US2008512A - Control system - Google Patents

Description

July 16, '1935. M. M. MoRAcK 21,008,512

CONTROL SYS'llEM Filed March 22, 1934 Y CRn-mm.

GRID VOLTAGE Inventor:

b q ttowned.

Patented July 1e, 193s PATENT 4ori-'ici'.

comor. SYSTEM Marvin M.l Morack, Schenectady, N. Y., assigner to General Electric Company, a corporationof New York Application mmh zz, 1934i, serial No. 716,817 s om (ci. 11s-ago) This invention relates to control systems, more particularly to systems for controlling the opera.- tion of electroresponsive devices and the like, and

it has for an object the provision of a simple, re-

located electroresponsive device, such for example as a relay or contactor, and it has for a further object the provision of means for causing the device to operate in one manner at a certain value of control voltage, and to operate in an- T, other manner at a substantially similar but slightly different value of control voltage; e; g. to cause a relay or contacter to pick up. and drop out atsubStantialIy the same `value of control voltage.

In one aspect this invention relates to illumination control systems and the like. For example, in certain modern theatre dimming systems,

' the dimming apparatus comprising electric valves and saturable reactors are located behind or beneath the stage, and the dimming controllers are remotely located on a panel or control desk in the auditorium from which the operator has a. view of the performance. Electrical connections are of course provided between the controllers and the dimming apparatus. These systems usually incorporate black-out contactors for disconnecting a bank of lamps and its associated saturable reactor from the power supply when a rapid darkening or black-out is desired. These black-out contactors are located near the dim- 35 ming apparatus. The provision of additional control connections between these black-out contactors and the control desk, which are undesir` able because of circuit complications and added 4 characteristic, e. g. the varying voltage or current of the control desk connections or of the dimming apparatus for actuating the contactor. 'Ihis necessitates -a contactor which will pick up and drop out at substantially the same value of the controlling characteristic. Otherwise there wouldbe a. range of the controlling characteristic over which the contacter might be .either open or closed depending on the previous 50 operation. Accordingly a further and more spe- I cilc object of this invention is the provision of a remote control system in which means are provided for causing a contactor to pick up and drop out at substantially the same value 'of an electrical characteristic f a controlled device or exexpense, are averted by utilizing an electrical isting connections betwn a. controlling and a controlled device.

A further object of the invention is the provision of means for causing a large contactorv to pick up and drop out at substantially the same value of the controlling characteristics in systems in which the control power is very limited.

In carrying the invention into effect in one form thereof, an electroresponsive device is connected in the output circuit of an electric valve, which is capable, when fully conducting, of passing sufcient current to operate the electroresponsive device. A bias voltage is applied to the control grid of the valve and means are provided for applying an alternating voltage to the output circuit and an alternating voltage of reverse phase is applied to the grid so that at a critical value of bias voltage the valve is either fully conducting and the electro-responsive device operated or the valve is non-conducting and the device is not operated. A variable control voltage is utilized to vary the bias through a. range including this critical value.

In illustrating the invention in one form thereof, it is shown as embodied in a remote control theatre dimming .system for controlling the black-out contactar.

For a better and more complete understanding of the invention reference should be had to the following specification and to the accompanying drawing in. which Fig. 1 is a simple diagrammatical illustration of an embodiment of the invention, Fig. 2 is a chart of operating characteristics of a portion of the circuit, which facilitates an understanding. of the invention, and Fig. 3 is a modification. A

Referring now to the drawing, a. translating circuit, such for example as the circuit I0 across which is connected a plurality of load devices i1- lustrated asa bank of lampsl I .is supplied with power from a suitable source of alternating voltage represented in the drawing by the two supply lines I2, I3'. The circuit IU is connected to the conductor I2 through the contacts lla of the black-out contactor I4 when the latter is energized and the contacts Ila are bridged by the movable contact member Hb. The other side of the lamp circuit Ill is connected through the alternating current winding Ilot a saturable re- 'actor I8 to the lower side I3 of the supply source. As shown, the saturable reactor is provided with `a. direct current saturating winding I1 to which direct current of variable magnitude'is supplied by suitable means illustrated as an electric valve Y n. simougntms valve may be o: any mim type it is preferably of the three-electrode type vinto the envelope of which a small quatity of an ionizable medium such for example as mercury vapor is introduced after exhaust. The presence of this ionizable medium within the tube serves to convert the usual electronic discharge into an arc stream thus constituting the tube an electrostatically or grid controlled arc-rectifier. The anode la. is connected through the direct crrent winding ll of the reactor to the upper supply line l2 by means of a switching device 2l when the stationary contacts of the latter are bridged by the movable contacts.

The cathode i817 is connected to the lower supply line i3, as shown. As thus connected, the valve i8 rectii'les alternate halves of the alternating voltage, e. g. it recties the positive half cycles and this rectified voltage is supplied to the direct current Winding il of the reactor. Current iiow is sustained in the reactor winding during the inverse half cycles by means of a two electrode valve 2G, the anode 2da, of which is' connected to one terminal of the winding il and the cathode 20h of which is connected to the opposite terminal through a current limiting resistance 22.

By varying the magnitude of the current owing in the control winding il, the reactance of the reactor and the reactive drop across the alternating current winding l5 thereof are varied in proportion. This of course results `in varying the voltage drop across the lamp bank ll and hence the intensity of illumination of the lamp. In order to vary the current in the direct cur- Y rent winding of the reactor, it is necessary to vary the voltage -of the gridlac of the electric valve.

This is accomplished by means of a variable voltage device 23 illustrated as an inductance capacitance potentiometer comprising a condenser 24 and a variable inductance 25 connected in parallel relationship with each other and in series relationship with a condenser 26 to the supply source l2, I3. 'I he grid ic of the electric valve I8 is connected through a rectifying -device 27 and the alternating current winding of a scene fading saturable reactor 28 to an intermediate point of the variable inductance 25.' Since the rectifying device 2l is included inthe grid connections, the voltage supplied to the grid is partially rectified. The magnitude of this voltage may be varied as desired by varying the inductance 25. Thus, by varying the inductance 25, the grid voltage and anode current of the valve i8 are varied, which as previously pointed out results in varying the intensity of illumination of the lamp bank l I.

In certain installations a plurality of control devices similar to the device 23 are utilized for presetting a number of lighting scenes. In such installations, a "scene fader 30 controlling thev An illumination control system similar to the foregoing is fully disclosed and claimed in U. S. Patent 1,904,485, Livingston. The 'present invention may however be applied to other forms of illumination control systems. When a rapid darkening or black-out of the lamp ll is desired,

the black-out contactor M is utilized to disconneet the lamp Il and the saturable reactor i6 from the supply source l2, I3. Inicertain im` stallations, the dimming apparatus, such for ex-l ample as the saturable reactors, electric valves and fading reactors illustrated in the drawing on the right hand side of the dotted line 3l is located in some convenient place behind the stage or beneath it, Whereas the `control devices 23 shown at the left-hand side of the dotted line si are usually arranged on a control panel or desk in the auditorium. With the arrangement shown in the drawing only a single conductor 32 is required between each control device 23 and its associated dimming apparatus. If independent control of the black-out contactors lll were provided this would require an additional conductor from each black-out contactor to the control panel. This is undesirable because of the added cost and complication of circuits. If dependent control of the black-out contactcrs is provided it must not interfere with the normal control of dimming apparatus by the control device 23. To meet these requirements the operation of the black-out contactors ll is controlled in accordance with the magnitude of the voltage existing in the grid control connections 32 at a point physically near the dimming apparatus and remote from the control devices 23. In order to insure the necessary non-interference with the regular dimming operation, the control of the black-out contactors is so arranged that it does not become effective until near a limit of the range of the variable control voltage by means of which the regular dimming operation is accomplished. Obviously, the contactor le should pick up and drop out at substantially the same value of control voltage or otherwise there would be a range of control voltages within vwhich the contactor Mlniight either be open or closed depending upon the previous operation. If this were the case the control would be erratic, uncertain and unsatisfactory. It is, therefore, a requirement that the black-out contactors ll shall pick up and drop out at substantially the same` critical value of the control voltage.

Since 'direct current contactors are less noisy than alternating current contactors, the former are greatly preferred in theatres and similar installations. The contactor i4, therefore, is a directcurrent contactor. Since the control volt-A age is alternating, means are provided for translating the eiect of this alternating control voltage to the operating coil of the black-out contactor. As shown, directeur-rent is supplied to the operating coil of the black-out contactor by means of an electric valve 33 which is of the three-electrode type into the envelope of which a small quantity of an ionizable medium such for example as mercury vapor or other inert gas is introduced after exhaust. The presence of this ionizable medium serves to convert the usual electronic discharge into an arc stream thereby constituting the valve an electrostatically or grid controlled arc rectifier. The valve is provided with an output circuit which includes the anode 33a, a resistance 29 with which the operating coil of the black-out contactor I4 is connected in parallel, and the cathode 33h. The grid 33e of valve 33 is connected to the control conductor 32 through suitable rectifying means illustrated as a two electrode rectifying valve 3Q. A

Alternating voltage is supplied to the output circuit of the valve 33 by means of a supply transformer 35, the primary winding 35a, of which is connected to the supply source l2, i3, through the switching device A2l andthe terminals of the secondary windings 35h of which are connected ous values of grid voltage which if exceeded in a positive direction will initiate a discharge in the anode and cathode, In other words, if at any instant during the positive half cycle of anode voltage the grid becomes more positive than the critical grid voltage for that instant, the valve will fire and remain conducting through the remaining positive half cycle of anode voltage. It vwill thus be seen that by applying an alternating voltage to the grid and varying its phase relationship with respect to the anode voltage the instant at which the valve becomes conducting during eachv positive half cycle can be controlled and the average value of the current flowing in the output circuit can be similarly controlled and varied as desired.

In the present case, however, variation of the output current is undesirable becausethe contactor I4 does not have substantially identical pick-up and drop-out characteristics. This is averted by arranging the connections such that the valve is either fully conducting'or non-conducting at a critical value of control voltage. Thus, when the valve is fully conducting the black-out contactor I4 is operated, and when the valve is non-conducting the black-out contactor I4 is entirely deenergized. To accomplish this an alternating voltage of substantially reverse phase with respect to the anode voltage -is supplied to the grid 33e of the valve by meansof the secondary bias winding yo, which is energized from the primary winding 35a. The right hand terminal of the secondary winding 35o is connected both to the grid 33C of the valve 33 and to the right hand anode of the rectfying. device 34, Whilst its opposite terminal is connected through a filter comprising a resistance and a parallel condenser 4| to the cathode of the valve 34. The rectifying valve 34 furnishes negative half waves of voltage which are partially smoothed out by the lter 40, 4| and the secondary winding 35C supplies an alternating voltage to the grid. If the instantaneous value of this alterhating voltage and the partially rectified voltage are added, the resultant voltage wave will have a. form similar to that represented by the curve 42. It will thus 'be seen that this grid voltage represented by the curve 42 may be considered as consisting of-an alternating voltage superimposed upon a. negative D. C. bias voltage. Thus, as

y p shown, the grid voltage 42 is always more negative than the critical grid voltage 31 during the positive half cycle of anode voltage. Consequently, there is no point during the positive half cycle 'of anode voltage at which the valve 33 will fire and thus it remains non-conducting as long as this condition of grid voltage obtains.

However, as previously pointed out, the grid of valve 33 is connected through the rectifying valve 34 and conductor 43 to the control conductor 32. The connections are so chosen that positive half cycles ofthe alternating voltage of the control Conductor 32 are rectified by the rectifying valve and this rectied voltage is smoothed by the filtenshown as comprising the condenser 44 and parallel connected resistance 45. When the instantaneous values of this rectified voltage are added to the instantaneous values of the grid voltage 42, the net effect isto make the grid voltage more positive. As shown, diagrammatically, the net eilect is to displace the grid voltage curve 42 upwardly. If the magnitude of the control voltage is sufficiently high, the grid voltage curve 42 will be displaced to a position indicated by the curve 4G. It will be noted that during the positive half cycle of anode voltage, the grid voltage represented by curve 45 is always more positive than the `critical grid voltage and consequently the valve 33 conducts during the entire positive half cycle of anode voltage. This means that the valve,33 is conducting the maximum` amount of current and the black-out contactor I4 is operated to its closed position.

With the above understanding of the apparatus and its organization in the completed system in mind, the operation of the system itself will readily be understood from the following description:

It is assumed that the switching device 2| is operatedto itsclosed position, thereby completing the output circuit of the electric valve I8 AWhich supplies saturating current to the reactor o I6. If the lamp bank I I is to have a high intensity of illumination, the variable voltage device 23 is operated to increase the control voltage in the conductor 32 to a high value and thus to cause a large current to ow in the saturating winding of the 'reactor I6. This produces a decrease in the voltage drop across the alternating current winding i5 of the reactor and increases the voltage across the lamp bank II. Since the voltage winding I5 of the reactor and increases the voltage of the valve 33 as represented by the voltage A curve 45 is positive at the beginning of the positive half cycle of anode voltage, represented by curve 36. Consequently, the valve 33 conducts maximum current, the black-out contactor I4 is operated to its closed position thereby completing the connection of the lamp bank II and the saturable reactor I6 to the source I2, I3.

If it is. now desired to dim the lamps II rapidly to a complete black-out, the variable voltage device 23 is operated to decrease thevoltage in the control conductor 32. This decreases'the saturating current .of the reactor, thereby increasing its reactance and the voltage drop across its alternating current winding which in turn results in decreasing the voltage across the lamp bank II. The decreasing voltage in the control connection 32 decreases the positive voltage supplied to the grid 330 of the valve 33. This has the effect of displacing the grid voltage curve 46 in a downward direction. However. the current flowing in the output circuit of the valve 33 is not decreased, but on the contrary the valve 33 remains fully conducting and the black-out contactor I4 remains picked up. The explanation of this is that although the grid voltage of the valve 33 is being decreased which is represented by displacing the grid voltage curve 4S downward1y, the grid voltage is still positive at the beginning of the half cycle of anode voltage and consequently the valve conducts during the entire half cycle. The electrical constants, of the circuit are so chosen that when the control voltage in the-conductor 32 has been reduced suiliciently to reduce the intensity of illumination ,of the lamp blank II to approximately 10% of full intensity, the grid voltage of the valve -33 is represented by the curve 48.

Itwill be observed that this curve crosses the critical -grid voltage curve at the beginning of the positive half cycle of anode voltage. Consequently, the valve 33A is still fully conducting and CII the black-out contactor is maintained in its closed position. I

However, if the control voltage ofthe connection 32 is now reduced a very slight amount the grid voltage of the valve 33 will also be reduced inproportion and the curve 48 willnow lie below the critical voltage curve during the entire positive half cycle of anode voltage. In othezrwords, at no time during the positive half cycle of anode voltage, does the grid become suiciently positive to initiate a discharge and consequently the valve 33 is non-conducting and the black-out contactor 'It is deenergized causing its movable contact member Mb to descend to its lower open position to disconnect the reactor I6 and the lamp bank II from the supply source I2, I3.

It will thus be seen that the operation of the valve 33 is extremely critical at a predetermined value of the control voltage of the conductor 32, that is to say, a very minute change in the voltage at this critical value varies the conductivity of the valve 33 from full-on to full-ofi.

Similary during the reverse operation, i. e. in-

' creasing the intensity of illumination of the lamp bank II, the operation of the valve 33 is also very critical at this same predetermined value of control voltage. As long as the control voltage is so low that the grid voltage of the valve 33 lies beneath the curve 48 the valve 33 remains entirely non-conducting and the black-out contactor remains unoperated. However, when the control voltage is increased to the point at which the grid voltage of the valve 33 is represented by curve 48, the valve 33 becomes fully conducting and the contacter I4 is operated to its closed position to connect the reactor I6 and the larnp bank I I to the supply source. I

The arrangement illustrated also improves the operation of the illumination system in the. following respect: The alternating current winding 28a of the scene' fading reactor 28 is connected to the source I2, I3 through the rectifying device 21 and consequently in the absence of a provision of further means only half cycles of current would pass through the reactive winding of the fader reactor. 'Ihe best operating characteristics of the reactor are obtained, however,

, when a full wave alternating ,current iiows Cil through the reactive winding. By connecting the rectifying valve 34 across the' conductor 32 and the side I3 of the supply source in the manner illustrated the inverse half cycle of alternating voltage is rectied and consequently both half cycles of alternating current flow through the reactor winding 28a `therebyimproving its operation.

The modification illustrated in Fig. 3 is similar to the system of Fig. 1, but ditt'ers therefrom primarily in the provision of means for eiecting operation of the controlled device in response to a D. C. control voltage. In certain electrical systems, s uch as rectiers and directcurrent transmission systems, there is a -need for a relay which will eiect certain control operations as a function of the direct current. particularly in'high voltage systems of this'char acter, to insulate the controlling relays from the system voltage. A simple arrangement for accomplishing this object is illustrated in the modiication of Fig. 3. In this modication, it will be observed that the electric valve 60 and its output circuit including the secondary winding Sla of the supply transformer 6|,resistor 32 and contactor 63 are identical with the electric valve .33 of Fig. 1 and its output circuit. The grid circuit It is desirable,

of the electric valve 60 is substantially identical type rectifying device 64, such for example as a copper oxide rectier. A filter device, illustrated as comprising a resistance 65 and a condenser 66 connected in parallel with each other, is provided for the purpose of smoothing the rectified bias voltage.

The direct current circuit, the current of which is to be utilized forcontrolling the operation oi the relay 63, is represented by the plus and minus lines 61. As shown, the grid circuit of the electric valve 60 is connected to this direct current circuit, but insulated therefrom, by means of a saturable core type reactance device B8. The alternating current winding 68a of the saturable core reactor is supplied with alternating voltage from the secondary winding GIC of the supply transformer. A` suitable rectifying device, illustrated as a surface contact rectifying bridge 10 is included in circuit between the secondary winding `6Ic and the alternating current winding 68a of In operation, the impedance of the alternatin current winding 68a of the reactor varies inversely with the direct current flowing in the saturating windings 68h and 68e. For example, as the current in the saturating winding increases the impedance of the alternating current winding decreases and the rectified voltage appearing across the resistance. 'II increases. The increase in the voltage across the resistance 1I has the same eiect as an increase in voltagev across the resistance has in the system of Fig. 1. In other words, a variation in the current flowing in the D. C. circuit 61 has the same controlling effect on the electric valve 60 and its associated relay 63 that a variation in the voltage of thecontrol connection 32 in the system of Fig. 1 has on the electric valve 33 and its associated contactor I 4. For currents in the saturating winding of the reactor which are lower than the desired trip value of the relay, the actual grid voltage of the electric valve 60 is more negative than the-critical grid voltage. As the current which iiows in the saturating winding increases, the positive bias across the resistor 'II increases until the actual grid voltage curve Aintersect's the critical grid voltage, thereby causing the valve 60 to become ionized at the beginning of each positive half cycle of anode voltage. The valve 6B, therefore, becomes fully conducting and energizes therelay 63, causing it to close its contacts. SimilarlyLwhen the current owing in the saturating winding becomes less than the vcritical value, the grid voltage of the valve 63 is 2,008,512 cuit between the'recufying bridge and the man resistance ll of the grid circuit. This variable resistance 'N serves to vary or adjustthe setting of the relay, i. e. the current in the saturating winding of the reactor at which the relay is energized.

Although in accordance with the provisions of the patent statutes, this invention is embodied in concrete form, it will be understood that the apparatus and connections shown are merely illustratve and the invention is not limited thereto since alterations and modiilcations will readily suggest themselves to persons skilled in the art Without departing from the true spirit of the invention or from the scope of the annexed claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A control system comprising an electroresponsive device, an electric valve having an ionizable medium, a control grid and an output circuit connected to said device capable of passing suicient current to operate said device when said valve is fully conducting, means for supplying a negative bias voltage to said grid, means for.sup plying an alternating voltage to said output cir- -cuit and an alternating voltage of reverse phase to said grid, and means for supplying a positive bias voltage to said grid to reduce said negative bias voltage so that said valve is rendered fully conducting and said electroresponsive device is operated for certain values of said bias andsaid valve is rendered non-conducting and said` electroresponsive device is not operated for all other values.

2. A control system comprising an electric .l valve containing an ionizable medium, and having an output circuit and a control grid, an electroresponsive device connected to said output circuit and capable of actuation when said valve is fully conducting, a half wave rectier connected in the grid circuit for supplying a negative bias voltage thereto, means for supplying an alternating voltage to said output circuit and an alternating voltage of reverse phase to said grid, and means for supplying a positive voltage to said grid to oppose said negative bias voltage so that said valve is fully conducting and said electroresponsive -device actuated for certain values of so that said valve is fully conducting and saidelectroresponsive deviceis operated for certain values of said bias and said valve is non-conducting and said device is not actuated for all other values, and means for supplying a variable positive voltage to said grid to decrease said negative bias tothe value at which said valve becomes fully conducting.- j

4. A control system comprising van electric valve containing an ionizable medium, a cathode, an anode and a control grid, an electroresponsive device connected in the anode circuit of said valve, a source oi' alternating voltage, connectlons from said source to said anode circuit for supplying an alternating voltage thereto,

connections from said source to said grid including a rectifying' device for supplying a negative bias and a superimposed alternating voltage of reverse phase with respect to the anode voltage so that said valve is conducting and said l electroresponsive device is actuated for certain values of said bias voltage less negative than a predetermined critical value and said valve is non-conducting and said device not actuated for all other values of said bias, and means for supplying a variable positive"'voltage to said grid to vary said bias voltage through all of said values. 5. A control system comprising an electric valve containing cathode, anode. control grid and an-ionizable medium, an electroresponsivedevice connected ln the anode circuit of said valve, a source of alternating voltage, connections from said'source to the anode circuit of said valve for applying an alternating voltage thereto, connections from said source to said grid including a half wave rectifying device for supplying to said grid a negative bias and a superimposed alternating voltage of reverse phase with respect to I said anode voltage whereby said valve is non-conducting for certain values `of said bias and fully conducting for all other' values,- and means forl supplying an alternating current of variable effective value-to said rectifying device whereby a variable positive voltage is supplied to said grid for decreasing said bias to cause said valve to become conducting.

6.v A control system comprising an electric valve containing cathode, anode, control grid and an lonizable medium, an electroresponsive device connected in the anode circuit of said valve, a source of Aalternating voltage, connections from said source to said anode for supplying 'an alternating voltage thereto, a rectifying device having a plurality of anodes, connections including one of the anodes'of said rectifying device from said source to said grid for supplying to said grid a negative biasv and a superimposed alternating voltage of reverse phase with respect to the anode voltage of said valve so that said valve is fully conducting for certain values of said bias and non-conducting for all other values, and connections from another anode of said rectifying device to a source of alternating voltage of variable effective value for decreasing said negative bias to render said valve conducting.

7. A relay control system comprising a source of alternating voltage, an electric valve containing'cathode, anode, control grid and an ionizable medium, a source of alternating voltage, a relay having its operating coil connected in the anode circuit of said valve, a rectifying device in the ternating voltage thereto, connections from said source to said anode circuit for supplying an al- 'ternating voltage thereto, connections from said source to said grid and said rectifying device for applying vto said grid a negative bias voltage and a super-imposed alternating voltage of reverse phase with respect to said an'ode voltage so that said valve conducts suicient current to actuate said relay for certain values of said biasA and is non-conducting for all other values of said bias, a source of variable control voltage and connections from saidcontrol voltage source to said grid for supplying a variable positive voltage to said grid ,to vary saidbias through all of said values so that said relay is actuated for certain values of said control voltage and deenergized for all other values.

8. A control system comprising a source'of vato be controlled by' said voltage, connections from said source to said device, an electric valve containing an ionizable medium and having a control grid and an output circuit, an electroresponsive'device connected in said output circuit and operable to exert control over said controlled device when said valve is fully conducting, means for supplying a negative bias to said grid, means for supplying an alternating voltage to said out- 10 put circuit and an alternating voltage of reverse phase to said grid so that said valve is fully conducting for certain values of said bias and nonconducting for all other values, and connections from said grid to said control voltage connections for supplying a variable positive voltage to said grid to vary said bias to provide actuation of said electroresponsive device for certain values of said control voltage and deactuation for all other values.

MARVIN M. MORACK.

CERTIFICATE 0F CORRECTION.

'9mm No'. 2,008,512. my 16, 1935.

MARviN M. MoRAcK.

It is hereby certified that error appears in the printed specification of the above numbered patent requiringcorrection as' follows: Page 3, second column, line 33, for the words "winding 15 of the reactor and increases the" read in the control conductor 32 is high, the grid; andlne' 68, for "blank" read bank; page 5, line 11, first column,

for "illustratve" read illustrative; and vsecond column, line 57, claim 7, strike out the syllable and words "ternating voltage thereto" and insert instead grid circuit of said valve; and that the said Letters Patent should be read with these corrections therein that the same may conform to the re'cord of the case in the .Patent Office.

g Signed and sealed this 27th day-of August, A. D. 1935.

Les] ie Frazer v1(3gal) Acting Commissioner of Patents.

CERTIFICATE 0F CORRECTIGN.

'Pmm No; 2,008,512. July 16, 1935.

MARVN M. MoRACK.

l lt is hereby certified that error appears in the printed specification of the above numbered patent requiring'correction as' follows: Page 3, second column, line 33, for the words "winding 15 of the reactor and increases the" read in the control conductor 32 is high, the grid; andiine' 68, for "blank" read bank; page 5, line 1l, first column,

for "illustratve" read illustrative; and second column, line 57, claim 7, strike out the syllable and words "ternating voltage thereto" and insert instead grid circuit of said valve; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the IPatent Office.

i Signed and sealed this 27th day-of August, A. D. 1935.A

Leslie FrazerA v(grml) Acting Commissioner of Patents.

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