£40450 3, ?daiq
Aug. 6, 1935. 2,010,002 G. P. ADAIR VOLUME CONTROL SYSTEM Filed July 3, 1931 6 Sheets-Sheet 'l IN VEN TOR. £40450 3, ?daiq 0%?‘ AT TORNEYY: . BY 5, Aug- 6, 1935- G. P. ADAIR 2,010,002 VOLUME CONTROL SYSTEM Filed July 3, 1951 6 Sheets~Sheet i: Ivar-4 INVENTOR. gee/19w 3. Mai/v, BY by d‘?g 0‘ ~ ATTORNEY 2 Aug- 6, 1935. G. P. ADAIR 2,010,002 VOLUME CONTROL SYSTEM Filed July 3, 1951 6 Sheets-Sheet 3 |. \ \ .._._.._______ ___ Ilu . / 5% INVENTOR. gem/5w 55'” Wall/u, BY 6 ATTORNEY “H0 Aug. 6, 1935.’ 2,010,002 G. P. ADAIR VOLUME CONTROL SYSTEM Filed July 3, 1931 6 Sheets-Sheet 4 BY 7 ' ATTORNEY 6‘, 1935. G, P_ ADAlR ‘ 2,010,002 VOLUME CONTROL SYSTEM Filed July 3, 1931 6 Sheets-Sheet '6 ' INVENTOR. Qwmgw .5’, 91241430, BY (2 ATTO’RNEY . Patented Aug. 6, 1935 2,010,002 UNITED STATES PATENT OFFICE 2,010,002 VOLUME CONTROL SYSTEM George P. Adair, San Antonio, Tex. Application July 3, 1931, Serial No. 548,695 6 Claims. (Cl. 250-20) (Granted under the act of March 3, 1883, as amended April 30, 1928; 370 O. G. 757) My invention relates broadly to volume control ciation of the coupling coils in an ampli?er sys systems and more particularly to a circuit ar 10 20 25 30 3 40 tem is variably controlled in accordance with the rangement for automatically controlling the cou amplitude of the incoming signaling energy; Fig. pling of the electrical circuits in a radio receiving 6 is a schematic view showing a modi?ed form of system for correspondingly controlling the volume coupling control mechanism which may be em of the reproduced sound. ployed in the control system of my invention; Fig. One of the objects of my invention is to pro 7 illustrates a further modi?ed form of coupling vide a circuit arrangement for reducing fading control mechanism embodying the principles of effects in radio broadcast receiving circuits by my invention; Fig. 8 illustrates another form of automatically controlling the coupling of the cir variable capacity device operative in accordance cuits in the receiver in accordance with the with changes in amplitude of signaling energy for amplitude of the incoming signaling energy. correspondingly controlling the ampli?cation Another object of my invention is to provide a characteristics of the receiving apparatus for volume control circuit for radio broadcast re governing the volume of the reproduced sound; ceivers which operates independent of frequency Fig. 9 shows a further form of coupling control for maintaining the volume of sound reproduced mechanism which may be employed in the cir by the radio broadcast receiver constant in cuits heretofore described; Fig. 10 illustrates a dependent of fading effects. still further modi?ed form of variable capacity Still another object of my invention is to pro mechanism controllable in accordance with the vide a device for controlling the coupling of the operation of the signal receiving circuits in the circuits in a radio broadcast receiver at the system of my invention; and Fig. 11 shows a same time indicating conditions of resonance of further modi?ed construction of variable capacity the receiving circuit for the reproduction of control device employed in the fading compensa sound at constant volume independent of fading tor of my invention. e?'ects. The volume control system of my invention is A further object of my invention is to provide a adaptable for installation in existing types of construction of mechanical actuator for control radio broadcast receivers. The coupling control ling the volume of reproduced sound in accord mechanism may be assembled as a separate unit ance with variable conditions of fading in the and incorporated in existing equipment at very circuits of the radio broadcast receiver wherein reasonable cost. The control circuit of my inthe parts are arranged to avoid appreciable wear vention may be built into the radio receiving ap and to give gradual control over the entire range paratus as part of the production program there of the receiving apparatus. of at very reasonable cost. The control mecha Other and further objects of my invention re nism of my invention comprises a variable im side in the circuit arrangement for and the con pedance device, the effective value of which is struction of the variable coupling device used in electromagnetically or electrostatically controlled association with the ampli?er circuits of a radio in accordance with the amplitude of the incom broadcast receiver as set forth more fully in the ing signaling energy. The operation of the vari speci?cation hereinafter following by reference to able impedance device is so damped that there is the accompanying drawings, in which: no tendency toward amplitude distortion. The Figure 1 illustrates one circuit arrangement for effective variation in impedance may be made a radio broadcast receiver equipped with a volume as fast as desired to meet special conditions. 5 10 " ‘ 35 I control circuit in accordance with my invention; may arrange the variable impedance device in Fig. 2 shows a modi?ed form of variable coupling termediate the stages of ampli?cation or the va 45 system employed in interstage relationship in an riable impedance devices may be arranged be ampli?er system embodying my invention; Fig. 3 tween antenna and the ampli?cation circuit shows a simpli?ed form of radio receiving circuit of the receiver, or etween the ampli?er system including a variable coupling device for control the sound reproducing circuit. ling the transfer of energy through the ampli?er In applying the volume control system of my in accordance with my invention; Fig. 4 illus invention to existing types of receiving circuits, ’ trates an electrostatic control apparatus for vary all voltages and currents of the high frequency ing the coupling between the stages of an ampli cascade circuit remain unchanged so that they ?er system in accordance with my invention; Fig. may be adjusted for maximum efficiency thereby 5 shows an arrangement of automatic coupling reducing to a minimum signal distortion and so control apparatus in which the inductive asso called “cross-modulation” due to improper ad (a Cl '2 2,010,002 justment of tube voltages which is inherent in 33, 37, 38 and ‘58 as shown. I also arrange suitable voltage changing control circuits. In applying’ capacities 29, ilii, 4!, as, at, $3 and ill as shown. the variable impedance system of my invention to The anode potential for the tubes 3 and it is sup existing receiving circuits there is no tendency for plied from terminals 35 across by-pass condenser the receiving circuit or ampli?er to oscillate or 35‘. The output from the high frequency recti?er flutter as is the case with other systems of volume tube 3 leads from anode 3d through condenser £2 . The system of my invention provides to output terminals 5-38 as shown. In the operation of my volume control system signal strength than has been possible with previ as illustrated in Fig. l, variations in the amplitude ous volume control systems. The volume control of the incoming signaling energy due to fading device of my invention operates independent of conditions are caused to slowly modify the ampli tude ‘level of the space discharge in the tube 2. frequency oi‘ the actuating signal. This in turn, reacting upon the condenser iii Referring to the drawings in more detail, ref erence character 6 designates the input terminals produces variations in the potential applied to of the receiving apparatus which includes a high the grid of the tube ii, thereby controlling the frequency ampli?er stage 2, the output .of which flow of current in the armature winding ll which is connected to the high frequency recti?er stage is in the output circuit of tube (1. At the ex 3 through a variable coupling system. The output treme of fading conditions the output signaling circuit of the high frequency ampli?er tube 2 ' energy from tube 2 is for the most part trans connects to the input’ circuit of a control tube it ferred to the grid 3c of the detector tube 3. as will be hereinafter described in detail. The ‘This ‘is accomplished when the armature i6 is in the ?gure and when the coupling system enclosed within the dotted line i d the position shown is actuated under control of the control tube 65 capacity area 23 is opposed to the capacity area As the amplitude level of the incoming signal for'variably controlling the transfer of energy to rises, however, more and more current ?ows ‘the high frequency‘ recti?er tube 3. The input circuit l connects through coupling through the armature winding ll, thereby caus condenser l3 and selectively tuned circuit 5 to ing the capacity area 23 to move toward its the control grid '20 and the cathode element 27) position shown in dotted lines, where-at the cou LID and 2% is through the ground connections’ 52. The output pling between the capacity areas 9“ circuit of tube 2 connections from anode 2d to the diminished and the strength of the signal applied input circuit of control tube it through condenser to the grid 30 is controlled. In Fig. '2 I have shown the application of the iii to control grid to. The anode potential for tube 2 is supplied through choke coil '5 and regu circuit arrangement of my invention to a screen lating resistance it which leads to the positive grid ampli?er and high frequency recti?er circuit. terminal or" the anode potential. A by-pass con In this arrangement high frequency ampli?er denser 9 connects across the anode potential tube 2' includes a screen grid 26 which is biased source to ground it as shown. A suitable biasing from the potential source 35 through regulating ‘potential is supplied to the grid of control tube resistance 28 having-a by-pass circuit to ground 6340 through resistance i l. The heater cathodes 2a through condenser 36. High frequency recti?er of’tube 2, do of tube fl, and 3a of tube 3, are sup~ tube 3’ includes the screen grid element to which pliedwith heating current through circuit 5' from .is biased from potential source 35 through re any suitable potential source. The output circuit sistance 3! having a path to ground through of control tube It connects from anode lid to the condenser 32 connected therewith as shown. The control. automatic control over a much wider range of 15 20 25 40 winding ll on the rotatable armature it of the biased to such value through. resistances-iii, 3'1, trol apparatus within the dotted line it. The armature iii is pivotally mounted for rotation be tween the magnetic pole‘pieces of the magnetic system 55. Rotatable armature it carries the 50 movable pointer or indicator is which is normally biased by means of the spring 59 to occupy the full line position shown in Fig. l. The pointer or in 38 and 39 that the tube 2' operates on the lower knee of the characteristic plate current ‘curve. dicator is has one end thereof movable over a 25 is connected to ground '2 as shown while ca in‘ O ' " ' high frequency ampli?er tube 2’ has its grid 20 45 electromagnetic device forming part of the con graduated scale 26}. The other end of the indi cator or pointer 58 carries the plate member ‘23 by means of armv 22 in insulated relation to the indicator r pointer it? through insulated joint ‘ii. The plate 23 is electrically connected through ?exible conductor (‘-5 with the control grid 3c of the high frequency recti?er tube 3. The plate member 23 may be capacitatively related to either ?xed plate or ?xed plate 25. The capacity area 10 ‘When a signal of a value greater than that re quired to give the desired output is impressed on the system the plate current in the output of to ‘tube L3 is increased thereby increasing the energi Zation of actuating winding ii and decreasing the capacitative coupling between high frequency‘ vampli?er tube 2’ and high frequency recti?ertube by shifting capacity area 23 out of juxtaposi tion with capacity area thereby maintaining the output‘ the same as it was originally or as nearly so as desired. The degree of control de pends upon the adjustment of the condenser ii! and the bias on grid 530 of control tube 5 through (50 bias resistance ii. The control also depends on the shape, dimensions and characteristics of the vcoupling device. In order to secure extreme pacity area 2% connects to the output circuit of accuracy in operation of control tube 65. a manual high frequency ampli?er tube 2 at the anode 2d adjustment may be provided at the input circuit (3-3 as illustrated. The input circuit of high’ fre of control tube 4 for obtaining the most satis quency recti?er tube 3 is selectively tuned by factory results from the system as a whole. The circuit 3% one end of which is connected to con use of the grounded capacity area 25 is for the trol grid 50 and to the movable capacity area 23 purpose of making the rate of change of coupling through flexible lead (55 and the other end of greater and also preventing signal pick-up from which is connected to ground, completing the cir any source other than through tube2’. cult to the cathode 3b of the high frequency recti ' I While I have shown the grounded capacity area ?er tube 3'. In order to furnish the required op I realize that other modifications may be em erating potential on the several electrodes of the ployed and I do not intend that my invention be 75 tubes, I provide the resistance'elements 25, El, 'limitedto the precise arrangement shown. 3 In Fig. 2 the dotted line position |8b of the circuit. As the signal strength increases above a indicator l8 shows one extreme condition where predetermined value, the plate current or tube 4 maximum coupling between the output of high decreases and the charge on movable condenser frequency ampli?er tube 2’ and the input of high plate 23 increases in like polarity to the charge frequency recti?er 3' exists. The dotted line on ?xed plate 24, resulting in the repulsion of position l8a of the indicator represents a con plate 23 which moves to the dotted line position dition where minimum coupling exists between 23a thereby decreasing the capacity between the output of high frequency ampli?er tube 2' plates 23 and 24 and decreasing the coupling be and the input of high frequency recti?er tube 3’. tween high frequency recti?er tube 3 and high Fig. 3 shows a simpli?ed arrangement of volume frequency ampli?er 4. When the current strength control circuit where the control tube 4 connects rises in the plate circuit of tube 4 the charge of directly in the input circuit to the receiver. The plate 23 is smaller, thereby decreasing the repul input circuit of the ?rst radio frequency ampli sion force and enabling the plate 23 to auto ?er stage 2 is capacitatively coupled through matically resume the full line position shown, the relatively movable plates 23 and 24 with the restoring the normal coupling relationship. tuned input circuit 50 of the receiver. I have In Fig. 5 I have shown a modi?ed form of the illustrated the receiver connected to antenna 5| circuit of my invention in which an electromag and ground 52, the antenna circuit including pri netic coupling system is employed. In this ar mary winding 53, electromagnetically coupled rangement I have shown the input circuit of high 20 with a secondary winding forming part of the frequency ampli?er tube 2 completed through tuning circuit 50. A tap is taken from the tuned suitable potential source 64 for supplying prede input circuit 513 to the variable condenser ID in termined bias potential on the grid 20 of tube 2. circuit with the grid 40. The winding ll of the The control tube 4 has the grid electrode 40 electromagnetic device is connected as heretofore thereof biased from suitable potential source 65. described in the output circuit of control tube 4. The output circuit of control tube 4 connects in The ?xed plate 24 of the variable capacity sys the manner heretofore described through winding 10 15 20 25 tem connects to one side of the tuned circuit 53. The ?xed plate 25 connects to ground l2 as shown. The flexible lead 45 connects the movable capacity area 23 to the control grid 20 of the high fre quency ampli?er tube 2 as shown. The desired potentials for the tube electrodes are obtained by means of resistors 26, 21, 55 and 51. Condenser ll of the magnetic actuator. The indicator arm it of the magnetic actuator and the shafts 66 and 6‘! are mechanically coupled one to another by electrically insulating couplings 2| or 68 as thereby shifting arm l8 and decreasing the capacity coupling between electrical circuit 50 and tuned circuit 5. Plate potential for tube 2 is obtained through impedance elements 1 and 8 coupling system. The output circuit of high fre 40 quency ampli?er 2 is completed through the in ductances comprising variometer 69. By varying shown. The shaft 66 carries variometer windings which are inductively related to the inductance 75 which connects to the input of the high fre 51 is connected in shunt with resistor 56 as shown. quency recti?er tube 3. The shaft 51 carries The tuned circuit 5 is connected to the input of varicmeter windings ‘l0 inductively related to 35 the high frequency ampli?er tube 2 as shown. the inductance IS. A control circuit including The maximum coupling condition is shown in full shunt connected impedance ‘Ha connects across line position. When the signal strength increases the inductance 10 for effecting adjustment of and the plate current for control tube 4 increases compensating for changes in inductance in the which lead to the source of plate potential. the spacial relation of the sets of inductances 59 The and it’ with respect to inductance ‘I5 the coupling output terminals 60 lead to the succeeding stage between the output of high frequency ampli?er 2 of high frequency ampli?cation or to a high and the input of high frequency recti?er 3 is frequency recti?er. varied. Condenser ‘i5 is employed to tune induct~ In Fig. 4 I have shown a modi?ed form of cir cuit embodying my invention in which the elec trostatic attraction and repulsion between plates 23-44 is utilized for changing the effective cou pling between the output of high frequency am pli?er tube 2 and the input of high frequency rec ti?er tube 3. In this arrangement the arm I8 Cl Ll is pivotally mounted and biased by means of spring I!) and carries movable plate 23 which may be shifted from juxtaposition with respect to ?xed plate 24 to a position adjacent ?xed plate 25. The circuit from the output of control tube 4 60 leads through radio frequency choke coil 6| to the indicator or pointer l8 to which movable plate 23 is directly connected. The plate poten tial. for control tube 4 is supplied tnrough resist ance 26 and radio frequency choke coil 62 from the source of potential 35. Condenser 12 is dis posed in series between one side of the choke coil 62 which connects with arm I 8 and movable capacity area 23 and the tuned circuit 36. The required operating potential for cathode 3b is supplied through resistance 63. Under conditions of fading the signal strength varies in the output circuit of high frequency ampli?er 2. The bias is arranged on 4c so that the control tube 4 is op erating on the upper knee of the characteristic curve and an increasing signal reduces the plate ance ‘l5. As the current in the plate circuit of control tube 4 increases the coupling between in ductances 59 and 7B and inductance T5 is de 50 creased, thereby maintaining the output current at terminals 48 substantially constant. The variable capacitative device which I have heretofore described for controlling the coupling between different portions of the circuit may take 55 many different forms. I have illustrated a num ber of such forms in Figs. 6 to 11. In Fig. 6 the rotatable armature l6 carries a shaft member to which there is attached a spider Tl carrying separate capacity areas 18, 19 and 30 displaced 60 approximately 120° and adapted to shift angu larly within set of stationary capacity areas dis placed 60" apart. Each stationary capacity area is displaced 60° from the adjacent stationary capacity area. The adjacent capacity areas be long to different sets. One set of stationary capacity areas is indicated by reference character 8| connected to bus connector 84. The output circuit of one of the coupling systems connects between terminal 85 and ground l2. The suc ceeding circuit 36 to which energy is to be trans ferred is connected between ground l2 and the capacity areas 18, 19 and 8!]. Tne motive mech anism for driving the sets of movable plates is actuated in the manner heretofore described. 4 2,010,002. In Fig. 'l I have shown a multiple set of rotor systems may be used for controlling the coupling plates 83 mounted upon shaft 22 extending from between different circuits in accordance with con insulated coupling 2i which plates are adapted ditions existing in the circuits. I‘have found the to.‘ be variably interleaved with oppositely disposed system of my invention as described herein highly sets of multiply arranged stator plates. One set practical and while I have described my invention of stator plates is represented at 8'? connected in certain preferred embodiments, I desire that with the circuit from which energy is to be trans it be understood that modifications may be made ferred through lead 85. The other set of stator and that no limitations upon my invention are plates is represented at BSconnected to ground l2. intended other than are imposed by the scope of 10 By moving shaft 22 so that plates 83 overlap to the appended claims. a maximum extent with plates 8?, the transfer The invention herein described may be manu of energy to tuned circuit 3Eiis increased to a factured and used by and for the Government of maximum. However by moving rotor plates 86 the United States of America, for governmental to a position where the overlap is a minimum the purposes, without the payment of any royalties 15 transfer of energy is a minimum. The plates 86 are automatically actuated in the manner hereto What I claim as new and desire to secure by fore described. ' I Letters Patent of the United States is as follows: 1. In a signal receiving system, the combina In Fig. 8, I have shown a form of variable ca pacity device wherein the movable capacity area tion of a signal receiving circuit, an electron tube 20» is in the form of 'a rod member 89 curved about ‘system, a'control tube connected’ with said sig the center of angular movement as shown. The nal receiving circuit, variable coupling means in stationary capacity areas are shown at 9,8 and ill terposed between said signal receiving circuit in the form of tubes concentrically disposed about and said electron tube system, and electrostatic the movable capacity area 89. The actuation of means controlled from the output of said control the movable capacity area is brought about in the tube for, actuating said variable coupling means manner described in connection with the previous in accordance with conditions ‘existing in the ?gures. output of said control tube for variably control In order to increase the stability of the variable ling the transfer of energy from said signal re capacity coupling device I may employ an ar ceiving circuit to said electron tube system. , 30 rangement as shown in Fig. 9, wherein a movable 2. In a signal receiving; system, a signal re— indicator arm i8 is linked through insulated cou ceiving circuit,v an electron tube system, a control pling 2i with the connecting rod 92 which con~ tube operated from said signal receiving circuit, nects with piston 93 in cylinder 94. Cylinder 94 and electrostatic means controlled by the output connects through tube 95 with the riser 936 which of said control ‘tube for varying the coupling be is ‘concentrically surrounded by the tubular ca-' tween said signal receiving circuit and said elec pacity area 9? in spaced relation to riser 96. tron tube system in accordance with conditions Movement of indicator 'armit by reason of the in said signal receiving circuit. thereon; 40 actuation of the electromagnetic mechanism tends to displace the mercury in cylinder at to variably ?ll the riser 96 thereby controlling the eifective capacity between the coupling connec tion 85 leading from theinput system and the coupling connection 65 leading to the output sys tem. In lieu of the rotary electromagnetic actuating ' ’ _ > 15 20 25' 30. 3. In an amplifier system, apliirality of elec-' tron tubes each having input and output circuits, a control tube’ connected to'the output circuit of one'of said electron tubes, a movable actuator operated by' said control tube, a resonance indi cator controlled by said movable actuator, a mov able condenser plate shiftable to different posi tions by said movable actuator and connected 45 mechanism I may employ a solenoid actuator hav with the input circuit of a succeeding electron ing an actuating coil 98 as shown in Fig. 10, the tube, a stationary condenser plate connected with actuating coil being connected in the output cir~ the output circuit of theprcceding ‘electron tube, cuit of the control tube in a manner similar to an independent stationary condenser plate posi the connection of the winding H. Armature 98' is operated by solenoid winding 98 to move lon gitudinally extending shaft we to the right or left, thereby displacing the tubular capacity area iti with respect to the ?xed concentric cylinders 55 “32 and H33. The amount of displacement is measured by indicator if!!! operative over scale I85. The input system is connected through lead 85 withcap'acity area 562. The output system is ' connected through lead 155 with tuned circuit 35. tioned opposite said ?rst mentioned stationary 50 plate and connected to ground potential, said movable actuator operating to shift said movable condenser plate through positions adjacent or in termediate said stationary condenser plates for controlling the transfer of energy from the pre 55 ceding electron tube to the succeeding electron tube in accordance with the characteristics of the signaling energy in the circuits of the pre ceding electron tube. V 60 Maximum transfer of energy occurs between 4. An ampli?er system comprising a plurality 60 leadstb and 45 when capacity area It’! is shifted of electron tubes each havinginput and output concentrically within capacity area Hi2. Mini circuits, control means connected with the out mum transfer of energy occurs when capacity put circuit at one of said electron tubes, a mov . area lBl is shifted concentrically within cylin able actuator operated by said control means, said movable actuator carrying a resonance indie 65 der I03. cator and a movable capacity area shiftable to .Fig. 11 shows a further modi?ed form of sole noid actuator in which. the solenoid winding 98 operates upon armature 99 through insulated link 2! to displace piston 93 operative upon the top of apool of mercury within the cylinder 94. The control‘syste'm is provided with all required electrical shielding in order to avoid undesired in terference with the operation of the circuits. I realize. that other forms of variable impedance either of two limiting positions, the stationary capacity areas disposed adjacent each of the lim iting positions of said movable capacity area, one’ of said stationary capacity. areas being con nected tothe output circuit of one of said elec V tron tubes and'said movable capacity area being > connected to the input circuit of the succeeding electron tube, the ‘other of said stationary capac ity areas being; connected to ground, said mov 75 2,010,002 5 able actuator operating to variably shift said characteristics of signaling currents in the cir movable capacity area into juxta-position with cuits of said preceding electron tube for repulsively either or" said stationary capacity areas in ac controlling the spacial relation between said mov cordance with the operation of said control capacity area and said ?rst mentioned sta means for variably controlling the coupling be able tionary capacity area and correspondingly con— tween said electron tubes. trolling the coupling between said electron tubes. 5. In an amplification system, a plurality of In a volume control system for radio receiv electron tubes each having input and output cir ing6. circuits having a plurality of successively cuits, a control circuit connected with the output coupled stages, means for impressing upon one 10 circuit of one electron tube, a pivotally mounted of said stages incoming signaling energy the am arm carrying a capacity area connected in the plitude level of which varies according to exist 1O output of said control circuit and shiftable to ing fading conditions, means controlled by am either of two limiting positions, a stationary ca pli?ed output energy derived from the particu pacity area disposed adjacent the path of said 15 movable capacity area at the opposite limits of lar stage last described for varying the coupling between said stage and a succeeding stage, said movement of said movable capacity area, one of last mentioned means comprising a stationary 15 said stationary capacity areas being connected capacitive element connected with the output cir with the output circuit of one of said electron cuit of one of the stages, a movable capacitive tubes, the other of said stationary capacity areas 20 being connected to ground and said movable ca pacity area being connected to the input circuit of the succeeding electron tube, means for apply ing charges of the same polarity to said ?rst men tioned stationary capacity area and said movable 25 capacity area, the magnitude of said charges be ing varied in accordance with the operation of said control means in correspondence with the element connected with the input circuit of the succeeding stage and a stationary grounded ca 20 pacitive element adjacent the ?rst said station ary capacitive element for maintaining a sub stantially uniform reactance between said mov able capacitive element and the stationary ca~ pacitive elements in different positions of said 25 movable capacitive element. GEORGE P. ADAIR.
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