US1678203A - Communicating and switching system - Google Patents

Description

July 24, 1928. 1,678,203

E. H. SMYTHE COMMUNICATING AND SWITCHING SYSTEM 2 smfionqr efgsox azbna amfi sheets sheet I 2 3 I 2 3 l 2 3 l -2 3 Switching v Station 5,

H0 sumo" 4 to Shanon Safionl Slafiom? I 2 3 I 2 3' Switching Station A.

//7 vemor: Edw/h f1 Smy/he.

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July 24, 1928. 1,678,203

E. H. SMYTHE COMMUNICATING AND SWITCHING SYSTEM Filed Feb. 27, 1923 7 Sheets-Sheet 5 Qyerororfs posih'on l by A/fj/ July 24, 1928.

E. H. SMYTHE COMMUNICATING AND SWITCHING SYSTEM 7 Shets-Sheet 4 /n vemor: Edwin f1. Smy/he by Filed Feb. 27, 1925 July 24, 1928.

E. H. SMYTHE COMMUNICATING AND SWITCHING SYSTEM Filed Feb. 27, 1925 7 Sheets-Sheet 5 'lllllllHl lllll llllllllll lllll ||IIII IIIIll lllll lll llll'l 8 W, rm ms mfi 8 M f July 24, 1928. 1,678,203

E. H. SMYTHE COMMUNICATING AND SWITCHIMG SYSTEM Filed Feb. 27, 1923 7 Sheets-Sheet 6 o 2 0 9% N W Tw July 24, 1928.

E. H. SMYTHE COMMUNICATING AND SWITCHING SYSTEM Filed Feb. 2'7, 1925 -7 Sheets-Sheet '7 Wye/War. aw/2v H Smy/he by Patented July 24, 1928.

UNITED STATES 1,678,203 PATENT OFFICE.

EDWIN H. .SMYTHE, 0F EVANSTON, ILLINOIS, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A. CORPORATION OF NEW YORK.

commumoa'rnve AND swrrcnnve svs'rnm.

Application filed February 27, 1823. Serial No. 621,635.

This invention relates to an improved communicating and switching system, and more particularly to a system of the type in which carrier waves or oscillations, either free or guided, are employed as the medium for conveying energy for effecting the operation and control of electric signaling switch-.

ing and communicating apparatus located at separated points in the system. The invention especially relates to a system in which carrier currents, in the form of high frequency waves guided upon wires, are used as the medium of electrical connection between the electrical devices and apparatuses located at different points in the system. These carrier currents are assigned such frequencies that they are non-interferring with one another, so that each carrier serves as a connecting medium separate and distinct from the other carriers superposed upon the line or medium over which the carriers are transmitted. Each of the carriers is modulated in accordance-with the voice currents or other signaling or controlling currents that are to be carried. The carriers thus modulated are transmitted over the toll or long distance line and are passed through individual receiving arrangements each of which is selective of one carrier frequency to the exclusion of the others, and each of which serves to reproduce from the modulated carrier the voice, signal or controlling currents with which the carrier was modulated at the distant station.

The apparatus which is used to transmit and receive by means of any one high frequency wave as distinguished from that which is used for transmitting and receiving by means of a wave of a different frequency is commonly known as a high frequency or carrier channel, or as a frequency selective channel.

The present invention is directed particularly toward providing a system in which a connection may be built up, link by link,

between two tol or long distance stations through intervening stations or toll centers and the lines extending between them without the necessity of employing human agencies at the intermediate stations or centers; also toward providing a system in which a connection thus built up between two toll stations through intervening interme-.

diate stations may include a series of carrier channels each of which may be characterized by the employment of a different frequency from the frequency employed. by one or' The resent invention has for its princi pal ob ect's to provide an or anizatlo'i in which the extending of a tol connection through one or more intermediate toll centers is accomplished by machine switching methods; in which machine switching methods areemployed for effecting the intercon nection of carrier channels with each other; in which the automatic interconnection of the carrier channels with each other takes place at carrier frequency levels; in which any station of the system is enabled to complete connection with any other station of the system'through the agency of a series of carrier channel links automatically built up through one or more intervening carrier switching stations or toll centers and utiliz-- ing any of the carrier channels that happens at the time to be available in each of the series of-links; in which where succeeding carrier channels in the series operate at diverse frequencies, these-diverse frequencies are automaticall transformed at each succeeding point 0 interconnection to the frequency at which the next succeeding channel of the series is adapted to operate the channel frequencies thus bein automatically matched with each other; in which a plurality of matching frequencies is applied at the point of interconnection of two diverse channels of the series, and modulating means employing the matching frequencies, so that whatever the frequency level of the incoming frequency band may be, one of the products of modulation will be at the frequency level at which the outgoing channels is adapted to operate, means being provided to suppress the other modulation products; in which the matching frequencies applied show in skeleton form a at the point of interconnection of two diverse frequency channels are higher than the frequency level at which the outgoing channel is adapted to operate in order to invert the relative positions of the side bands and prevent the passage of unmodulated matching frequencies into the outgoing channel; in which the control of the building up and holding of the connection between the stations and the control of the display of the calling and supervisory signals is effected by means of a continuously applied audible frequency modulation of the carrier, which, however, is barred from the talking circuits at the terminal stations; in which the switching circuits and apparatusat the intermediate stations or toll centers are designed and adapted to minimize interference effects and facilitate the shielding of the high frequency circuit paths; and in which the control of the building up, calling, su- 'pervising and connection holding operations at the terminal stations 'is effected over a path entirely separate and apart from the talking circuit .of the terminal station.

'The foregoing are some of the principal objects that the system of the presentinvention attains. The foregoing and other features are all included in the present embodiment of the invention, but are capable of separate use in systems where it maybe desirable toemploy only part of the features herein disclosed. The 'manner in which the above mentioned and other objects and features are attained and realized will be exlained in the following description of the preferred form of the invention, and the structures and combinations involved in the realization of the various novel'features will be more particularly defined in the appended claims. 1

The preferred embodiment of the invention is illustrated in the accompanying drawings, in which Figs. 1 and 2 taken together portion of a comprehensive carrier multip ex toll switching system having a plurality of terminal stations and a plurahtv of intermediate switching stations or toll centers; and in which Figs; 3, 4, 5, 6 and 7 taken together .and arranged in the manner indicated in Fig. 8 show diagrammatically as much of the apparatus and circuits of one of the terminal stations and of one of the intermediate switching stations as is necessary to disclose the manner in which the apparatus and circuits of the entire system may be or d.

Reference will first be made to t e portion of a comprehensive carrier multiplex interconnecting system illustrated in skeleton form in Figs. 1 and 2. The portion of the entire system shown in Figs. 1 and 2 includes station 1, station 3 and station 5, which may be terminal toll or long distance stations, each of which operates in conjunction with a telephone exchange in which local subscribers lines are served; and switching station A and switchingstation B, which may be intermediate switching stations or toll centers into which the toll or long distance lines from the various terminal stations or other intermediate switching stations converge, and in which the various entering lines are adapted for interconnection by mechanisms under the control of the operators at the remote terminal stations. Toll or long distance lines extending to station 2, station t and station 6 arealso indicated as having connection with the intermediate switching stations A and B.

As diagrammatically indicated in the portion of the comprehensive system illustrated in Figs. 1 and 2 there may their respective subscribers stations to springjacks where the toll operator may have access to them by means of pairs of connecting cords and plugs. From each terminal station a toll line extends to the associated intermediate or switching station; and

as shown in the present embodiment of the be local lines at each of the terminal stations extending from 3. In the case of all of the toll lines entering switching station A from station 1, station 2 and station 3, and switching station B respectively, channel 1 in each case is arranged to receive upon the upper side band of the 10,000 cycle modulated carrier wave and to transmit upon the lower side band of the 23,333 cycle modulated carrier wave; chahnel 2 is arranged to receive upon the upper side band of the 13,333 cycle carrier and to transmit upon the lower side band of the 26,666 carrier, and channel 3 is arranged to receive upon the upper side band of the 16,666 cycle carrier and to transmit upon the lower side band of the 30,000 cycle carrier. At the distant end of each of the toll lines centering in switching station A, the frequency selective construction and adjustment of the channel apparatus is such that the transmission is efi'ected upon the saine frequency hand which is used for reception at switching station A, while reception at the distant station is effected over the. same frequency band that is employed for transmission at switching station A. That is, at the distant or terminal station channel 1 transmits over the upper side band of the 10,000

carrier and receives over the cycle modulated of the 23,333 cycle carrier,

lower side band channel 2 transmits over the upper side band of the 16,666 carrier and receives over the and side ban lower side band of the 30,000 carrier.

In the case of the toll lines centering in switching station B, and'connecting that station with station 4, station 5, station 6 and switching station A, the carriers and side bands employed for transmitting and receiving over the various channels are the same at switching station B as are employed at the terminal stations 1, 2 and 3 associated with switchin station A; and theparriers s employed for'--tlie" 'various channels at the distant ends of the toll lines centering in switching station B are the same as those'that are employedfor transmitting and receiving over the corresponding channels in the case of switching station A. The reason for using the he uencies indicated above in connection with t e channels at the various terminal and intermediate switching stations and the manner in which these frequencies are adapted to each other in the building up of a communicating and controlling connection between difierent terminal stations by way of one or more intermediate switching stations will be explained in the detailed description of the system that will follow. a

In the present embodiment of the invention, as shown diagrammatically in Figs. 1 and 2, four toll or long distance lines center in each of the intermediate or switching stations; and as each of these four toll or long distance lines is provided with three frequency selective channels, means are provided at each switching station for inter- 7 connecting any channel of any one of the toll lines with any channel of any of the other three toll lines entering the switching station. This is accomplished by terminating each channel of each of the four toll lines upon the movable terminals of an interconnecting switch, which movable terminals have access to stationary terminals representing the various channels of each of the other toll lines. These interconnecting switches are designated S to S, inclusive, at each of the two switching stations A and B. At switching station A channels 1, 2 and 3 of the toll line extending to station 1 ter-' minate upon interconnecting switches S, S and S respectively; channels 1, 2 and 3 of the toll line extending to station 2 terminate on switches S, S and,S; channels 1, 2 and 3 of the toll line extending to station 3 terminate on switches S, S and S; channels 1, 2 and 3 of the toll line extending to intermediate or switching station B termi-' nate on switches S, S and S At switching station B interconnecting switches S S and S are similarly identified with channels 1, 2 and Set toll line extending to station4; interconnecting switches S, S and S are identified with channels 1, 2 and 3 extending to switching station 5; interconnecting switchesS', S and S are identlfied with channels 1, 2 and 3 of the toll line extending to station A; and interconnecting switches S, S and S are identified with channels 1, 2 and 3 of the toll line extending to station 6.

- No attempt has'been made in the skeleton diagram of Figs. 1 and 2 to show the electrically actuated mechanisms at the switching stations and the controlling devices at the terminal stations by 'means of which" the operations of the switching station mechanisms are governed, these elements being fully disclosed in the detailed circuit dia gram of Figs. 3 to 7, inclusive. The skeleton diagram of Figs. 1 and 2 merely attempt to indicate the general scheme of interconnection employed between a certain.

or switching stations of the system. The significance of what is shown merely in outline in Figs. 1 and 2 of the drawings will be clearly apparent after the detailed circuits as shown in the other figures of the drawing are explained. It is sufficient at this time tosay, with regard to the arrangement shown in Figs. 1 and-2, that the operator at any orient the terminal stations indicated, by makin connection with an available channel 0% that terminal station, obtains control over the interconnecting switch identified with that channel at the associated switching station; and that thereupon this interconnecting switch is caused to operate, under the control of the operator at the distant callin station, to extend the channel. selected by tie operator at the calling station to any available or non-busy channel of the toll line extending to the called terminal station, or in the direction of the called terminal station through the medium of a toll line extending to the next intermediate or switching station through which the completed connection must ass.

At switching station A, as shown in igs. 1 and 2, there is an oscillator O, which produces a base frequency of 3,333 cycles, and this base frequency is vsupplied over the toll lines to the various other switching and terminal stations of the system. At each at the terminal stations this base frequency, acting through the medium of a harmonic generator, produces the various harmonic frequencies of the'base, including 10,000, 13,333, 16.666, 23,333, 26,666 and 30,000 cycles, and these harmonic frequencies are supplied to the transmitting branches of the various channels to be modulated by the voice currents and to the receiving branches of the various channels'to'demodulate'the corresponding incoming carrier side bands and reproduce the voice currents with which the carriers are modulated at the distant sta-- ing station for the purpose of separating out a relatively low controlling frequency that is employed to control the operation of the interconnecting switches at the switch ing station. The remaining harmonic frequencies produced at the intermediate or switching stations from the base frequency are the frequencies 33,333,' 36,666, $0,000,

43,333 and 46,666; and these frequencies are all applied to modulators included one in each of the various channel transmitting branches to change the frequency level of the carrier side band received over the channel from the calling station to a level such as to permit it to ass out over the transmitting branch of t e channel that the interconnecting switch identified with the calling station has selected for -use in establishing the next link in the completed connection.

- Thus as the connection is built up from one station to the next in the series, the interconnecting switch of the calling toll line acts under, the control of the calling station operator to select the line extending from the switching station to the terminal toll station wanted, or in the direction of such terminal toll station, the interconnecting switch automatically selects an idle channel of the toll line-that is to constitute the next link in the connection, .and the harmonic frequencies applied to the modulating apparatus in the transmitting branches of the two channels thus interconnected operate to change the carrier wave frequency level of the incoming branch of each channel to match it with the frequency level of the outgoing branch of whichever channel has been automatically selected to extend the connection to the next toll point.

As will subsequently appear from the detailed explanation of the organization and operation of the system, the system is capable of substantially indefinite extension so.

far as the inclusion of terminal stations and switching or intermediate stations is c0ncorned, successive switching stations and their associated terminal stations employing modulating, demodulating and matching-frequencies either of the value of those in 'cated in connection with switching staments of such tion and its associated terminal stations,

or of the value of those indicated in connection with switching station B and its associated terminal stations. Ofcourse it will be understood, and will be evident from the detailed explanation of the system that follows, that the particular frequencies that have been indicated are merely typical, and that if desired for any reason other sets of frequencies may be substituted for-them.

Reference will now be had to the detailed.

representation of certain of the circuits of two of the terminal stations and one of the intermediate or switching stations that are shown in Figs. 3 to 7, inclusive, of the drawing. In order to present a concrete ex-' ample it has been assumed that these circuits are the circuits of terminal stations 1 and 3 and certain of the circuits of switching station A indicated in Figs. 1 and 2 of the drawing. Inasmuch as stations 1 and 3 are identical in the matter of the arrangement of their circuits andthe carrier frequencies emlpoyed, Fig. 3 and Fig. 4 will be assumed in the followin description to represent the circuits of both which stations a connection will be assumed to be completed by way of the intermediate switching station or toll center station A. Referring now to Figs. 3 and 4:, the terminal toll station illustrated is assumed to have access to and to be accessible by the local telephone lines of an ordinary telephone exchange. These local lines are represented as being of the central energy type and as extending from the local subscribers stations, as SS, by way of metallic line circuits 101, 102 and 103 to terminal apparatus at thecentral oflice. The terminal apparatus of each line is represented as comprising the usual cut-off relay 10, line relay 11 and line lamp LL; and each line is further assumed to be provided with an answering spring'jack AJ and one or more multiple springjacks LMJ. The circuit arrangelines are well known in the art and need not be specifically described. Obviousl any other desired t e of line for estab 'shing communication b tween the central ofiice and the subscribers stations may be substituted'for the type of line par ticularly illustrated.

At the central 'ofiice or toll station two operator s positions are indicated, and there may be as many operators positions pro-' vided as are required to handle the local and-long distance or toll trafiic. Each of the two operators positions illustrated is.

of these stations, between and a toll plug TP the tip and ring contacts of which are united by means of a pair of talking conductors with an interposed repeating coil RC. Each connecting cord circuit has a local supervisory lamp LSL controlled by the connect local line through the medium of a supervisory relay '12, and a toll supervisory lamp TSL that is under the control of the channel with which the associated'toll plug TP is brought into connection. Each connectin cord circuit also is provided with a ringmg'key RK, a listening key LK' and a calling key CK; These keys are for the purpose of signaling the local subscribers stations and bring the operators telephone set OTS and the operators dial OD, common to the cord circuits of each position, into operative relation with the particular cord circuit with which the listening key LK and the calling key CK are associated. The operators dial OD may be of the well knownt pe disclosed in Patent 1,161,854, issued l ovember 30th, 1915 to O. F. Forsberg. On account of certain speed limiting factors in the circuits controlled by the dial, which will appear in the detailed description that follows, the speed governing mechanism of the dial is adjusted so as to retard the rate at which the dial in its operation opens and closes the circuit that it controls.

The carrier multiplex toll line through which the terminal station has communica-- tion with the other stations of the carrier multiplex system is generally of the type quency hybrid coil disclosed in Fig. 49 in the Colpitts and Blackwell paper entitled, Carrier Current Telephony and Telegraphy, published in the Journal of the. American Institute of Electrical Engineers for." April, May, and June, 1921. Referring to- Fig. 4 of the application drawing the toll line comprises a pair of metallic conductors 104 that enter the terminal station b way of a hi h fre- FH provide with the usual network N for balancin the toll line and producing a conjugate re ation between the windings of the high frequency hybrid coil. From the hybrid coil there is connection by way of the apparatus of channel 1, channel 2 and channel 3 to springjack terminals associated with eachof the three channels, these being designated respectively TJ 1 2 and TJ at operators position 1 is in use either for an outgoing or incoming connection, and the calling lamps CL are lighted by incoming calls over their respective channel's. v

Associated with each channel there are two relays and 16 responsive to the insertion of a plug in one of the corresponding, channel springjacks and controlling the energization of the carrier channel and the applicationto the carrier of the low controlling frequency; a relay 17 that responds to the condition of use of the channel and causes the lighting of the associated busy lamps; a relay 18 that responds to the application ofthe calling frequency at the dis tant end of the channel and controls the calling, supervisory and busy lamps; and two relays 19 and 20 that operate under the control of the other relays to energize the transmitting branch of the channel and apply to it the carrier frequency and the lower calling and controlling frequency. The high frequency hybrid coil HFH of the tollline has a main transmitting branch and a main receiving branch, and from each, of these'main branches there is a'sub-branch to each of the three channels. Interposed in each of these sub-branches is a band respectively, so constructed and adjusted as to pass the frequency bands comprising in each case the appropriate side-band of the modulated, carrierwave of the frequency respectively indicated by the numerals placed above the rectangles representing these filters, the frequency bands for the various channels being in accordance with what has already been statediin connection with the description of Fi s. 1 and 2 of the drawing. These and the ot er band filters designated as BF throughout the system are of the general type disclosed in Patent 1,227,113, issued May 22,. 1917, to G. A. Campbell and also described, with respect to the specific manner in which they are used in a carrier multiplex system of this type, in the Col itts and Blackwell article that has been re erred to. The organization of only one of the three channels, channel 1, is il u The other two channels are organized in the same manner as channel 1 excepting in the matter ofthe-s ecific frequency values on which they are esigned to operate, and are merely indicated by brokenline rectangles.

and MJ MJ and 'MJ at operator's posi-As specifically illustrated in the case of chantion2. All of the channel springjacks at the two positions have associated with them busy lamps, designated BL, BLand BL, respectively, for the three" channels; and each of the toll line terminal springjacks at operators position. 1 has-also associated with it a calling lamp designatedCU, CL and CL, respectively, for the three channels. The'busy lamps, as will be explained, are

lighted whenever the corresponding channel nel 1, the frequency band that passes through the band filter BF included in the rece1ving branch is carried intothe balancedde- .tector BD where it is joined by the corresponding carrier frequency selected from the common distributing circuit 105 by the tuned circuit TCE Demodulation takesplace in the balanced? detector BD ,.and the products of demodulation are-amplified in the amplifier A and are carried to the low pass strated in Fig. 4.

filter LPF and the band filter BF". The low pass filter LPF, in the, present instance, is constructed and adjusted to have an upper cut-off limit of 200 cycles per second and the band filter BF is constructed and adjusted to pass with substantially negligible attenuation a frequency band extendmg preferably from 300 to 2300 cycles per second. The relatively low callin and con-- trolling frequency conveyed by tlie carrier passes through the low pass filter LPF is rectified and amplified in the rectifier R and energizes the relay 18. The essential voice frequencies pass through the band filter BF and are carried to the mid-points of the line windings of the'low frequency hybrid LFH. One endof the line windings of the low frequency hybrid coil is connected through condensers with a low frequency balancin network N and the other end is connect through condensers and through an artificial line AL of the well known H type by way of conductors and 51 to the tap and ring or talking springs of spring jack TJ and also to the corresponding sprin 'ac s of such other multiple spring jacks J as ma beassociated with this particular channel:

The series winding of the low frequency hybrid LFH is connected by way of the band filter BF with the primary winding of the transformers 21. The band filter BF i. of. the Campbell type previously referred to, and is constructed and adjusted to pass a band of frequencies ranging from 300 to 2300 cycles per second and to cut off frequencies below and,v above these limits. The outer terminals of the secondary winding of the transformer 21 are connect. respectively with the grids of the two tiiermonic .vacuum tubes of the balanced modulator BM and the inner ends of the two halves of the secondary winding of the transformer are connected with movable contact points of'the potentiometer 22 the midoint of which is connected by way of the battery and the potentiometer 23 with ground and the positive terminal of the filaments of the two thermionic tubes of the balanced modulator BM. The potentiometer 22 serves to apply to the grids of the balanced modulator tubes at the proper potential the 150 cycle calling and controlling frequency, and the potentiometer 23 serves to apply to the grids of the balanced modulator tubes at the proper potential the carrier frequency which this channel is adapted to employ for transmitting purposes, in the present instance the frequency of 10,000

cycles per second. The organization of the modulator IBM is in accordance with the disclosure of Patent 1,343,306, issued June 15,

1920, to J. R. Carson, the modulator being of what is commonly known as the balanced effect of the potential variations in the divided input circuit' of the modulator is balanced and neutralized in the output circuit excepting when the low frequency voice current fluctuations are impressed bywayof the band filter BF and the transformer 21 or when the low frequency calling or controlling current variations are impressed by way of the potentiometer 22. In either of these cases the fluctuations in the output circuit of the modulator are no longer balanced and consequently oscillations are produced in the output circuit having an amplitude proportional to the'instantaneous value of the low frequency speech or low frequency controlling modulating currents. The mov-,

circuit being by way of normally open contacts of the relay 19, and the secondary side of the tuned circuit being connected with the extremities of th potentiometer 22. The tuned circuit acts not only to insure the application to the balanced modulator BM, of the pure control frequency, but also to prevent the backing up into the common distributing circuit 106 of the other frequencies that are present in each of the balanced modulator circuits, namely, the carrier frequency of each of the various channels and the voice frequencies with which the carrier is to be modulated. The carrier wave employed in the transmitting branch of the channel is derived from the common distributing circuit by way of a tuned circuit TC which is constructed and adjusted to be selective of the particular carrier frequency that the channel employs. The connection with the primary side of the tuned circuit T0 is by wayof alternate contacts of the relay 20, and the secondary side of the tuned circuit has connection with the extremities of the fpotentiometer 23.

The output side 0 the balanced modulator, BM is connected by way of the windings of the transformer 24 and a high pass filter HPF with the input side of the balanced amplified BAR The high pass filter HPF acts to prevent the passage of the amplified andunmodulated low controlling and voice frequencies through the balanced amplifier BA inorder to prevent these frequencies from overloading the amplifier. The balanced amplifier BA is of the so-called in connection with Fig.

push-pull type as illustrated and described 49 of the Golpitts and Blackwell paper reinbefore referred to, and the high pass filter HPF is of the sort also illustrated and described in the Colpitts and Blackwell paper. Theoutput of the balanced amplifier BA is connected by way of the band filter BF with the main transmitting branch that joins the various channel transmitting branches with the high frequency hybrid coil HFI-P.

As the tubes of the balanced modulator BM and the balanced amplifier BA are required to be active only when the associated channel is in use, the circuit of the A battery for heating the filaments of the tubes is controlled incontacts of the relay 20 which are normally open and are closed only when the relay 20 is energized by the taking of the channel for use. The circult of the A battery is illustrated as extending in parallel branches to the filaments ofthe balanced modulator and amplifier tubes; but it will be understood that-this is merely a diagrammatic showing, and that the enercircuits of the filaments of the tubes may e connected with each other and the associated circuits in accordance with the standard practice of the wiring of these devices. Inasmuch as it is desirable that the receiving branches of the various channels be normally in a responsive condition in order that they may constantl be in readiness to respond to a call on inated at a distant station, it is assumed t at the thermionic tubes associated with the receiving branches have their filaments normally ener- 'zed, and therefore the A battery circuit or these tubes and means for controlling it are not shown.

The apparatus by means of which the carrier frequences for use in the various chan-. nels are produced is shown diagrammatically at the bottom of Fig. 3 of the drawing.

The receiving branch that extends from the high frequency hybrid coil HFI-I to the receiving branches of the various channels also has another branch that has interposed in it a tuned circuit STC which is tuned to permit the passage through it of the base frequency 0 3,333 cycles per second suplied over the carrier multiplex toll line rom one of the remote stations, as will be described, and also includes a shunt made anti-resonant to the 3,333 frequency and which therefore permits the substantially unattenuated passage of the other frequencies in the series circuit of the high frequency hybrid coil receiving branch. The tuned .circuit STC is connected by way of conductors 107 with the input circuit of the amplifier A (Fi 3) and the output generator is connected with the input circuit of the amplifier A and the output circuit of amplifier A is connected with the distributing circuit 105. This train of apparatus acts to generate and amplify a series of harmonics of the base frequency 3,333 in the manner described in the Colpitts and Blackwell paper previously referred to, the nature and arrangement of the apparatus being in accordance with the disclosure contained in Patent No. 1,446,752 of February 27, 1923 to Burton W. Kendall.

The low control frequency, 150 cycles per second in the present embodiment of the invention, is produced in the oscillator 0 (Fig. 3), which oscillator may be of any usual and well known thermionic type with a feed back connection from its output into its input circuit. The sinusoidal oscillations produced in the oscillator() are amplified in the amplifier A andthence pass through the tuned circuit TC, which serves to suppress the undesired frequencies, to the primary winding of the transformer 14. This transformer is provided with a grounded shield between the primary and secondary windings to prevent unbalance to ground through the transformer of the two sides of the distributing circuit 106 which is connected with the secondary winding of the transformer. The distributing circuit 106, as has been described, has branch connections with the different channels through which the 150 cycle control frequency is applied for the modulation of the channeltransmitting branch carriers under the control of relays such as relay 19 of channel 1.

The organization of the circuits and apparatus at the intermediate or switching station will now be described by reference to Figs. 5, 6 and 7 of the drawing. Fig. 6 illustrates diagrammatically a. part of the switches at the switching station through the medium of which interconnection is made between the various channels, and Figs.- 5 and 7 show the channels identified with two of the four carrier multiplex lines that are assumed to terminate at the switching station, the circiiits of one of the channels of each of the two carrier multiplex lines being illustrated diagrammatically in detail and the other two channels of each of these two lines being merely indicated by broken line rectangles. The organizations of the circuits of the different channels of windings of the hybrid coil are connected with the network N which exercises the usual balancing function. From the high frequency hybrid coil HFH the main receiving branch has connection by way of the band filters BFQBF 11 and BF with the separate receiving branches of channel 1, channel 2 and channel 3, and the main transmitting branch entering the high frequency hybrid coil has connection by way of the band filters BF BF and BF with the transmitting branches of the three channels. The main transmitting branch also is connected by the anti-resonantly shunted tuned circuit STC with the. apparatus illustrated in Fig. 6 that generates the base frequency of 3,333 cycles per second and the various desired harmonics of this frequency. The band filters BF to BF inclusive, are of the type disclosed in the Campbell patent previously referred to. The filters BF", BF and BF are constructed and adjusted to pass the upper speech and control frequency side bands of the 10,000 cycle, 13,333 cycle and. 16,666 cycle carrier waves, re-

spectively, and the filters BF BF and BF are constructed and adjusted to pass the lower'speech and control frequency side bands of the 23,333, 26,666 and 30,000 car-- rier waves respectively.

Referring now to the typical channel circuit arrangement illustrated in connection with channel 1 of Fig. 5, the receiving branch extends directly from the band filter I BF to normally open contact points of ,two

controlling relays 35 and 36, the functions of which controlling relays will be described hereinafter. The transmitting branch of the channel extends from other normally open contact points of these relays to the input circuit of 'the balanced modulator BM The output side of modulator BM extends by way of a Campbell type band filter BF, adapted to pass the 23,333 cycle carrierwave and its lower speech and control frequency side band, to the input circuit of the amplifier A", and the output circuit of this amplifier is connected with the band filter BF, previously referred to, which is interposed between the channel transmitting branch and the common transmitting branch of the high frequency hybrid coil EFT-l The balanced modulator BM is of the type disclosed in Patent 1,343,306 to Carson previously referred to in which the currents from the normally open contact points of the controlling relays 35 and 36, and the modulating frequencies are supplied to the divided input circuit by way of the common distributing circuit 108 that extends from the harmonic frequency generating apparatus illustrated in Fig. 6. As will subsequently be described, there are present iii the common distributing circuit the harmonic frequencies having the values of 33,333, 36,666, 40,000, 43,000 and 46,666 cycles per second. Consequently the frequencies that are supplied to the balanced modulator BM by way of the serially connected portion of the modulator input circuit simultaneously modulate all of the frequencies supplied byway of the common distributing circuit 108 to the common portion of the divided input circuit of the modulator. The thermionic tubes of the balanced modulator BM are chosen of sufficiently great capacity to carry the peak voltages of the combined modulating harmonic frequencies and the frequencies to be modulated without causing the grids of the tubes to go positive. If this occurred it would have a tendency to produce improper intermodulations between the various frequencies and to cause distortion in the modulator. But by exercising care to choose thermionic tubes of a type having ample capacity from among t ose which are available and well known in the art, the modulation which occurs between the frequencies which enter by way of the series 100 portion of the input circuit and each of the multiplicity of harmonic frequencies which enter by way of the parallel portion of the input circuit may be made to be the same as through the other harmonic modulating frequencies were not present.

It may be well at this point to explain the manner in which this modulating organization operates in conjunction with the channel switching apparatus to establish opera- 11 tive relation between incoming and outgoing channels and change the incoming frequency to the frequency at which the apparatus of the outgoing channel is adapted to operate. I It is apparent that in a general scheme of interconnection such as is contemplated in the present invention any incoming channel receiving branch must be capable of being extended to any outgoing channel transmitting branch regardless of the diverse'frequencies of the different channels that may be brought into connection. Referring spe cifically for example to channel 1 as illus trated in Fig. 5 of the drawing, in order to pass through the band filters BF and BF in the transmiting branch of the channel the transmitted frequencies must lie within the range of the 23,333 cycle carrier wave and its lower speech frequency side band. Assuming that the speech frequency side band to be transmitted has a breadth of 2,200 cycles, this means that in order to pass out of the transmitting branch of channel 1 the modulated current must lie within the range comprised between 23,333 cycles and 23,333- 2,200 cycles per second. But the frequencies which enter the switching station by way of the receiving branches of the various channels of the carrier multiplex toll lines that center in the station are the upper side band of the 10,000 cycle carrier wave for channel 1, the upper side band of the 13,333 cycle carrier wave for channel 2, and the upper side band of the 16,666 carrier wave for channel 3 of each of the carrier multiplex toll lines. None of these frequencies, therefore, as they enter the switching station, are

at the proper level to pass out by way of the transmitting branch of channel 1, or by way of the transmitting branches of channels 2 and 3. In order to pass out by way of the transmitting branch of any of the channels the level of the incoming .frequencies must be changed; and moreover the extent of the change necessary depends upon the difi'erence in frequency level between the incomin frequencies and the frequency at whic the outgoing transmitting branch is adapted to operate. Furthermore, as the received mod ulations of the carrier of each channel are in the upper speech frequency side band of that carrier, and as the transmitted modulations outgoing from the switching station must lie in the lower speech frequency side band of the carrier, in the embodiment of the invention herein disclosed, the interconnection of onechannel with another at the switching station must be effected in such a way as to invert the relative position of the side band with its carrier.

All of the above mentioned transformations necessary to the operative indiscriminate interconnection of the channels are effected by providing for the modulation, in

' each channel transmitting branch, of the frequencies received from the interconnected channel by a group or multiplicity of frequencies lying above the received frequencies by a definite,' predetermined amount in the frequency scale.

To illustrate the actionwe may assume that the switch mechanism at the intermediate station has operated to connect channel 1 of one of the carrier multiplex lines with channel 1 of the line illustrated in Fig. 5. Under these circumstances the receiving channel-of the first mentioned line is connected with the transmitting branch of channel 1 of Fig. 5, and consequently the 10,000 cycle carrier wave with its upper speech frequency side band is being applied to the series input circuits of the balanced modulator BM of channel 1. At the same time there is applied to the parallel input circuit of the modulator 13M the harmonic frequencies 33,333, 33,333, 40,000, 43,333 and.

products of the received voice modulated carrier with each of the five harmonic waves as followsz- As may be seen from the foregoing the result is a plurality ofmodulation products of which the second in the series, namely that represented by 23,333-, represents the 23,333 cycle carrier with its lower speech side band which is adapted to be transmitted through the band filters BF and BF interposed in the output circle of the modulator BM? All of the other modulation products in the series lie outside of the imits of the band filter BF, and conseuently are suppressed and do not pass to t e amplifier A which is interposed between the two band filters BF and BF. It may be remarked that the amplifier A. is employed for the purpose of relieving the balanced modulator BM as far as possible of the work of amplification which would tend to overload it and therefore prevent it from properly performingits primary function of separately combining without distortion the multiplicity of frequencies present in its input circuit. It may also be remarked that there will be present in the output circuit of the modulator a number of frequencies other than those noted in the table given above, these frequencies including the amplified 10,000 cycle carrier with its upper side band ofthe interconnected receiving channel as well as the group of harmonic modulating frequencies 33,333 to 46,666, inclusive, which are not entirely suppressed in the balanced modulator. But it may be seen that these frequencies also are outside of the frequency range of the band filters BF and BF? interposed in the out-' put circuit of the modulator BM so that these undersired frequencies are also suppressed anddo not pass into the common Transmitting branch of the carrier multiplex If it is assumed that it is channel 2 of one of the other carrier multiplex lines that has been brought into connection with channel 1 of Fig. 5, then-it will be the 13,333

cycle carrier with its upper side band that adapted to pass through the band filters BF? and BF in the output circuit of the modulator BM and that the other products of modulatipn will be suppressed in these filters and barred from entering the carrier multiplex line. Futhermore, as stated in connection with the case first considered, the other undesired frequencies which are present in the output circuit of modulator BM are outside of the range of the filters BM and BM and are also suppressed.

If it is assumed that it is channel 3 of one of the other carrier multiplex lines that is connected with channel 1 of Fig. 5, it is the 16,666 carrier wave with its upper speech frequency side band which is imposed upon the series circuit of the input side of the balanced modulator BM The sum and difference modulation products which then appear in the output side of the balanced modulator are 50,000+, 16,666, 53,333+, 20,000+, 56,666+, 23,333, 60,000+, 26,- 666, 63,333+ and 30,000-. It-,will be noted that the sixth in this series represents the frequency bearing the speech modulation that is adapted to pass through the filters BF and B13 interposed in the output circuit of the modulator, and that the frequencies of the other modulation products are such that they are suppressed in these filters.

A further fact which appears from an inspection of the groups of modulation products recited in connection with the three assumptions that have been made as to the particular channel that has been connected with the outgoing or transmitting branch of channel 1 of Fig. 5, is that each of the three groups noted contains the modulation products represented by the numbers 26,- 666-, and 30,000- as well as that represented by the number 23,333. That is to say, whenever the receiving branch frequency 10,000+, 13,333+ or 16,666+ modulates the plurality of harmonic modulating frequencies 33,333 to 46,666, inclusive, among the modulation products there is a speech modulated carrier frequency so located in the frequency scale as to be capable of passchannels is included in the connection;

and such other amplification or modulation products as are present in the output circuit of the modulator are so positioned in the frequency scale as to be barred out of the main transmitting branch of the carrier multiplex line by the interposed band filters.

It seems well at this point to digress long enough from the description of the organization of switching station A illustrated .in Figs. 5, 6 and 7 to explain the way in which the above described manner of matching the diverse frequencies of the different interconnected channels is applied at switching station B. As illustrated in Figs. 1 and 2, the channels of the lines that center at switching station E employ the same carrier frequencies as the frequencies employd by the channels of the lines that center at switching station A excepting that the carrier and side band frequencies associated with the transmitting branches of the channels at switching station A are associated with the receiving branches of the channels at switching station E, and vice versa. Consequently the modulation that occurs in the transmitting branches of the channels at switching'station B must be of a character that will transform the incoming lower side bands of the carriers 23,333, 26,666 and 30,000 into the upper side bands of the out going carriers 10,000, 13,333 and 16,666. This is effected by applying to the common portion of the divided or parallel input paths of the balanced modulators in the transmitting branches the same harmonic modulating frequencies that have been described as being employed to perform the frequency transformation in the balanced modulators of switching station A,-that is, the frequencies 33,333, 36,666, 40,000, 43,333 and 46,666. The intermodulation of these five frequencies with the carriers 23,333, 26,666 and 30,000 with their lower side bands, one or another of which is applied to the series input circuit of the balanced modulator by way of the receiving branch of the interconnected channel, gives rise to sum and difference modulation products-in the output circuit of the balancedmodulator, just as described in the explanation of the modulating action at switching station A. All of the sum or additive products of modulation are far above the frequency range of the band filters for passing the 10,000+,

13,333+ and 16,666+ frequencies to the transmitting branches of the high frequency hybrid coils of the respective carrier multiplex lines. The difference or subtractive products of, modulation of the five harmonic modulating frequencies with the three incoming or receiving branch carrier frequencies and their lower side bands include the 3, there appear in the difference modulation products in the output circuit of the modulator the frequencies 10,000+, 13,333+ and 16,666+. In other words, no matter which channel at switching station 13 is connected to any one of the other channels, the modulation ofits receivingbranch frequency by the five harmonic modulating frequencies applied to the balanced modulator in the transmitting branch gives rise among the different modulation products to a carrier .wave and upper side band that is so positioned in the frequency scale as to be capable of passing out through the band filters interposed in the output circuit of the mod ulator; and furthermore that the cut-off limits of the band filters interposed beyond the balanced modulator in the transmitting branch of each of the three channelsis such as to bar the passage of all modulation product frequencies other than the particular speech modulated carrier which the transmitting branch of that channel is designed and adapted to transmit.

It may be seen from the foregoing discussion that the modulating frequency required to match the carrier side bands in an interconnection between channel 1 and channel 1 is 33,333, between channel 1 and channel 2 is 36,666, between channel 1 and channel 3 is 40,000, between channel 2 and channel 2 is 40,000, between channel 2 and channel 3 is 43,333, and between channel 3 and channel 3 is 46,666. This is under the assumptions liereinbefore Set forth with respect to the carrier frequencies employed for the different channels and the relation of the speech frequency side bands to their respective carriers. Of course, a change in these factors will necessitate corresponding changes in the values of the modulating frequencies for matching the channel frequen cics with each other. It is obvious that in the case of the interconnection of any one of the channels with any one of the other channels only one of the multiplicity of matching frequencies, that is the frequencies from 33,333 to 46,666, inclusive, is usefully employed. The particular one which is needed is automatically selected and made effective by virtue of the frequency selective characteristics of the apparatus in the output side of the balanced modulator BM, the other frequencies and their modulation products being automatically suppressed and eliminated in this apparatus. It is to be understood that it is deemed to be within the scope of the presentinvention to efiect the selection of the particular matching frequency desired by means of a selection automatically effected before the matching frequency is applied to the modulator, as, for instance, by selective circuits operating to make effective one or another of the matching frequencies in the movement of the channel selecting switch upon which eachchannel terminates. But it has been found that the normal application of all of the matching frequencies to all of the modulators, and the suppression and elimination of the undesired modulation products in the output circuit of the modulator, permits of a very substantial simplification of the switching apparatus; and therefore this form of the invention is disclosed as the preferred embodiment.

It may further be noted that the modulation which effects the matching of the carrier frequency of one channel with another may be effected in modulators of well known types other than the particular balanced or carrier suppression type indicated at BM the filters in the output circuit of the modulator being relied upon to suppress the undesired products of modulation. But the balanced modulator of the type, that has been described is preferred for the reason that it is adapted in itself to suppress or materially reduce in quantity a portion of the undesired modulation products, and is furthermore Well adapted to handle with out distortion the relatively large amounts of energy that are present in the multiple modulation that the preferred form of the invention renders desirable.

Further reference may now be made to the organization of the receiving branches of the channels, as typefied in connection with the receiving branch of channel 1 shown in Fig. 5. As has hereinbefore been stated, the receiving branch extends in two conductors from the band filter, as BF", interposed at the head of the channel to normally open contact points of the two controlling relays, as 35 and 36. It is over this path' that the voice modulated frequency bands pass between the two interconnected stations. For switching control purposes a branch extends from this pathby way of the amplifier A to the balanced detector BD This detector is of the. type disclosed in Patent 1,343,306 to Carson issued June 15, 1920, and is suppliedfrom the common distributing circuit 109 by .way of the tuned circuit TC? with the harmonic demodulating frequency of 10,000 cycles corresponding with the carrier frequency of the associated channel branch. The output circuit of the balanced detector 13D is connected with the input circuit of the rectifier R by way of the tuned circuit TC which is so constructed and adjusted as to pass the control frequency of 150 cycles per second which is separated out by the demodulating action which takes place in the detector BD After amplification and rectification in the rectifier It the 150 cycle control frequency is effective to energize the relay connected in the output circuit of the rectifier, and the relay 25 acts as the medium through which the distant station controls the selective operation of the intermediate station switching mechanism and its restoration to normal when disconnection occurs. This switching mechanism and its associated controlling circuits will now be described.

Under the conditions that have been assumed there are at switching station A twelve switches, each one serving as the active terminal of a different one of the twelve channels of the four carrier multiplex lines centering at the station. In Fig. 6 only four of these switches are diagrammatically illustrated, the switches shown being designated S, S S and S As the titles on Fig. 6 indicate, switches S and S are the incoming terminals of channels 1 and 2 respectively of station 1, switch S is the in- .coming terminal of channel 3 of station 3,

and switch S is the incoming terminal of channel 1 of switching stationB. Each of the switches may conveniently be of the sin gle motion rotary type in which brushes or wipers, in the present instance 1, 2, 3, 1, 5

and 6, respectively, are moved by electromagnetically driven step by step mechanism over corresponding rows of stationary contacts or terminals and are spring returned to their normal position upon the withdrawal of a holding pawl by the associated release magnet. Such mechanism is diagrammatically illustrated in Fig. 5, in which the stepping magnet 33 operates the brush moving ratchet wheel 41 through the medium of the stepping pawl 42 and the release magnet 3* when energized lifts the holding pawl 43 and also at the same time disengages the stepping pawl 42 from the teeth of the ratchet wheel. Rotary switches of this type are well known in the art and require no special description or illustration. Obviously one ordinarily skilled in the art may readily and without the exercise of invention substitute other of the well known types of switches for the single motion stop by step rotary switch that is indicated. It may be remarked, however, that a type of switch like that indicated, in which the groups of contact terminals associated with all of the channels of the carrier multiplex lines may be reached, as will be described,

of the invention which secures asimple,

compact and also readilyshielded wiring arrangement for the channel conductors that operate at the diverse carrier frequencies required. 7

Each of the brushes. 1 to 6, inclusive, of the switch is adapted in its movement to engage a corresponding series of contact terminals; The relation between brushes 1 and'2 and the contact terminals which they are adapted to engage is such that the brush engages each succeeding contact of the series before it disengages the preceding contact, In the case of the brushes 3, 4:, 5 and 6 the relationship issuch that each brush disengages one contact before it comes into engagement with the next contact of the series. In the first or resting position of the switch brushes as also in the second position, only brush 1. engages a stationary terminal In their 3rd, 4th and 5th positions the brushes are in engagement successively with three groups of contacts serving as terminals of channels 1, 2 and 3 respectively of station 1. In the 6th position brushes 1, 2, 5 and 6, engage terminals concerned with the transmission of a'busy tone signal to the calling operator incase all of the channels of station 1 are in use. The 7th, 8th, 9th and 10th groups of stationary contact terminals serve similar purposes in connection with the channels of station 2. The 11th, 12th, 13th and 14th groups of sta: tionary terminals serve like purposes in connection with the channels of station 3, and the 15th, 16th, 17th and 18th groups of ter-- minals engaged by the set of brushes serve like purposes in connection with the channels of switching station B. Corresponding groups of contact terminals in all the switches of the series are multiplied together, andthe multiple wires of each group are connected with conductors extending to the channel apparatus of the associated channel. As it is never necessary for any of the channels to complete connection with another channel of the same carrier multiplex line, the multiple wiring between corresponding groups of stationary terminals in the various switches does not have connection with the groups of terminal contacts in the switches serving as the active terminals oi the channels of the corresponding carrier multiplex lines. -Thus as shown in Fig. 6, the multiple conductors representing the incoming paths to channel 1 and channel 2, respectively, of station 1 are not extended to connect with the stationary contact group engaged in the 3rd and 1th positions of the brushes of switches S and S and corresponding omissions are made at switches S and S in the connections of the multiple conductors that serve as the incoming paths to channel 3 of station 3 and channel 1 of station B respectively.

The row of contact terminals engaged by brush 1 has to do with the selection, under the control of the operator atthe remote calling. station, of the particular carrier multiplex station or group of channels to which it is desired to extend connection; the stationary contacts engaged by brush 2 have to do with the automatic selection of a particular available or non-busy channel of the group that the distant operator has selected; and the stationary contacts engaged b the brushes 3, 4, 5 and 6 have to do with the transmission of the speech modulated carrier frequencies between the connected terminal stations. This will appear clearly in connection with the description of the operation of the system hereinafter. It will be observed that the order of the switch brushes and their respective rows of contact terminals shown at one side of Fig. 6 is inverted relatively to the arrangement shown at the other side of Fig. 6. This involves no change in the relative circuit arrangements of the different switches, but is merely for the purpose of symmetry in the two halves of the drawing shown in Figs. 5, 6 and 7.

There still remains to be made a brief reference to the apparatus illustrated at the bottom of Fig. 6 for generating the base frequency that is to be transmitted to the other stations of the system and the "harmonic frequencies that are used at theswitching station for effecting the frequency changes and demodulations to which reference has hereinbefore been made; also the apparatus for impressing a busy tone upon one of the carriers to be employed in case all of. the channels of a selected group are found to be in use. The base frequency of 3,333 cycles, in the present instance, is generated in the oscillator 0 which is preferably of any usual and well known thermionic type with a feed back connection from its output into its input circuit. The sinusoidal oscil lations produced in the oscillator O are amplified in the amplifier A, have the magnitude of their voltages greatly increased in the transformer 47 and are then impressed upon the input circuit of the harmonic gen erator HG The magnitude of the impressed oscillations is such as greatly to overload the thermionic tube of the harmonic generator, with the result that the repeated oscillations are distorted into an extended series of currents the frequencies of which are the harmonics of the base frequency current. The above described action is in accordance with the disclosure contained in said Kendall Patent No. 1,446,752. The harmonics thus produced consist of an extended series having relatively uniform energy values. The base frequency of 3,333 cycles is separated out by means of the antiresonantly shunted tuned circuit STU and is applied -to the distributing circuit 100 from which it is supplied by way of shunted 7 tuned circuits, as STC' of Fig. 5, to the main transmitting branches of the difierenb carrier multiplex lines centering in switching station A, The group of harmonic frequencies 10,000, 13,333 and 16,666 is separated out by means of the band filter BF and is applied to the distributing circuit 109 from which the different frequencies of the group are supplied for the demodulation of the carriers of the channels of the various carrier multi lex lines by means of tuned circuits as T 6 of Fig. 5. It will be understood of course that the harmonic frequency taken 011' by the tuned circuit TC of each of the various channels centering in the switching station is such as in each case to correspond with the carrier frequency of the receiving branch of that channel. The harmonic frequencies 33,333, 36,666 40,000, 43,333, and 46,666 are separated out by means of the band filter BF and are applied to the distributing circuit 108 from which they are supplied to the balanced modulators, as 3M of the transmitting branches of all of the channels centering in the switching station for the purpose of matching the channel frequencies, as hereinbefore described.

Any one of the three frequencies 10,000, 13,333, or 16,666, the 10,000 cycleirequency being used in the particular-embodiment disclosed, is selected by means of the tuned circuit TC (Fig. 6) and is applied to the parallel input circuit connection of the balanced modulator BM for the purpose of being modulated by a characteristic frequency or tone which is derived from any suitable source and is applied to the series input circuit of the balanced modulator BM. The modulation products are amplified by means of the am lifier A connected in the output circuit of t e modulator BM; and the 10,000 cycle carrier frequency with its upper side band carrying the busy tone modulation is selected by the band filter BF and is. applied to the distributing cir cuit 130 which has a branch connection with the pairs of stationary contact terminals adapted to be engaged by brushes 5 and 6 of the various switches in the 6th, 10th, 14th and 18th positions that the switch brushes are adapted to occupy in the successive sta es of their movement.

The operation of, the system will now be described in detail. It will be assumed that connection is to be established between one of the local lines centering in station 1 and one of the local lines centering in station 3 by way of switching station A, as shown generally in the skeleton diagram of Figs. 1 and 2 and as shown more specifically in

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