Telephone Cables

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wansea ara htan mouth k Plymouth Maidenhead Derb Marlborough Guildford Particulars of Representative Types of Loaded Main Underground British Cables. * ioo-lb. conductors used for telegraphs. t Not phantomed. $ In course of construction.

latter. The effect was to reduce materially the attenuation constant and increase the range of speech in loaded cables.

The improved dielectric was used in a cable laid in Aug. 1912 between St. Margaret's Bay, Dover, and La Panne, Belgium. This cable contained four copper conductors, each weighing 160 lb. per nautical mile and insulated by a dielectric weighing 150 lb. per mile (as compared with 300 lb. per mile in the 1910 Anglo-French cable). The variation of attenuation with frequency is much less in the 1912 cable than in the earlier one.

The Anglo-Belgian cable had another special feature, namely, the provision of loading coils for a third circuit superposed on the two physical circuits. The loading coils for all three circuits were placed together at intervals of one nautical mile.

A similar cable with some further improvements in dielectric and loading coils was laid across the Irish Sea between Nevin, Carnarvonshire, and Howth, co. Dublin, in 1913.

A submarine telephone cable of the continuously loaded type was laid across the English Channel by the French Government in 1912, between the same points as the 1910 coil-loaded cable. The weight per nautical mile of dielectric is the same in both cables, but each copper conductor of the former weighs 300 lb. per mile as compared with 160 lb. per mile in the latter. The transmission efficiency of the cables is practically equal, but the continuously loaded cable provides an additional circuit by superposing. Experiments conducted on this cable in 1914 proved the possibility of obtaining four circuits from a continuously loaded 4-wire submarine cable by introducing an improved method of balancing the electrostatic capacity of the conductors. The fourth circuit has not yet been successful in a coil-loaded cable.

Several additional coil-loaded telephone cables were laid across the English Channel during the war period. Details of these cables are given in the Table.

With equal weights of conductor and dielectric, the relative transmission efficiencies of (a) coil-loaded and (b) continuously loaded 4-wire submarine cables are as 100 to 75, but the latter may provide four circuits as compared with three in the former. Experience has shown that the maintenance and repairs of coil-loaded cables are attended by difficulties which are not met with in continuously loaded cables.

The introduction of telephone relays has made it possible frequently to use submarine cables of a less efficiency than the coil loaded cables previously required. Consequently, it is practicable to increase the use of continuously loaded cables, and the modern tendency is in that direction. (W. No.) United States The more important improvements made in the United States during 1910-21 are briefly described below.

Exchange Cables.-Improvements in the design and the methods of manufacture of cables for use in local exchanges made it possible greatly to increase the number of wires of a given size in a sheath of given size. By employing wires of smaller diameter than those heretofore used the maximum number was still further increased. Cables containing either goo wires No. 19 A.W.G. ( 0359 in. diam.), 1,800 wires No. 22 A.W.G. ( 0253 in. diam.), or 2,400 wires No. 24 A.W.G. ( 0201 in. diam.) were extensively used in 1921. The improvements which rendered practicable these cables of maximum diam. have been employed also in cables of fewer pairs, thus enabling their diams. to be decreased and their costs reduced. Cables containing the smaller sizes of wire were used as extensively as was justified by their economic balance in relation to other portions of the plant. This resulted in the employment of considerable amounts of No. 24 A.W.G. conductor cable.

For a long time cable sheaths were made of lead alloyed with about 3% of tin, unalloyed lead not having the requisite strength and resistance to corrosion. Extensive research, directed toward finding a cheaper but no less effective alloy, resulted in 1912 in the adoption of lead alloyed with a small amount of antimony. Readjustments in the thicknesses of sheaths and in the composition of the insulating and binding paper produced still further economies.

Loading Coils in Exchange Service.-Many thousands of trunk circuits in multi-office exchanges and circuits connecting large cities with suburban points have been equipped with loading coils, pro British Submarine Telephone Cables. viding transmission of such a grade as would require from 5 to TO times as much copper in the cable circuits if loading were not employed. Loading coils have been materially improved by constructing the cores of several rings, each of which is made by compressing finely divided particles of iron with a binding material which acts as insulation between the iron particles. There may be as many as thirty thousand million of these particles in the core of a cable loading coil. These cores are more uniform and stable than the wire cores formerly used and are much less affected by excessive currents which may accidentally come into the circuit.

Long-Distance Telephon

Open Wire. - At the beginning of the decade 1910-20, the limits of telephone transmission were about 1,200 to 1,500 m. in open wire. These limits were extended rapidly so that in 1921 practically all parts of the continental United States were placed in communication with each other over distances of 4,000 m. and upwards, employing overhead wires no larger than those used to give the restricted service of 1910. These improvements were made with only slight changes in the lines and equipment and with no change whatever in the subscriber's station apparatus. They depended upon the development of satisfactory repeaters with their associated apparatus and methods of use. The form of repeater generally employed in 1921 was the 3-element thermionic tube. Devised primarily for radio purposes, it was so adapted as to become a remarkably effective repeater. This required that a large amount of auxiliary apparatus be invented and developed and methods devised for balancing the lines and making them suitable for the operation of this apparatus. The amplifier or repeater receives the minute attenuated telephone currents and sends out currents of exactly the same form but greatly enlarged. The transmission gain which may be obtained with vacuum-tube amplifiers in two-way operation depends on the electrical conditions of the line in which the amplifiers are used. This has a great effect on line design.

Transcontinental Telephony

By the development of methods by which the loading coil could be applied to the heaviest gauge wires and such wires, when equipped with loading coils, could be operated on the phantom principle, it became practicable, in 1911, to provide telephone service between New York City and Denver, Col., and greatly to improve the transmission of speech between cities less far apart. By the application of the phantom principle to such circuits the available facilities were largely increased so that, between the important telephone centres, notable improvements in service were accomplished. On Jan. 25 1915, the transcontinental line of the Bell System was formally opened for business and after that time commercial service was given between the cities on the Atlantic Coast and those on the Pacific Coast. The service in 1921 was handled over a group of 4 non-loaded wires equipped with telephone repeaters. By using the 2 side circuits and the phantom circuit formed by these wires, 3 simultaneous transcontinental connexions may be established. By means of the addition of compositing apparatus to the circuits the 4 wires which carry 3 telephone circuits also carry 4 telegraph circuits. These 4 telegraph circuits may be arranged to transmit 8 simultaneous messages. The line from New York City to San Francisco is 3,400 m. in length.

Long-Distance Telephon

Cables. - By 1906 a cable 90 m. long was successfully operated between New York and Philadelphia, but, in the then state of the art, that cable could not be used for connexions extending beyond New York or Philadelphia. In 1911, an underground cable was designed capable of giving a satisfactory conversation between Washington and Boston. By 1912, a section of this new cable was laid from Washington to Philadelphia, there connecting with the earlier type of cable to New York. During 1913, a section of the new cable was laid between New Haven and Providence, connecting at New Haven with an earlier type of cable extending to New York and connecting at Providence with an earlier type extending to Boston. Although talking over the whole distance from Boston to Washington was not possible so long as stretches of cable of the older types had to be employed, yet by using the underground in connexion with the overhead, the seaboard cities from Washington to Boston could no longer be isolated by storms destroying the overhead lines. During 1913, the advances in the art of loading and balancing underground circuits together with the repeater developments made it possible to talk satisfactorily by underground wires from Boston to Washington, a distance of 455 m. even though 47 °A of the total cable in the line was of the types formerly suitable for short-haul working only. In 1912, talking by underground wire for the first time between New York and Washington represented the longest distance achieved. By 1913, this distance had been doubled. The Boston - Washington cable was several times longer than any other in the world. There were in 1921 several cables working along the Boston - Washington route. During 1919, the extension of the toll cable system from Philadelphia to Harrisburg, Pa., was completed. Taken in combination with the cables already working between Boston and Washington, this gave a through toll cable route from the important points on the eastern seaboard as far west as Harrisburg. In 1921, this cable was extended from Harrisburg as far W. as Pittsburgh, a distance of 192 m. from Harrisburg and 304 m. from Philadelphia. For the greater portion of the distance the cable was supported aerially on poles. The composition of this cable was as follows: Section Quads No. 16 A.W.G. Quads No. 19 A.W.G.

Harrisburg - Ligonier 16 125 Ligonier - Pittsburgh 19 120 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 io 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 fre= quencies 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 ioo,000 listeners standing in an open space of more than to 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 longdistance 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 at the central office is set in motion and controlled by a dial, associated with the substation set, and rotated by the subscriber. The " step-by-step " type of equipment makes use of a series of selectors in each of which contact is made by means of a central arm that can be raised to any desired level and rotated, at that level, to the proper one of a series of terminals arranged in the arc of a circle. This type of equipment is mostly used in the smaller cities and for automatic private branch exchanges. The " panel " type of equipment has been developed to a point where it is now being installed on an extensive scale in a number of the larger cities of the United States. On account of its importance a brief description is given.

Panel Type System.-The panel type equipment is so named because the multiple of the selectors is built in panels. The selectors have, in general, capacity for 500 lines or trunks. The multiple of these selectors consists of punched brass strips about 3 ft. long and one in. wide piled one above the other with insulation between. Since 3 connexions are necessary for each line or trunk, 1,500 of these strips are provided. The strips are divided horizontally into 5 groups or panels of too lines or trunks each and are mounted on frames having capacity for 60 selectors each, 30 on each side. The selector consists of a tube running vertically, close to the banks, the tube being equipped with a set of brushes for each bank. The brushes normally are held mechanically so that they do not engage the terminals. At the bottom of the tube, a friction clutch is provided which, by engaging constantly rotating shafts, can cause the tube to be raised or lowered. The brushes are multipled together by wires within the tube, these wires being attached at the top of the tube to feeder brushes which move over insulated feeder strips. The process of selection consists in first mechanically tripping the desired brush into engagement with its multiple bank, next in moving the tube carrying the brushes upward to choose the desired group within the bank, and finally continuing the movement upward to choose the desired subscriber's line or an idle trunk within the selected group. The subscribers' lines appear on the multiple of panel type selectors known as " line finders." The function of the line finder is to make connexion with calling subscribers' lines. It corresponds to the "A" operator's answering cord and the subscriber's answering jack in the manual system. The brushes of the line finder are attached to the brushes of a panel type " district selector " and also to the brushes of a small selector known as a " sender selector." As soon as a calling subscriber's line has been picked up by a line finder, the sender selector selects an idle " sender " out of a common group. When the calling subscriber dials, the pulses are registered in the sender which controls the setting-up of the connexion and is then freed. The sender may be likened to the operator of the manual system. The sender causes the district selector to choose a trunk to the desired office, or, if more than 500 outgoing trunks from the office are required, causes the district selector to pick out an idle " office selector " of the panel type which selects the desired trunk. The trunk incoming to the full mechanical office ends in the sender of an " incoming selector " of the panel type whose function it is, under control of the sender, to pick out an idle panel " connector " having access to the group of 500 lines in which the called subscriber's line may be found. Controlled by the sender, the connector then selects the called line.

Calls from a machine-switching to a manual office are completed over " call indicator trunks." As the calling subscriber dials his call, the district or office selector picks out an idle trunk to the desired office. This trunk ends in a plug before a " B," or incoming trunk operator in the called office. When a call appears on that trunk, the " B " operator depresses a display key associated with that trunk, whereupon the number which is desired in that office is quickly transferred by the sender to a bank of numbered lamps appearing before the " B " operator, and the " B " operator thereupon plugs the trunk into the desired subscriber's line.

For completing calls from a manual to a full mechanical office " key indicator mechanism " is employed. This is a mechanism which indicates to the " A " operator an idle trunk to the desired office which ends at that office at an incoming selector. The " A " operator by using a small io-button key-set is enabled to control the incoming selector to make connexion through the aid of connectors with the called line desired.

Other Improvements.-In the United States there is a large and growing use of the telephone for communications essential to the operation of both steam and electric railways. The problem of minimizing the disturbing effect upon telephone circuits produced by induction interference from electric light and power circuits has resulted in careful, coordinated work by the power and telephone engineers. Important improvements have also been made in local and toll line operating efficiency.

Organized research has not only enabled the limits of telephony to be greatly extended but, at the same time, improvements and economies have been made in every department of the business.

Telephone Development of the World-January z 1910, 1914 and 1920. (Some of the figures-for the most part those for small places not shown separately-are necessarily in part estimated.) Bibliography. - Manual Telephone Systems: J. Poole, The Practical Telephone Handbook; K. B. Miller, American Telephone Practice; W. Aitken, Manual of the Telephone; J. E. Kingsbury, Telephone and Telephone Exchanges. Automatic Telephone Systems: Smith & Campbell, Auto. Telephony; R. Mordin, Strowger Auto. Telephone; Professional Papers of the Institution of Post Office Electrical Engineers: J. Hedley, W. E. Co.'s Semi-Auto. System; B. O. Anson, W. E. Co.'s Auto. System; Papers from the Journal of the Institution of Post Office Electrical Engineers: H. W. D., " Dudley Auto. Tp. Exch." (Jan. 17); G. F. 0., " Theoretical Principles of Traffic Capacity of Auto. Switches " (Oct. 20); W. J. Bailey, " Lorimer Exch. at Hereford " (July 13); W. J. Bailey, " Epsom Auto. Exch." (vol. 5, 1912); J. Hedley, " Auto. Exch. Darlington " (vol. 7, 1914); R. L. Bell, " Auto. Switches in Split Order Wire Wkg." (vol. 7, 1914); P. V. Christensen, " No. of Selectors in Auto. Tp. Systems " (vol. 7, 1914); " Coin Box and Call Meter for Auto. Exchanges " (vol. 8, 1915); J. Hedley, " Developments in the Strowger Auto. System " (vol. 8, 1915); A. K. Erlang, " Solution of Problems in Theory of Probabilities, Auto. Exchs." (vol. 10, 1917); F. McMorrough, " Grimsby Exch." (vol. 9, 1916); A. B. Eason, " Relay Auto. Tp. System " (vol. 13, April 20 1920); G. F. 0., " Comparisons of Auto. Exch. Systems " (vol. 12, 1919); Proc. American I. E. E.: W. Lee Campbell, " Traffic Studies in Auto. Switchboard Telephone Systems " (March 1914).

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