US1471638A - Telephone system - Google Patents

Description

Oct. 23, 1923. 1,471,638

D. F. WHlTlNG TELEPHONE SYSTEM Filed Dec. 8, 1918 //4 van) 0 Dana/a E Wh/f/nq y Patents Get. .23, W23.

DONALD F. WRITING, OF NEW YORK, N. Y., ASSIGNOR TO WESTERN ELECTRIC COM- PANY, INCORPORATED, OENEW YORK, N. Y., A CORPORATION OF NEW YORK.

TELEPHONE SYSTEM.

Application filed December 28, 1918. Serial No. 268.757.

To all whom it may concern:

Be it known that I, DONALD F. WRITING, a citizen of the United States, residing at New York, in the county of Bronx, State of New York, have invented certain new and useful Improvements in Telephone Systems, of which the following is a full, clear, concise, and exact description.

This invention relates to telephone systems, and moreparticularly to such systems in which a plurality of sets of different frequency alternatingcurrents are propagated over a transmission line. While the invention is capable of use in connection with higher frequencies, it is particularly applicable to'systems in which currents of frequenries within the talking or voice range and currents of a frequency or frequencies outside this range, usually and preferably below those of the talking range, are transmitted from one line section to another line section. 3

It has previously been proposed in systems of this character to amplify both talking and signaling currents while such currents were bein transmitted from one line section to anot er. One method proposed involves the use-of separate amplifiers, one for amplifying talking currents and the other for amplifying signaling currents. These amplifiers are connected in separate paths joining the two line sections. In order to prevent interference between the two amplifiers, it is n'ecessar to include in one path apparatus for exc uding signaling currents and'in the other path apparatus for excluding talking currents. While such a system isefiicient in the transmission and amplification ofboth. talking and signaling currents, a large amount of apparatus is re-.

uired. Another method proposed involves tie use of a single amplifier and associated apparatus for transmitting and amplif mg both talking and signaling currents rom one line section to another line section. In such prior systemsemploying but a single amplifier, it has not been-possible to efiiciently'tra-nsmit both the talkingand signaling currents Without designing the system primarily for the efiicient transmission and amplification of current of one character and providing supplementary apparatus in the form of electromagnetic switching devices responsive to current of the other char- .clectromagnetic switching devices.

acter transmitted over one line section to alter the relation of the amplifier and its associated apparatus for temporarily transforming the system from one efficient in transmitting and amplifying current of the one character to one efficient in transmitting and amplifying current of the other character. This, it will be readily seen, renders it impossible for such a system to be used for the simultaneous transmission and amplification of both talking and signaling currents. Furthern'iore, the electromagnetic switching devices connected with the line section involve the use of extra apparatus which is, of course, energy consuming and therefore reduces the amount of signaling current availablefor the control of signaling devices.

It is therefore the object of this invention to provide a system which overcomes the difficulties experienced with the systems heretofore proposed. While the system according to the present invention overcomes the obje: tions to and difficulties experienced with previous systems, it also,-by employing a sin'gle ampli er, transmits and amplifies both talking and signaling currents from one line section to another as efiiciently as current of either character may be transmitted and amplified singly, and accomplishes this Without the introduction of any A system according to the presentinvention employing but a single amplifier, not only permits the most efiicient transmission of current of either character without the introduction of electromagnetic switching devices but, on account of the elimination of these switching devices, it permits of the simul taneous and uninterrupted transmission and amplification of currents of both characters.

In attainin the object of this invention, two sections 0 a transmission line are joined through transformer means preferably in the form of separate transformers for talking and signalin currents and a cooperating amplifier which transmits and amplifies both talking and signaling currents. The input circuit of the amplifier is connected with one line section through the separate talking and signaling current transformers,

and the output circuit of the amplifier is flection losses resulting from the transmission of currents from a circuit of one imped-' ance to a circuit of another impedance. A condenser cooperating with the primary winding of the signaling current transformer to form a closed circuit resonant t signaling currents, is connected in series with means included in bridge of one line c section for controlling the amplification of the amplifier. In order that thegreatest degree of amplification of talking currents may be obtained, the secondary winding of the signaling current transformer is preferably connected .between the secondary windings of the talking current transformer and the cathode (or heating element or filament) of the amplifier.

For a more complete understanding of this invention, reference should .be had to the following description considered in con-- nection with the accompanying drawing, illustrating one embodiment thereof.

As shown in the drawing, L represents a section of a transmission line which is joined with a second section L through an amplifier 5. As usual, this amplifier comprises a cathode orfilament electrode 6 which is heated by means of a grounded battery 7, current being supplied therefrom through a regulating resistance 8; a grid or modifying electrode 9 connected in the input circuit; and a plate electrode 10 connected in the output circuit. The input circuit of the amplifier 5 is connected with the line section L through separate talking and signaling current transformers. The talking current transformer comprises two equal primar' windings 11, 11 and a secondary winding 12, 12, while the signaling current transformer comprises a rimary winding 13 and a secondary winding 14.. Primary winding 13 is connected in series with and between the primary wind ings 11, 11, while the secondary winding 14 is connected between the cathode or filament 6 and the secondary winding of the talking current transformer. The secondary windings being in series. The windings of the talking current transformer are wound on one core, while the windings of the signaling current transformer are wound on another core. A battery 15 is provided for maintaining a negative potential on the modifying elect-rode or grid 9. An adjustable potentiometer comprising equal resistances 16, 16 is connected in bridge of the line section L for controlling the amount of current admitted to the primary windings 11,11 of the talking input transformer. A condenser 17, preferably of rather large .capacity, connected in the potentiometer bridge is also connected in parallel with the primary winding 13 and affords a ath of low impedance to talking currents etween the two sections of the potentiometer and between the two primary windings 11, 11 of ing currents of lower frequencies being beneficial.

Sincethe condenser 17 is connected in parallel with the primary winding 13 of the signaling current transformer, a closed resonant circuit is provided which is tuned to the signaling frequencies. To assist in the tuning of the closed circuit to signaling currents and for purposes which will hereinafter more fully appear, a condenser 18, preferably of low capacity, 18 connected in parallel with the secondary winding 14 s of the signaling current transformer. The natural characteristics of closed resonating circuits are such that currents of the resonant frequency incoming over the line section L and impressed upon the terminals of the resonant circuit through parallel paths by way of the sections of the potentiometer and the primary windings of the talking current transformer set up in this closed circuit oscillatory currents like in character to, but greater in magnitude than the impressed currents. These local oscillatory currents of great -magnitude cause. the terminals of this circuit to assume an effec tive impedance to signaling currents of the resonant frequency comparable to the impedance of the line section. L and the parallel paths which include thepotentiometer 16 and the primary windings 11 of the talking current transformer. This effective impedance to the signaling currents of the resonant frequency is much higher than the impedance to currents of this character presented by either the condenser 17 or the primary winding 13 of the signaling current transformer. In a similar manner and due to the coupling between the primary and secondary windings 13 and 14 of the signaling current transformer, oscillatory currents of the signaling frequency and of comparatively great magnitude are induced in the local oscillatory circuit comprising the secondary winding 14 of the signaling current transformer and the condenser 18 connected in parallel therewith. On account of the oscillatory currents of'great magnitude flowin in this closed circuit compris- 7 secondary winding.

The impedance of the secondary windings Ill" . newness 12, 12 of the talking current transformer is very low compared to the impedance between the cathode or filament 6 and the modifying electrode or grid 9 of the amplifier 5, the latter impedance being practically infinite. Therefore, the impedance of the secondary winding of the talking current transformer causes no deleterious effect upon the action of the signaling current transformer. However, should the secondary windings of the talking and signaling current transformers be-connected directly into the line section L the impedance of the secondary winding of the talking current transformer would then interfere quite seriously with the proper operation of the signaling current transformer, because of thecomparatively low impedance of the linesection L The impedance of the secondary winding let of the signaling current transformer causes no deleterious effect upon the action of the secondary winding of the talking current transformer because the condenser 18 of low impedance to the talking currents bridges this secondary winding 14. An adjustable resistance 25 connected in parallel with the condenser 17 serves as a means for regulating the amount of oscillatory current permitted to pass into the primary winding 13.

The output circuit of the amplifier 5 is connected with the line section L through an output transformer 19, which is included for the purpose of reducing reflection losses to a minimum. A battery 20 is provided for maintaining a positive potential on the plate 10, and an impedance coil 21is connected in circuit with the battery and in bridge of conductors 22 and 23 of the output circuit to prevent the shunting of the amplified talking and signaling currents by the battery 20. A condenser 24: connected in series with conductor 22 prevents the flow of direct current from the battery 20 through the primary winding of the output transformer 19.

Since the line wires comprising the line section L extend for a considerable distance over or near the ground, there exists between these line Wires and ground, capacities distributed along the line wires. The impedances of these) capacity paths to ground are high in comparison to the impedance of the line wires and therefore do not interfere to an appreciable extent with till the transmission of low voltage talking currents over the line section L There also exists between the primary winding 13 and the secondary winding 1 of the si naling current transformer and between the primary windings and the secondary winding of the talking current transformer, capacities of even smaller magnitude than the capacities existing between the line wires of line section L and ground. These minute capacities, while of small importance so far as currents of low potential are concerned,

are of considerable importance and need to be taken into consideration so far as currents of high potential are concerned. Since the currents in the circuit including the secondary winding of the talking current transformer are of high potential, precaution must be taken to uard against the minute capacities interfermgwith the currents of high potential in this circuit. Since the impedauces presented by a given capacity to currents of different frequencies are inversely proportional to the frequency of the currents, it is readily seen that these minute capacities exert very small influence upon the low frequency signaling currents but exert a much greater influence upon the higher freuency talking currents. On account of this act it is very desirable for the successful propagation of both high and low frequency currents, that the secondary windings of the talking and signaling current transformers be connected in the manner shown in the should the secondary winding of the signaling current transformer be connected between the sections of the secondary winding. of the talking current transformer or between the modifying electrode of the amplifier and the secondary winding of the talking current transformer, the propagation of talking currents of high otential would be seriously and detrimenta ly affected. Since the lower terminal of the secondary winding of the' talking current transformer is effectively grounded and since the cathode of the amplifier is grounded, the lower terminal of the secondary winding of the talkingcurrent transformer is connected with the cathode of the amplifier with respect to talking currents.- The n per terminal of the secondary winding oft e talking current transformer being connected with the modifying electrode or grid 9 of the amplifier, the secondary winding 14: of the signaling current transformer does not interfere in any manner whatever with the propagation of talking current. One terminal of the secondary winding 14 is connected with the cathode or filament 6 of the amplifier through the very low impedance of the battery 15, while the other terminal thereof is connected through the secondary winding of the talking current transformer with the modifying electrode or grid 9 of the amplifier. Since the secondary winding of the talking current transformer offers an impedance to low frequency signaling currents which is very low compared to the infinite im edanoe between the cathode and the modifying electau trode of the amplifier, or to the closed resonating circuit which includw the secondary winding 14: and the condenser l8, it will be seen that this secondary winding 12 has very little, if any, deleterious effect upon the propagation of signaling currents. It will be seen, therefore, that there is no interference between the two transformers, the talking current transformer operating efficiently to impress upon the incoming terminals of the amplifier, potentials of the talking current frequency, and the signaling current transformer operating efficiently to impress upon these terminals potentials of the signaling current frequency.

It is essential when employin amplifiers in cOnnection with two-way te ephone repeating systems that the capacity balance to ground of the line wires of the incoming line section be maintained as perfect as possible.

If such a capacity balance is not maintained,

high frequency disturbances occur within the repeating system which have a deleterinus effect upon the operation of the entire system. Due to the position of the primary winding 13 with respect to the equal primary windings 11, 11 and to the connection of the secondary winding 14 to ground through the battery 15 and'the cathode 6, and aided by the paths of low impedance to highfrequency currents across these windings 13 and let made possible by the presence of the condensers 17 and 18, the small capacity of low impedance to high frequency currents, such as talking currents, existing between the primary and secondary windings 13 and 14, has the effect of vrounding the mid-point of the primary winding 13 with respect to talking currents.

What is claimed is:

1. In a telephone system, in combination, two line sections, an amplifier for amplifying both talking and signaling currents and having its output side connected with one line section, separate talking and signaling current transformers connecting the input side of the amplifier with the other line section, and a condenser in shunt to the primary of said signaling current transformer, said condenser, with said primary, forming a circuit tuned to the frequency of the sigthe amplification of the ampli-.

Lenses 3. In a telephone system, in combination, two line sections, an amplifier for amplifying both talking and signaling currents having its output side connected with one line section, means connecting the input side of the amplifier with the other line section, means associated with the amplifier for regulating the talking current admitted thereto, an impedance, and a capacity of low impedance to talking current in series with the current regulating means and cooperating with the impedance to alter the effect of the current regulating means to signaling current.

4. In a telephone system, in combination, two line sections, an amplifier for amplifying both talking and signaling currents having its output side connected with one line section, separate talking and signaling current transformers connecting the input side of the amplifier with the other line section, means associated with the amplifier for regulating the talking current admitted thereto, and a capacity of lo impedance to talking current in series with the current regulating .means and cooperating with the primary winding of the signaling current transformer to alter the effect of the current regulating means to signaling current. I

5. In a telephone system, in combination, two line sections, an amplifier for amplifying talking and signaling currents having its output side connected with one line section, separate talking and signaling-current transforn'iers connecting the other line section with the input side of the amplifier, means in bridge of said other line section controlling the amplification of the amplifier, and a condenser in the bridge cooperating with a primary winding of the signaling current transformer to present to signaling current an effective impedance higher than that presented by either the condenser or the primary winding of the signaling current transformer.

6. In a telephone system, in combination, two line sections, an amplifier for amplifying talking and signaling currents having its output side connected with one line scction,separate talking and signaling current transformers connecting the other line section with the input side of the amplifier, a potentiometer connected in bridge of said other line section for controlling the amplification of the amplifier, and a condenser in the potentiometer bridge cooperating with the primary windin of the signaling current transformer to form a closed circuit resonant to signaling current whereby the potentiometer shunt is substantially of infinite impedance to signaling current.

7. In a telephone system, in combination, two line sections, an amplifier including a cathode for amplifying talking and signaling currents and having its output circuit dill aarneee connected with one line section, a talking current transformer having its primary winding connected with the other line section and its secondary winding connected through a path of low impedance to alternating currents with the input circuit of the amplifier, and a signaling current transformer having its primary winding connected with said other. line section and its secondary inding in the input circuit of the amplifier with a terminal of this secondary winding connected through a path of low impedance to alternating currents with the cathode of the amplifier.

8. In a telephone system, in combination, two line sections. an amplifier including a cathode for amplifying talking and signaling currents and having its output side connected with one line section, a talking current transformer having its primary winding connected with the other line section and its secondary winding connected-through a path of low impedance to alternating currents with the input side of the amplifier, and a signalin current transformer having its primary wlnding and secondary winding connected in series with the respective windings of the talking current transformer and a terminal of its secondary winding connected through a path of low impedance to alternating currents with the cathode of the amplifier.

9. In a telephone system, in combination, two line sections, an amplifier including a cathode and a modifying electrode for ampli fying talking and signaling currents and having its output side connected with one line section, a talking current transformer comprising a primary winding connected with the other line section and a secondary windin having one terminal connected with the mo ifying electrode and the other terminal grounded through a low impedance path with respect to talking currents, a signaling current transformer having its primary winding connected with said other line section and a terminal of its secondary winding connected through a path of low'impedance to alternating currents with they cathode of the amplifier, and a condenser of low impedance to talking current connected in parallel with the secondary winding of the signaling current transformer. t

10. In atelephone system, in combination, two line sections, an amplifier including a cathode for amplifying talking and signaling currents and having its output side connected with one line section, a talking current trans former comprising primary windings 0on nected with the other line section and a secondary winding connected with the input side of the amp ifier, and a signalingcurrent transformer having its primary winding connected in series withand between the primary windings of the talking current transformer and its secondary winding connected between the cathode of the amplifier and the entire secondary winding of the talking current transformer.

11. In a telephone system, in combination, two line sections, an amplifier including a cathode for amplifying talking and signaling currents and. having its output side connected with one line section, a talkmg current transformer comprising primary and secondary windings each divided into two sections connecting the input side of the amplifier with the other line section, and a signaling current transformer having its primary winding connected between and 1n series with the primary sections of the talking current transformer and having a terminal of its secondary winding connected, through a path having an impedance to alternating currents small compared to the alternating current impedance of the secondary winding of said talking current transformer, with the cathode of the amplifier.

12. In a telephone system, in combination, two line sections, an amplifier including a ,grounded cathode for amplifying talking and signaling currents and having its output side connected with one line section, a talking current transformer comprising primary and secondary windings each divided into two sections connecting the input side of the amplifier with the other line section, and a signaling current transformer having its primary winding connected in series with and electrically balanced with respect to the primary sections of the talking current transformer and having its secondary winding connected between the grounded cathodeof the amplifier and the secondary winding of the talking current transformer. 1

13. In a telephone system, in combination, two line sections, an amplifier including a cathode and a modifying electrode for am' plifyingi talking and signaling currents and having its output side connected with one line section, a talking current transformer comprising a primary winding connected with the other line section and a secondary winding having one terminal connected with the modifying electrode and the other terminal grounded with respect to talking currents, and a signaling current transformer having its primary winding connected with said other line section and a terminal of its secondary winding connected through a path of low impedance to alternating currents with the cathode of the amplifier. 14. In a telephone system, in combination, two line sections, an amplifier'for amplifying both talking currents and signalin currents having its output side connecte with one line section, and "inductive connections between the other line section and the input side of the amplifier comprising means for transmitting both talking and signaling currents, said connections comprising an antiresonant circuit tuned to the frequency of the signaling currents.

15. In a telephone system, in combination, two line sections, an amplifier for simultaneously amplifying both talking and signaling currents, and means connecting the amplifier between the line sections forsimultaneously transmitting both kinds of currents from one line section to the amplifier and from the amplifier to the other line section, said means comprising an anti-resonant circuit tuned to the frequency of the signaling currents. 1

16. In a telephone system, a line adapte to be traversed by a pluralty of sets of alternating current of different characteristics, induction repeating apparatus for each of said sets, means comprising a condenser in shunt of the input side of one of said induction repeaters for by-passing around said repeater the set of currents to be efliciently repeated by another of the re eaters and forming with the shunted induction repeater a loop circuit tuned to the impulses to be efficiently transmitted by the shunted repeater, and an amplifier having an input circuit of high impedance when compared to the impedance of the line inductively connected with the tuned circuit.

17. In a telephone system, in combination, line sections, amplifying means joining the line sections for amplifying a plurality of sets of different frequency alternating currents, means associated with the. amplifying means for regulating the intensity of the current of one set admitted thereto, an impedance device, and a condenser cooperating with the impedance device inoperative relation to the current regulating means for altering the effect thereof to currents of another set.

18. In a telephone system, in combination, line sections, amplifying means joining the line sections for amplifying talking and signaling currents, means associated with the amplifying means for regulating the talking current admitted thereto, an impedance device, and a condenser cooperating therewith for altering the effect of the currentregulating means to signaling current.

19. In a telephone system. in combination. line sections, amplifying means joining the line sections for amplifying talking and signaling currents, means associated with the ampl fying means for regulating the talking current admitted thereto, an impedance device. and a condenser oflow impedance for talking current connected in circuit with the current-regulating means and cooperating with the impedance device to alter the effect of the current-regulating means to signaling current.

20. In a telephone system, in combination,

line sections, amplifying means joining the line sections for amplifying talking and signaling currents, means associated with the amplifying means for regulatlng the talking current admitted thereto, and a circuit tuned to signaling current and in operative relation to the current regulating means v for a1- tering the effect thereof to signaling current.

21. In a telephone system, in combination, two line sections, an amplifier for amplifying both talking and signaling currents having its output circuit connected with one line section, means connected in bridge of the other line section for controlling the amount of talking current admitted to the input circuit of the amplifier, and a loop circuit tuned to signaling current connected in said bridge for rendering thebridge of high impedance to signaling current.

22. In a telephone system, in combination. two line sections, a repeater for amplifying talking and signaling currents having its output circuit connected with one line sec- .tion, means in bridge of the other line section for controlling the amount of talking current admitted to the input circuit of the repeater. and a loop circuit tuned to signaling current associated with said other line sec't'on for rendering the bridge of high impedance to signaling current.

23. In a telephone system. in combination, two line sections over which talking and signaling currents. are transmitted, separate talking and signal ng current transformers joining the line sections. the primary windings of the transformers being connected in series. and means for regulating the intensity of the talking current admitted to the talkingcurrent transformers without affecting the intensity of signaling current admitted to the signaling current transformer.

24. In a telephone system, in combination. two line sections over which talking and signaling currents are transmitted. separate talking and signaling current transformers joining the line sections, the primary windings of the transformers being connected in series, and means associated with the transformers for regulating the intensity of the signaling current admitted to the signaling current transformer without afiecting the intensity of the talking current admitted to the talking current transformer.

25. In a telephone system, in combination, two line sections over which telephone and signaling currents are transmitted, separate talking and signaling current transformers joining the line sections. and means associated with the transformers for separately regulating the intensity of the talking and signaling currents admitted to the respective transformers.

26. In a telephone system, in combination.

two line sections, an amplifier for amplifying talking and signaling currents having its output circuit connected with one line section, transformer means connecting the input circuit of the amplifier with the other line section, and means associated therewith for separately and independently controlling the amplification of the amplifier to'both kinds of currents.

27. In a telephone system, in combination, a transmission line, an amplifier having a control element and connected in the transmission line for simultaneously amplifying both talking and signaling currents, said talking and signaling currents being both impressed on said control element, and means for separately and independently controlling the amplification of the amplifier to both kinds of currents.

28. In a telephone system, in combination,

line sections, amplifying means joinin the line sections for amplifying talking an signaling currents, separate talking and signal ing current transformers connecting the input side of the amplifying means with one of the line sections, the primary windings of the transformers being connected in series, regulating means connected in the last-mentioned line section and cooperating with the talking current transformer to control the intensity of talking current admitted to the amplifying means, and regulating means associated with the ringing current transformer for controlling the intensity of ringing current admitted to the amplifying means.

29. In a telephone system, in combination, two line sections, an amplifier for amplifying talking and signaling currents having its output circuit connected with one line section, separate talking and signaling current transformers connecting the input circuit of the amplifier with the other line section, the primary windings of the transformers being connected in series, apotentiometer in bridge of said other line section for controlling the amplification of the amplifier to talking current, a condenser connected in the bridge and cooperating with the primary winding of the signaling cur rent transformer to form a loop circuit tuned to signaling current whereby when signaling current is received the tuned circuit renders the impedance ofthe bridge high to signaling current, and means in the loop circuit for regulatin the amplification of the amplifier to signal ing current.

30. In a telephone system, in combination, two line sections, an amplifier for amplifying both talking and signaling currents having its output circuit connected with one line section, means connecting the input side of the amplifier with the other line section, means in circuit with said other line section controlling the amplification of the amplifier to talking current, and a loop circuit tuned to signaling current and rendered effective when signaling current is impressed upon the terminals thereof to alter the effect of the amplification-controlling means to signaling current.

31. In a telephone system, in combination, two line sections, an amplifier for amplifying both talking and signaling currents having its output circuit connected with one line section, means connecting the input circuit of the amplifier with the other line section, means in bridge of said'other line section controlling the amplification of the amplifier to talking currents, and a loop circuit associated with said bridge and tuned to signaling current for altering the effect of the amplification-controlling means to signaling current. v

32'. In a telephone system, a line adapted to be traversed by a plurality of sets of alternating current of different characteristics, induction repeating apparatus for each of said sets, means in circuit with the repeaters for regulating the intensity of one set of currents admitted thereto, means comprising a condenser in shunt of the input side of one of said induction repeaters for by-passing around said repeater the set of currents to be efficiently repeated by another of the repeaters and forming with the shunted.

repeater a loop circuit tuned to the impulses to be efficiently transmitted by the shunted repeater, whereby when currents of the frequency or frequencies to which the loop circuit is'tuned are received the tuned circuit presents a high impedance to the flow thereof through the regulating means, and a thermionic amplifier inductively coupled with the tuned circuit.

33. In a telephone system, in combination, two line sections over which talking and signaling currents are transmitted, separate talking and signaling current transformers joining the line sections, the primary windings of the transformers being connected in series, means comprising a condenser in shunt of the primary winding of the sig naling current transformer for by-passing talking current around said winding and forming with the shunted winding a loop a circuit tuned to signaling current. and means in the loop circuit for regulating the in tensity of the signaling current admitted to the signaling current transformer.

34. In a telephone system, in combination, a transmission line, an amplifier having a control element and connected in the transmission line for simultaneously amplifying both talking and signaling currents, said talking and signaling currents being both impressed on said control element and means associated with the amplifier for controlling the amplification thereof to one kind of current.

loo

7 ing impressed upon said control element and means associated with the amplifying means for controlling the intensity of one kind of current admitted thereto without affecting the intensity of the other kind of current admitted thereto.

36. In combination, a line shunted by two paths, one path comprising the primary winding of a transformer, the other path comprising a resistance, and a tuned circuit common to both of said paths and pre senting a high impedance to currents of a particular frequency. i

In witness whereof, I hereunto subscribe my name this 27th day of December A. D., 1918.

DONALD F. \VHITING.

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