{ "system": "GoGuides Verified Text", "api_version": "verified-text-v1", "status": "ok", "response_type": "verified_text_record", "source_key": "britannica_1926", "source_title": "Encyclopaedia Britannica (1926)", "license_code": "public_domain", "attribution": null, "license_url": null, "chunk_id": "1926:telephony:28ec155fe583", "title": "TELEPHONY", "section": null, "hash_alg": "sha256", "hash_sha256": "0ff533001e4806f9608e95487e2864979b872cd62bc97555876fe00069a22423", "normalizer": { "name": "ggnorm", "version": "1.0" }, "verified_text": "the steady sequence of improve- ments in exchange cable which began with the installation of the first cable of this type in 1882 has, in the years now under review, included a goo-pair, 22-gauge! cable in 1912; a 1,200-pair, 24-gauge in 1914; followed by the 450-pair, 19-gauge cable in 1918, which because of the large size conductors it contains is being extensively uscd for the building of long toll lines, as will be mentioned in the paragraph long-distunce telephony by cable. these increases in the number of conductors—increases of from 200% to 500% over the original value—have all been accomplished without any increase in the outside diameter of the sheath, which still remains, for the largest sizes, at 23 inches. along with these increases in number of conductors has gone a notable decrease in the first cost per mile of installed cable con- ductors. the same developments are also making possible corre- sponding economies in all smaller sizes of cable which, because of fewer conductors, do not require the maximum size of sheath. a change in cable sheath material has also been effected which makes possible an equally durable sheath at a lower cost. the new sheath consists of lead alloyed with 1% of antimony instead of 3% of tin as formerly. telephone repeaters ——one of the outstanding achievements of the period 1910-26 is the devclopment of long-distance wire telephony to the point that the voice can be carried over practi- cally any terrestrial distance. in this advance the telephone re- peater has played a dominant part. the repeater in its present form employs the three-electrode thermionic vacuum tube as the amplifying agent for telephone currents and also includes balancing networks, electric filters and means for controlling amplification, their association calling for great precision of de- sign and manufacture. the first and one of the most notable achievements, employing the thermionic telephone repeater, was the opening for public service, in 1915, of the telephone line between new york and san francisco, a distance of some 3,400 miles. in 1921, in order to cope with an increasing demand for trans- continental service, the loading coils were removed from this line and repeaters of an improved type were applied in place of the original six. this change made the circuits suitable for the application of carrier current facilities, and also resulted in a 1 the wire gauge here referred to is a.w.g. 747 large improvement in the clearness and volume of transmitted speech. the four wires of the original transcontinental line now furnish three telephone circuits and, over certain sections, as many as 20 telegraph circuits. some picture of the accuracy and amplifying power of the telephone repeater can be gathered from the fact that, in passing from new york to san francisco, the voice current is amplified over a million-million fold and yet ar- rives with all the clearness and intelligibility with which it starts. long-distance telephony by cable-—as the demand for long- distance telephone service increased, it was foreseen that ulti- mately the need would arise for an improved type of long-dis- tance circuit offering increased electrical stability and increased immunity from damage and disruption by storm. moreover, large densities of trafhe made desirable a long-distance circuit more compact physically than those in use. accordingly, re- search was undertaken along various lines with the object of re- moving the restriction which had always hmited the use of cable circuits to relatively short distances. the success of these re- searches is attested by the long telephone cable (over goo m.) which has been completed between new york and chicago at a cost of $25,000,000. it supplies approximately 250 telephone channels and soo telegraph channels. among the developments which have made this cable possible are: improved designs of cable, improved methods for minimising capacity unbalance, means for automatically controlling the amplification of repeat- ers (as the resistance of the wire circuit to which they are at- tached varies with temperature), new and very compact types of telephone repeaters and other office equipment and new sys- tems of telegraphy. one system of land cable telegraphy em- ploys a unidirectional line current of about.1/2oth the magnitude of the current employed in the standard grounded telegraph cir- cuits; and another, known as “ voice frequency ” carrier teleg- raphy, supplies 1o duplex channels in the frequency band required for a single speech channel. new systems of loading designed to reduce “ transient ” and “ echo ”’ effects have also contributed to the success of the cable. in this connection, it may be pointed out that whereas it has become economically desirable, as noted in a preceding paragraph, to remove all load- ing from certain long, open-wire lines, nevertheless long cables depend upon loading because of their intrinsically higher attenuation. the echo-suppressor.—one of the many novel develop- ments underlying the successful long-distance cable is the “‘ echo suppressor.”’ in talking over very long cable circuits an appreciable interval of time exists between the uttering of the speaker’s words and the return of reilected transmission or the ‘“‘ echo ” to his ear from the distant end of the cable. this time interval is in the neizhbourhood of ~ of a second and early investigations showed that, to the average telephone user, appreciable echoes having such a delay are not only very notice- able but extremely confusing. the voice-operated echo suppres- sor was, therefore, devised automatically to restrict transmission on the circuit to one direction at a time. the long telephone cable, possessing a high degree of electrical stability and placed either underground or on heavy poles, ts a type of long-cistance trunk line which will become increasingly important in the fu- ture, particularly between large cities. currier current telephony and telegraphy.—from the earliest days of both the telephone and the telegraph there have been many attempts to develop methods for the multiplex transmis- sion of messages employing relatively high frequency currents as “carriers. ’’ it was not unt.l the development of the thermi- onic vacuum tube, however, that carricr systems were brought within the realm of practicability. these tubes are used in carrier systems as repeaters or amplifiers, as generators of the different carrier frequencies and as “modulators” and ‘‘ demodulators ” for respectively combining the messages with and separating them from the carrier currents. these systems also employ extensively another important de- vice which was given to the electrical art contemporaneously with the vacuum tube repeater. this is the electric wave filter. 748 this remarkable device can be so designed as to select any pre- determined band of frequencies, rejecting all others without ab- sorption. the rdle that the filter plays in carrier systems as a means of separating one message from another is therefore obvious. both carrier telephone and carrier telegraph systems have been extensively applied since 1918. when multiplexing a pair of telephone wires, it is customary to add either three or four high frequency channels to the voice frequency channel, thus increas- ing the message capacity of the wires cither four or five fold. in the case of carrier telegraphy, the message capacity of a wire cir- cuit may be made much larger and systems are in use in which as many as to to zo carrier frequency channels are placed upon a single pair of wires. the shortest length of line which it is now economical to multiplex with carrier facilities lies between 150 and 200 m.; but it is expected that as further improvements and simplifications are made, the limiting distance will be brought down to a much lower figure. transmission measurement and maintenance the successful maintenance of vast networks of long-distance telephone circuits, both open wire and cable, has in large measure been made possi- ble by the development of accurate means for making transmis- sion measurements. great success has been attained in the de- velopment of measuring apparatus, which, although of an exact and complicated character, is so simple of manipulation as to be used continuously in extensive daily programmes of routine measurements. in this apparatus, use of vacuum tube amplifiers permits the measuring of minute quantities by instruments that are rugged and, wherever necessary, readily portable. | radio telephony.—since tgt11 the entire growth, if not the ac- tual birth, of the radio telephone art has taken place. like the developments in carrier systems this rapid progress has been largely due to the high standards attained in the design and man- ufacture of thermionic tubes. an early radio telephone achieve- ment was the transmission of speech across the atlantic ocean in 1915. the development of radio telephone apparatus was un- dertaken during the world war for such purposes as continuous communication between observing aeroplanes and artillery sta- tions behind the lines, and between the units of a destroyer flotilla. following the war, public interest in the possibilities of radio telephony was quickly awakened, with a consequent rise of broadcasting (g.v.). the development of radio telephony for com- mercial message uses may appear, in contrast to the rapid strides of broadcasting, to have lain dormant. however, notable progress has been made. immediately following the war, the develop- ment of radio systems for establishing ship-to-shore telephone communicafion began. in 1920 an experimental system was dem- onstrated by american telephone engineers which enabled a pas- senger on a ship several hundred miles at sea to communicate with any telephone subscriber on land through the latter’s regu- lar instrument. to accomplish this, the message was carried over the water by radio and at a coastal station was relayed to the land lines, which, in the case of one demonstration, were 4,000 m. long. the system was entirely two-way in its operation, in that it permitted the telephone subscriber and the person at sea to hold a conversation exactly as over the usual telephone circuit. thus far no very extensive installation of ship-to-shore radio telephony has been made, largely because of the numerous com- mercial difficulties involved, although a limited service of this sort is, it is understood, offered by some german steamships. transatlantic radio telephony.—in 10920 a serious study of transatlantic radio telephony was undertaken. the difficult character of the technical problems involved may be inferred from the fact that, due to the extreme variability of atmospheric conditions as they affect the transmission of radio signals, the sending power required to produce a given effectiveness of received signal may increase by sevcralthousand times within the space of afew hours. to ensure even reasonably reliable com- munication at all times of day and throughout all seasons of the year, therefore, necessitates transmitting stations of large reserve power and special receiving systems to reduce the interfering noise arising from atmospheric disturbances. telephony another difficult requirement to be met is that the transoceanic radio link must automatically relay messages to and reccive mes- sages from the wire lines in both europe and america, so that it will be as easy for two telephone subscribers, wherever placed, to converse as though each were located at one of the radio stations. to illustrate the progress made, the demonstration of march 7 1926 might be cited, during which some 20 press representatives in london had two-way conversations with their american con- freres.! in this test, transmission was effected by wire from new york to the radio transmitter located about 70 m. out on long island, where it was automatically relayed to the trans- atlantic radio link. wire telephony as an adjunct to radio broadcasting. —early in the development of radio broadcasting, it was apparent that the popularity of the programme would be greatly augmented by bringing to the broadcasting station events of public interest such as large orchestral programmes, sporting events and na- tional ceremonies. this virtual extension of the walls of the studio by the use of telephone lines is now a matter of almost daily occurrence, particularly in england and america. as many as 25 or more broadcasting stations are sometimes joined together by a wire network to disseminate the same programme. transmission of pictures (see photo-telegraphy).—over a long period of years, many schemes were proposed for the electri- cal transmission of pictures. although these early systems were in most cases physically operative, none proved practical. to be of commercial value, a system must be at once simple, rapid and accurate. it has remained for developments of late years to meet adequately all of these requirements. in april 1925 the first commercial picture-transmission service, a development of the bell telephone engineers, was introduced. this service, em- bracing the cities of new york, chicago and san francisco, made possible the transmission of pictures in either direction be- tween any of these points. the actual sending time of a 5-in. by 7-in. picture is but seven minutes. at the sending end, the pic- ture is required simply in the form of a positive transparency made by the ordinary photographic process, and after transmis- sion it appears as a negative film from which prints can be pro- duced without delay. as it is possible to transmit while the posi- tive transparency is still wet, the total time of transmission is reduced to a minimum. machine switching systems.—a retrospective examination of the manually operated switchboard discloses the fact that the tendency of development has been continuously in the cirection of increasing the number and extent of the switching operations that are performed electromechanically, thus decreasing the amount of time and effort required of the operator for the han- diing of a connection. when a point is reached where the opera- tions performed manually at the central office are entirely elimi- nated, except in the case of certain special classes of calls, the term “ automatic ”’ or “machine switching ” is applied to the equipment. in this equipment the subscriber’s telephone set is furnished with a finger-wheel or dial having ro finger holes. thesc holes are designated by the 10 digits, and for the larger cities also by letters. by inserting a finger in one of the ro holes, rotating the dial to a stop and then letting it return under the control of a spring, the calling subscriber sends to the central office mecha- nism a series of from one to 10 electric impulses. in this way the number is given by electrical impulses rather than called, the operation of the dial conveying to the central office mechanism the necessary guiding impulses for setting up the connection desired. main types.—there are three principal types of machine switching equipment in extensive use, viz.: the “ panel,” “ step- by-step ” and “ rotary ” types. these names refer to the me- chanical design of the devices termed ‘“‘ selectors,” which are supplied in the central offices to switch the subscribers’ lines into connection with each other. a selector consists of a set of movable terminals which can be connected with any one of 3 number of sets of fixed terminals and takes the place of the flexi- ble cords, plugs and jacks of the manual switchboard. for 1see nature, p. 386 (march 13 1926). telephony example, in the “ panel ” type of selector the movable terminals are driven up and down in a straight line over the fixed terminals, which are placed on a flat surface in the form of panels. the step- by-step system was first developed for use in small cities, but redesigned for use in london. the other two systems were de- veloped particularly for use in large cities. new magnetic materials.—among the new magnetic materials developed for telephone purposes, two deserve special mention. one consists of very finely divided iron dust cemented together under extremely high pressure to yicld a material of substan- tially the same density as iron, of only 0,$50 the electrical con- ductivity of iron. ‘the product has nearly the same initial permeability as iron, and yet because of its finely divided ingre- dients, and the consequent microscopic free poles distributed throughout it, the material possesses practically no residual magnetism even after exposure to strong ficlds. the second new magnetic material, also a development of the bell system engineers, has been called permalloy. it is so sensitive to mag- netising influences that it is completely saturated in the carth’s field and is making possible important advances in various branches of the communication art. for example, in co-opera- tion with the western union telegraph co., a submarine tele- graph cable loaded with permalloy has been laid between new york and the azores, the message capacity being about four times that of the standard type of cable. the permalloy in tape form is wound as a cylindrical covering around the copper conductor, and, because of its very high initial permeability at low magnet- ising field strengths, causes a more rapid building up of current at the receiving end, which in turn results in increased speed of sending. permalloy is also being employed in the magnetic circuits of many telephones and telegraph relays, telephone transformers and receivers. the use of the telephone-—coincidental with these technical advances in the telephone art, there has been a notable increase since 1910 in the use of the telephone throughout the civilised world. the accompanying table shows the number of telephones in the principal countries and continents at jan. t 1910 and at jan. 1 1925, the latest date for which comparative data are available. the number of telephones per too people, which is also shown in the table, affords a means of comparing the tele- phone development of areas differing widely in total popula- tion and in total number of telephones. it will be noted that the aggregate number of telephones in service throughout the world increased from about 10,400,000 to over 26,000,000 in the 15 years under consideration. table i. shows that, at the end of this period, 62° of all the telephones in the world were in the united states, 26°% were in europe and the remaining 12% were scattered among all other countries. great britain and northern ireland had, at jan. 1 1925, some- what less than 5% of the world’s total telephones, germany had slightly over 9° and france about 2-5% of the total. there was, at that date, one telephone for every 7 people in the united states, one for every 36 people in great britain and northern ireland, one for every 26 people in germany and one for every 58 people in france. sweden had one telephone for every 14 people, norway one for every 16 people and denmark one for every 11 people. ‘the scandinavians also rank high among euro- peans in respect to the use of the telephone. during a recent year there were 13r telephone conversations per capifa in den- mark, 113 in norway, 106 in sweden, 23 in great britain and northern ireland, 30 in germany and 20 in france. the cor- responding annual figure for the united states was 191 telephone conversations per capita. the telephone and the war.—the increment between igt1o and 1925 in the world’s total telephones does not, of course, represent an even growth throughout the period. the normal rate of increase was decidedly retarded in the principal telephone- using countries during the world war. the difficulty and high cost of securing and shipping telephone supplies was felt through- out the world. in the belligerent countries these conditions were intensified by the necessity of giving priority to war re- quirements and the telephone systems were further handicapped 749 by the depletion of their working forces to supply communica- tion technicians for the military and naval forces. for example, about 13,000 out of the 20,000 men in the engineering department of the british post office joined the army. after the united states entered the war, nearly 25,000 employces of the bell system joine.| the war service of the government. of this number 233 were women telephone operators who were recruited and sent overseas; and hundreds of others were in training for this service at the time of the armistice. an enormous amount of telephone con- struction was also carried out to meet the wartime need for extra telephone service in washington and at army posts, camps and cantonments, aviation fields and naval and coastguard stations. experience with the telephone in the world war practically revo- lutionised modern military practice in the maintenance of communi- cations (see signalling, army). the signal corps of the u.s. army, for example, constructed during the war 1,990 m. of permanent pole line with 28,000 m. of wire, erected 3,230 m. of wire on french polcs, and installed approximately 40,000 m. of combat lines, american military telephone exchanges on permanent lines in france num- bered 273, and those in the advance section 123, besides small tempo- rary field installations. about 1,600,000 long-distance telephone calls were handled by the signal corps, in addition to local telephone calls estimated at 47,000,000. post-war development—aafter the war, the accumulated de- mand for telephone scrvice in various countries was sufficient . to accelerate telephone expansion to the maximum rate permitted by the amount of telephone apparatus which could be made available. in japan, however, a further setback was experienced as a result of the disastrous earthquake of sept. 1923, which put out of service upwards of 70,000 telephones, or more than one-eighth of all the telephones in the empire. in the re-estab- lishment of service in the devastated area the opportunity is being taken to introduce “‘ automatic” or “‘ machine-switching ”’ systems in place of the manual equipment formerly used. a like change is gradually being made in many of the large urban telephone systems throughout the world. by the end of 1925 the number of automatic or machine-switching telephones in the united states exceeded 1,650,000. administrative changes.—some important changes in telephone administration have taken place in europe since 1)10. in great britain, on dec. 31 1911, the british post olfice took over the entire telephone system of the national telephone company. all public telephone service in great britain and northern ireland is now rendered by the post office, except at iiull and on the islands of guernsey and jersey, where the telephone systems are operated by the local authorities. upon the establishment of the irish free state, the british post o,hce transferred to the free state govt. its telephone system in southern ireland. in 1916 the netherlands govt. acquired the 19 exchanges of the netherlands bell telephone company. other local systems owned by private companies or by municipalities have been taken over by the govt. from time to time; but the local telephone service in the larger dutch citics ts still under municipal operation. the interurban lines are owned and operat- ed by the netherlands government. the swedish govt. in 1918 purchased the telephone system of the stockholm telephone com- pany. on the other hand, a tendency has been manifest in europe, since the war, toward the extension of private enterprise in prefer- ence to government ownership in telephone administration. the spanish govt. has transferred its publicly owned telephone system to a private company, which has been granted a concession for the development of a nation-wide telephone network. local tele- phone systems formerly operated by the govt. of italy have been ceded to private concerns under concessions covering limited territorial zones, the polish (sovt. has transferred its telephone properties in anumber of cities to a corporation in which it has only a minor- ity interest. elsewhere, notably in germany, official action has been taken looking to the establishment of a form of administra- tion for the national communication services which shall approxi- mate as closely as practicable the corporate organisation commonly employed by private enterprise. at jan. i 1925, approximately 71% of the world’s telephones were privately owned, and the remaining 29 % under public ownership. international telephone communication.—the improvement of international telephone communication has received much attention during the period under consideration. the bell system in the united states has been connected by submarine cable with the telephone system of cuba and its long-distance lines afford access to many points in canada. in europe, al- though the distances between the principal cities are less than those covered by circuits in everyday commercial use in the united states, long-distance telephony has encountered obstacles. differences of language, of equipment and of operating practices 750 have exerted a restrictive influence on long-distance communi- cation, particularly across international boundaries. european telephone authorities have, however, taken cognisance of this situation. an international conference was held at paris in march 1923, to organise a “‘ comite technique preliminaire pour la telephonie & grande distance en europe” in which bel- gium, france, great britain, italy, spain and switzerland were represented. various technical proposals were adopted looking to the betterment of long-distance telephony in europe. a per- manent secretariat and a continuing advisory international tech- nical committee were set up. at subsequent technical confer- ences additional countries have been represented and further progress has been made in the study of the technical problems involved. greater difficulties are presented, however, by the question as to what, if any, form of international organisation should be entrusted with the administration or supervision of international telephone circuits. table i. telephone development of the world, jan. 1 1910 and 1925 telephony, submarine england has long been connected with the continent and with ireland by submarine telephone cables and the scandinavian peninsula has enjoyed like facilities for communication with neighbouring european countries (see telephony, submarine). an event of considerable interest, however, in long-distance tele- phone extension was the establishment, during 1924, of telephone communication between spain and morocco by a cable spanning the straits of gibraltar. table i. telephone wire of the world, as of jan. 1 1925 (some of the figures are necessarily partly estimated ) (some of the figures are necessarily partly estimated) jan. i igio jan. 1 1925 c <| q nes ns lege es g | os fe | cuales foe su6 oa| se salty naa sa | 5a wa, | sx leqann 22 |6 oe |;eol/sbede sien os gy oot aad 5 a st et ao) om or z, = z. ee great britain 609,274| 1-3. | 11,264,024] 2-8 654,750 denmark 87,436] 3-2 307,977] 9-0 220,541 france 211,664) 0°5 660,127] 1-7 448,463 germany 968,101/ 1-5 | 2,385,177] 3-9 | 1,417,076 italy 63,131] 0-2 172,900] 0-4 109,769 norway . 57:945| 2°4 168,518) 6-1 110,573 sweden 174,055] 3:1 418,318) 6°9 244,263 switzerland “4 73,758] 2:0 189,429] 4°8 115,671 other countries in europe 506,636 0-2 1,328,895 o-4 822,259 total europe 2,752,000| 0:6 | 6,895,365] 1-4 | 4,143,365 japan 6-8 8 109,780} 0-2 | 4544,433] 0-9 434,653 other countries in asia . 44,220 o:o! 257;975 0°03 213,755 total asia . 154,000] 0-02} 802,408] 0-1 648,408 union of south , preti@hs! 24a, 28 13,650) 0-2 271,448] 1-0 57,798 other countries in africa . : 18,000] 0-02 82,055] o-l 64,055 total africa 31,650] 0-02 153,503] o-1 121,853 united states 6,995,692] 7-6 |16,072,758|14-2 | 9,077,066 canada - 8 239,000] 3:3 | 1,072,454/11-6 833,454 other countries in north america . 38,900] 0-1 155,007) 0-5 116,107 total north america . 7,273,592! 5°5 |17,300,219|11-2 |10,026,627 total south america . 75,000] 0-2 373,157| o°5 298,157 australia 81,040) 1-8 | 318,279) 5-5 237,239 new zealand . 29,680! 2-7 *120,097| 8-7 90,417 other countries in oceania 10,000| 0-03 75,480] o-1 65,480 total oceania 120,720] 0°3 513,856] 0-7 393,136 total world 10,406,962] 0-6 [26,038,508 1 exclusive of the irish free state. 2 march 31 1925. 2 june 30 1924. 4exclusive of 23,800 telephones temporarily out of service as a result of the earthquake. note: in the case of countries the boundaries of which have under- gone change, the figures for each year represent the number of tele- 1-4 phones within the boundaries of that year. 15,631,546 nt, miles of peas telepnone sae wire per 100 population great britain and n. [ireland . 4,966,746 10°8 denmark 7 : , ; fa 37 21-8 france 1,963,890 4°9 germany 7,417,263 12-0 italy 550,000 1-3 norway 410,628 14:9 sweden 921,509 i5°3 switzerland : , 482,091 12:3 other countries in europe 3,425,506 1-2 total europe . 20,880,770 2 japan . ae . 1,537,237 2-6 other countries in asia 690,441 o-l total asia : 2,227,678 0-2 union of south africa . 246,246 3:3 other countries in africa ; 214,452 q-2 total africa 460,698 0-3 united states 46,500,000 41-0 canada oe ee : : . 2,793,596 30°3 other countries in north america . 391,564 1-2 total north america 49,685,160 22°3 total south america 986,709 1-4 australia 1,266,825 21°8 new zealand . . 363,361 26°3 other countries in oceania 290,626 0-4 total oceania 1,920,812 2:8 total world 76,161,827 bibliography.—for facts regarding scientific and _ technical progress in the telephone art, consult recent volumes of jour. of amer. inst. of electrical engineers; trans, of amer. inst. of electrical engineers; electrician; physical review; bell system technical jour- nal; electrical communication. for facts regarding modern telephone engineering, construction, and operating methods, and regarding the telcphone’s status as affected by political and economic conditions, consult the bell telephone quarterly and also c. a. wright and a. f. puchstein, telephone communication (1925); n. p. weinbach, textbook of transmission in telephony (1924); 5. g. mcmeen and kk. b. miller, telephony (1922); h. j. van der bijl, thermionsc vacuum tube (1920); k. s. johnson, transmission circuits for tele- phonic communication (1925); a. b. smith and w. l. campbell, automatic telephony (1914); j. e. kingsbury, the telephone and telephone fixchanges (1915); f. g. c. baldwin, the history of the telephone in the united kingdom (1925); a. lincoln lavine, circuits of victory (1921). (w.s. g.) telephony, submarine.— progress in submarine cable telephony has been due partly to improvements in cable design and partly to the introduction of the telephone repeater which has, in turn, reacted on the design of cables. continuous loading.—in 1887 oliver heaviside, in the elec- frician, vol. 19, showed that the transmission efficiency of a tele- phone circuit could be improved by artificially increasing the inductance of the circuit. so far as submarine cables are con- cerned this had been effected by “ continuous loading” up to the year 1910. “ continuous ” or “ krarup ”’ loading is effected by winding wire or tape of magnetic material, generally iron, in a close spiral on the conductors of the circuit. unfortunately in addition to increasing the inductance of the circuit the effective resistance to alternating currents is also increased as well as the telephony, submarine mutual capacity between the conductors. in practice this has hitherto imposed a limit of approximately 20 millihenries per nautical mile as the maximum inductance which can be ob- tained in a continuously loaded cable. in order to obtain a greater increase in inductance “ coil loading ”’ has therefore been resorted to. in toro a coil loaded cable was manufactured and laid by messrs. siemens bros. & co. for the british post office between abbot’s cliff near dover and cape gris nez in france. details of the cable are given in the table on page 752. the load- ing coils are spaced at intervals of one nautical mile (“‘ naut ’’). efficiency of circuit.—the efficiency of a telephone circuit is measured by the ratio of the current received to the current sent into the circuit. this ratio is expressed by the formula e74! where e is the base of the napierian system of logarithms, £ is ‘the attenuation constant per unit length of circuit and j is the length of the circuit. the naut will be taken as the unit length for the purpose of this article. for the cable referred to above the value of 8 for the cable unloaded is 0-0524 and for the loaded cable 0:0166. therefore for the same efficiency the loaded cable could be rather more than three times the length of the unloaded cable. the value of 8 depends upon the electrical constants of the circuit. fora coil loaded cable the natural inductance is usu- ally small compared with the inductance added by the loading coil and may therefore be ignored. the following approximate pon ct ae hae ve formula may then be used b=—————— where r is the loop resistance of the circuit in ohms, c the mutual capacity between the conductors of the circuit in farads, l the inductance in henries and g the leakance in mhos, all per unit length of circuit. ri and l; are the effective resistance, and the inductance of the loading coils respectively. all these values are to be taken at the particular alternating current frequency under consideration. in the formula given the two quantities r,/l; and g/c represent properties of the loading coils and of the dielectric respectively. in the anglo-french cable already men- tioned, the value of the former is 60 and of the latter roughly 120 at 1,000 cycles per second. in the next cable laid, the anglo- belgian cable, the value for g/c was reduced to 12, at 800 periods per sec., by using balata in place of gutta percha. as regarcls the quantity r,/l; the value has been reduced to 4o. in addition to the coil loaded cables shown in the table two coil loaded cables have been laid from dungeness in england to audresselles in france, two from dover to sangatte, and one from st. margaret’s bay to la panne, belgium. in the abbot’s cliff-gris nez cable only the two physical circuits are loaded but in all subsequent cables additional coils have been introduced to enable a third loaded circuit to be provided by superposing. the addition of these coils slightly increases the attenuation constant for the physical circuits, on account of the additional resistance introduced by the superposed circuit coils. in the year following the laying of the abbot’s cliffi-gris nez coil loaded cable the french govt. laid between the same points a continu- ously loaded cable with gutta percha insulation, details of which appear in the table. the loading of the physical circuits automatically loads the superposed circuits. a fourth circuit operated as an earthed cir- cuit can be worked over all four wires on which the loop super- posed circuit is formed provided that the electrical balance of the conductors is such as to prevent undue crosstalk, and that interference from earth currents is negligible. this system has been worked over the vancouver-victoria, b.c., cable manufac- tured by messrs. w. t. henleys telegraph works co. telephone repeater —the growing demand for circuits for international communication is leading to the adoption of cables with more numerous conductors. this is rendered practicable by the developments which have taken place in the telephone repeater consequent upon the introduction of the three elec- trode thermionic valve. the use of the telephone repeater, which amplifies the speech currents, enables smaller conductors to be used and an increased number of circuits can thus be provided 7o1 in a given size of cable. an example of such a cable is the leba- tenkitten cable across the bay of danzig. this cable contains 12 telephone wires laid up in three groups of four wires (quads) and three telegraph wires all paper insutated. it is provided with two lead sheaths, as a precaution against the penetration of water. in order to assist the lead sheath in supporting the exter- nal pressure to which the cable is subjected a spiral of steel wire is placed between the cable core and the lead sheaths. further, the armouring, instead of the usual cylindrical wires, consists of wires drawn roughly z shape so as to interlock and form a prac- tically incompressible tube. ‘the ordinary jute serving over all completes the cable. the direct current resistance of the tele- phone conductors per naut loop is 20-9 ohms for the physical circuits and 10-45 ohms for the phantom circuits. the increase of resistance with frequency due to the loading is indicated be- low, where w= 2m x the frequency. the testing current was 2 ma. w) = 3,000 @) = 5,000 w= 7,000 physical circuit o445 ohms 1-17 ohms 2-17 ohms superposed circuit o-241 “ 0-538 “ 0-965 “ the rise in resistance as the frequency increases involves an increase in attenuation of the higher speech frequencies. frequencies up to 2,000 per sec. corresponding to a value of @=12,400 approximately, must be transmitted without seri- ous loss if good commercial speech is to be obtained. special attention is therefore paid to the resistance-frequency charac- teristics of cables with a view to the efficient transmission of the higher frequencies. in 1924 the first continuously loaded paper core cable between england and the continent was laid between aldeburgh, suffolk, and domburg, holland, for the anglo-dutch service. this cable contains 16 wires (four quads) providing eight physical telephone circuits and four superposed circuits. each conductor consists of a center wire 0-076 in. (1-93 mm.) in diameter surrounded by three copper strips each o-oro in. (0-254 mm.) thick; weight 165 ib. per naut. each conductor is wound with two layers of 0-008 in. (0-203 mm.) diameter iron wire; weight 87 lb. per naut. the loaded conductor is wrapped with paper to a diameter of 0-188 in. (4-77 mm.). two lead sheaths are provided, the external diameter of the outer lead sheath being 1°52 in. (37-6 mm.). the armouring consists of 24 cylindrical wires 0:2321in. (5-9 mm.) diameter. the overall diam- eter of the cable is 2-42 in. (61-5 mm.). no steel spiral is pro- vided between the core and the lead sheath, as the water in which the cable lies is comparatively shallow. the length of the cable is 82-242 nauts. it terminates in a telephone repeater station at aldeburgh and is extended on the dutch side by a continuously loaded underground cable to a repeater station at middelburg. it provides circuits between london and rotterdam, amsterdam and the hague. in the operation of the two-wire telephone repeater the cir- cuit on each side of the repeater station is balanced by a net- work having the same impedance as the circuit over a range of frequencies corresponding with the range of speech frequen- cies to be transmitted. unless the electrical constants of the circuit are uniformly distributed the curve connecting circuit impedance with frequency will not be smooth and an accurate balance becomes impracticable. the repeater could not then be worked at its highest efficiency. in the anglo-dutch cable the impedance frequency curve is so smooth that so far as the cable is concerned the highest amplification which the repeater is capable of giving could be used. other factors not connected with the submarine cable limit the amplification actually ob- tained. a second similar cable will shortly be laid between the same points by the dutch government. the success of the anglo-dutch cable has led to the decision to adopt a similar type of cable for the anglo-belgian and anglo-french cables in each of which 28 conductors will be provided in seven quads giving 14 physical circuits and seven superposed circuits. the make up of the cables will be as follows:—conductor, solid wire 0-08 in. (2-03 mm.) diameter; weight 118 lb. per naut. loading is one wrapping of iron wire 0-008 in. 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