a fraction of a mile.
Repeater development reacted on the loading art, requiring the development of loading coils of great magnetic stability and uniformity. Such stability is also particularly important on long circuits which are composited for telegraph operation in order to prevent the telegraph from interfering with the telephone transmission. Although these loading and repeater developments greatly extended the use of cables for long-distance transmission they made it imperative to keep open-wire circuits as free as possible from cable in order to prevent the electrical irregularity thus introduced from reacting on the repeater operation.
Submarine Cables.—In 1921, telephone communication was established with Cuba by means of submarine cables connecting Havana with Key West. These cables brought all of the principal places in the United States into telephonic communication with Havana and other important places in Cuba. There were in 1921 3 cables, each about 115 m. in length. Except at the terminating points, the cables were laid some miles apart in order to minimize the danger of simultaneous interruption as the result of accident. The average depth was about 3,000 ft. and in some places depths of more than a mile were reached. The main portion of each cable had a single conductor, two conductors being employed in the shore ends. The main conductor weighed 350 lb. per m. and consisted of 7 strands of copper wire. The conductor bore a wrapping of fine iron wire, this being covered with gutta-percha enclosed in copper tape which served as a return grounded conductor. By the use of multiplex methods each cable handled simultaneously one telephone and two telegraph messages. Each cable is expected ultimately to handle two or more additional telegraph messages. The use of single conductor cables, the telephone amplifiers, the terminal telegraph apparatus, and the devices for permitting the telephone and telegraph to operate simultaneously, all differed from earlier practice.
The largest submarine cable equipped with loading coils in 1921 was that which crossed Raritan Bay from Staten Island to New Jersey. It was upwards of 28,000 ft. in length, was loaded at 5 points and contained 37 quads of No. 16 gauge wires and 12 pairs of No. 22 gauge test wires. Each loading pot was approximately 16 ft. long and weighed 4 tons. The cable was laid in shallow water, the average depth being 10 ft. at mean low tide.
Carrier Current Telephony.—From the earliest days of the telephone and telegraph there were many attempts to develop multiplex transmission of messages. It was while working on the problem of multiplex telegraphy that Dr. Bell had his first conception of the structure of the original telephone. The long series of inventors, scientists and engineers who have contributed to the development of the multiplex art includes Gray, Edison, Mercadier, Pupin, Hutin, Leblanc, Stone, DeForest, Vreeland, Ruhmer, Squier, Wagner and others. In 1918 research experts and engineers of the Bell System completed the development of a commercial multiplex telephone and telegraph system and put it into operation between Baltimore and Pittsburgh. By means of this multiplex system, 4 telephone conversations may be had simultaneously over one pair of wires in addition to the telephone conversation provided by the ordinary methods. Thus, over a single pair of wires, 5 telephone conversations are simultaneously operated, each giving service as good as that provided by the circuit working in the ordinary way.
In telegraphy, as compared with the ordinary duplex telegraph circuit, this multiplex system permits at least a tenfold increase in messages. Although the commercial installations in use in 1921 provided only 4 additional conversations, the limitations as to number of telephone or telegraph messages on a single circuit were determined entirely by economic considerations. The operation may be considered to consist of combining the telephone current with high frequency current, transmitting this combination over a line wire, and, at the receiving end, removing the high frequency current and leaving the telephone current. The high frequency current serves as a “carrier” for the telephone current over the line.
Simultaneous transmission of several telephone currents is accomplished by means of selective apparatus by which one particular receiving channel is made easily receptive to one particular set of high frequency currents and, at the same time, acts substantially as a barrier to the currents of other high frequencies which are carrying telephone conversations other than those which the channel in question is designed to receive.
The operation involves the following steps: (1) Generation of carrier current, (2) Modulation, (3) Demodulations, (4) Separation of channels by selective circuits, (5) Repeaters for amplifying currents of carrier frequency at intermediate points.
(1) Generation of Carrier Current.—Carrier currents of various frequencies are for convenience obtained from well-known forms of vacuum-tube oscillators. In general the telephone multiplex frequencies run about 10,000, 15,000, 20,000 and 25,000.
(2) Modulation.—This term is applied to the process by which carrier current, produced by an oscillator, is so combined with voice currents from a telephone transmitter that the variations of the latter are impressed upon the former. The carrier and voice frequencies are applied together in the grid circuit of a vacuum-tube modulator together with a steady battery voltage.
(3) Demodulation.—This is a complementary process of modulation. Modulation may be thought of as elevating the band of essential speech frequencies to a position adjacent to the carrier frequency, and demodulation may be regarded as restoring this band to its normal position in the frequency scale.
(4) Separation of Channels by Selective Circuits.—When a number of channels, each employing a different carrier frequency, are operated simultaneously on a common line, each channel must be connected with the line through selective circuits which transmit only the range of frequencies assigned to that particular channel. Not only must the demodulator assigned to a given channel be prevented from receiving, from the line, currents of other channels, but the sending modulator must be prevented from putting on the line currents or frequencies outside of its assigned band. The appurtenances specially developed for accomplishing this selection in carrier current telephony are known as “band-pass electrical filters.”
(5) Reaction on the Telephone Plant.—Carrier currents have imposed new requirements as to transpositions and it has also been necessary to develop new types of loading coils capable of transmitting the carrier frequencies, and also extremely uniform in impedance over the whole frequency range.
Limitations.—From the nature of the apparatus and methods employed, the system is not practically advantageous on short lines. In 1921 it was being applied to lines of 250 m. or more.
Carrier telephone systems were in commercial operation between the following points: Baltimore and Pittsburgh, Harrisburg and Chicago, Harrisburg and Detroit, Boston and Bangor, San Francisco and Los Angeles.
Loud-Speaking Telephones.—By the use of vacuum-tube amplifiers in connexion with specially developed transmitters and receivers, supplemented by large projecting horns, the human voice may be magnified thousands of millions of times so that a public speaker can make himself heard by a vastly greater number of people than ever before. By the use of apparatus and methods of this kind developed by the Bell Telephone System, President Harding's inaugural address in 1921 was heard by over 100,000 listeners standing in an open space of more than 10 ac. before the Capitol.
Radio Telephony.—In 1915 the engineers of the Bell Telephone System succeeded in transmitting speech from Arlington, Va., to the Eiffel Tower in Paris, and, simultaneously, to the Hawaiian Islands in the Pacific Ocean. Two experimental radio telephone transmitting and receiving stations were erected on the Atlantic Coast, one near Asbury Park, N.J., and the other near Plymouth, Mass. By means of these stations, radio telephone communication was maintained between the commercial telephone system and two ships experimentally equipped, plying from Boston to southern ports on the Atlantic Coast.
In July 1920 regular commercial radio telephone service was established between Santa Catalina Is. about 30 m. from shore, and the mainland near Los Angeles, Cal., at the latter point making junction with the local and long-distance wires of the Bell System throughout the United States.
The circuit is provided with through-line ringing of a type which is free from interference and there is a superimposed telegraph circuit capable of forming a link in a duplex wire telegraph circuit. The volume and quality of telephone transmission are so good that the radio link is regularly connected, whenever required, with long-distance wire circuits. On several occasions conversations have been carried on between a steamship on the Atlantic and the Avalon office at Catalina Is. in the Pacific, using the transcontinental wire telephone line as the connecting link overland.
Machine-Switching System.—A retrospective examination of the manually operated switchboard discloses the fact that the tendency of development has been continuously in the direction of increasing the number and extent of the operating functions which are performed electro-mechanically and likewise decreasing the amount of time required of the operator for the handling of the connexion. When a point is reached where the operations performed manually at the central office are eliminated, except in the case of certain special classes of calls, the term “automatic” or “machine switching” is applied to the switching equipment.
There are two principal types of machine-switching equipment, the “step-by-step” and the “panel” type. In both the apparatus
