US2268034A - Control system - Google Patents

Description

Dec. 30, 1941. R HUNTER V 2,268,034

CONTROL SYSTEM Filed June 22, I938 MAXIMUM BR\GHT-Posiiion of dimmer contact arm- DM Patented Dec. 30, 1941 UNHTED oFfFica CONTROL SYSTEM Richard B. Hunter, Shorewood, Wis, assignor to Cutler-Hammer, 1110., Milwaukee, Wis., a corporation of Delaware 9 Claims.

This invention relates to control systems and more particularly to regulating systems.

More specifically the invention relates to voltage regulating systems, and although not limited in its application the invention is of particular advantage in connection with theater light dimming systems. In such a system it is desirable, especially in connection with the presetting of intensities of illumination for different scenes, that the intensity of illumination of a lamp circuit shall accurately correspond to the indication of a control device employed to vary the illumination.

However, in systems of the aforementioned character which employ series connected impedances, such as variable resistances and reactances, the voltage drop across the series impedance device changes with changes in the load, and consequently the position of the indicator is not always an accurate indication of the degree of illumination. 7

In prior systems this difficulty was partially overcome by utilizing saturable reactors of spe- V supplied to a lamp circuit of variable load is con-' trolled by means responsive to a control voltage and a second voltage derived from the controlled circuit, such systems employing electric Valve and other apparatusinvolving relatively high original cost and maintenance expense. Moreover, the aforenoted systems are, more or less, arrangements which compensate for certain undesirable characteristics of series impedances.

Accordingly it 'is an object of this invention to provide a novel-control system which overcomes the aforementioned difficulties and disadvantages.

Another and more specific object is-to provide an improved and simplified illumination control system having means for adjusting and maintaining the voltage of a lamp circuit, within a Another object is to provide an improved illumination. control system of the aforementioned character which aflords a desired characteristic of variation of the voltage supplied to a lamp'circuit,

Another object is to provide control means for a system of the aforementioned character which is simple, efficient and relatively inexpensive.

Another object is to provide a control system of the aforementioned character having extreme flexibility in respect of affording various desired features of manual and/or automatic control.

Other objects and advantages of the invention will hereinafter appear.

In practicing the invention in one form thereof I prefer to employ a variable autotransformer dimmer to control the voltage of and degree of illumination afforded by a lamp circuit; or

alternatively to control an operating condition of another or similar form of alternating current circuit. The autotransformer dimmer is controlled responsively to a control voltage provided by a suitable control device. The control device is preferably of a type which is adapted to afford indication of the desired operating condition of the circuit, such as a variable resistance, preferably of the potentiometer type, or a variable autotransformer. I a plurality of such control devices would ordinarily be employed for each lamp circuit, and the same would be arranged to provide for presetting of a corresponding number of intensities of iilumination for the lamps of such circuit for.different scenes.

It is to be understood that while I prefer to employ a variable autotransformer dimmer to control the lamp circuit, other devices having an across-the-line or potentiometer connection may be employed. However, as is well known, the

autotransformer has certaininherent 'advantages, in respect of enabling stable voltage control of a translating circuit, with a high: degree of efficiency, at any desired value substantially independently of the value of or changes in the load.

transformer dimmer with its inherent properties for affording the desired character of control of a lamp circuit possesses a very material advantage over series impedance devices.

It is known that variable autotrans'former' dimmers have been employed heretofore for controlling the voltage supplied'to a'lamp circuit. It is a further object of this invention, however, to use (so a variable autotransformer' dimmer in a novel In a theater dimming system manner in an illumination control system, wherein the jautotransformer dimmer is controlled automatically in response to a varying voltage supplied by an indicating control device to adjust and maintain the voltage and degree of illumination of a lamp circuit, within a wide range of lamp control system embodying my invention;

Fig. 2 is a similar view of a more complete form of illumination control system embodying my invention; and

Fig. 3 is a diagram showing by means of curves the typical character of control of the voltage supplied to the lamps which is obtained with the systems illustrated in Figs. 1 and 2 and the resultant variation of the intensity of illumination of the lamps.

As-is well known, the variable autotransformer affords substantially linear variation of the voltage supplied to the controlled circuit. This has been found to be a definite advantage in the control of lamp circuits, since the curve of illumination intensity variation of the lamps resulting from linear variation of the voltage supplied thereto closelyapproximates the optimum curve of variation of illumination intensity as enunciated by what is variously known as "Fechners law or Weber's law. This law states generally that the sensational effect does not increase proportionally to the stimulus, but as the logarithm of the stimulus. Thus supposing the stimulus to be 10, 100 or 1000 times increased, the sensational efiect will not be 10, 100 or 1000 times, but only 1, 2 and 3 times greater. The curve CP of the diagram, Fig. 3, represents the variation of illumination intensity of the lamps in candlepower from black-out to full-bright when the lamp voltage, represented by curve LV, is varied linearly, or substantially so, from substantially zero to maximum. It will be apparent that the curve C? closely approximates the desired curve aforementioned.

In order to facilitate the description and understanding of certain features of the invention,

an elementary form of illumination control system is shown in Fig. 1, Referring thereto, the numeral I designates a group of electric lamps which are adapted to be supplied from a suitable source, such as the source of alternating current represented by thelines L L The voltage supplied to the lamps, and consequently the degree of illumination of the latter, is controlled by means of a potentiometer impedance device, shown as the variable autotransformer dimmer 2 which is connected across lines U, U. A movable contact arm 2 connected with lamps I, is associated with autotransformer dimmer 2 and said arm is adapted to be adjusted automatically in response to unbalance of a balancing control circuit. The latter comprises one or more variable sending potentiometer impedances 3, l, a variable receiving potentiometer impedance 5,

and a balanced relay 6. The impedances 3, 4, I are shown as autotransformers and each has associated therewith a movable contact arm 3.

4 5', respectively.

The contact arms 2 and 5 are mechanically connected with and electrically insulated from each other and are thus adapted to be moved simultaneously by the motor M through the medium of suitable reduction gearing 7. Motor M is a split field motor having directional field windings F and F which are adapted to be selectively energized through the medium of the stationary contacts 6 or 6 and the armature contactor 6 of relay 6.

Relay 6 is of the two coil type and its contactor 6' is normally in the neutral position shown, but when there is a difierence between the voltages across the two coils 6 6 the contactor is attracted in one direction or the other to energize motor M through one of its fields. Motor M thereupon operates in the proper direction to move contact arm 5 to restore balance of the voltages across the coils 6, 6 and simultaneously to move contact arm 2 to change the intensity of illumination of lamps I. Unbalancing of. relay 6 is normally effected by operation of means for rendering the sending impedances 3, 4 effective selectively, said means being shown as a double throw switch 8. Meanwhile the contact arm of the inactive sending impedance may be moved to preset another scene. It will be further apparent that relay 6 may be unbalanced by moving the contact arm of the active sending impedance.

Further details of the operation of the system shown in Fig. 1 will be better understood from the following description. Thus the control circuit is shown as being in a state of balance at a point to provide for supply of maximum voltage to the lamps! and consequent maximum brilliancy thereof. That is, contact arm 3 is at a point adjacent to line L with respect to sending impedance 3 and switch 8 is in the position to render arm 3 eflective. Contact arm 5 has been moved to a position with respect to receiving impedance 5 corresponding with that of arm 3 by motor M in response to operation of relay 8. Contact arm 2 consequently is in a position with respect to autotransformer dimmer 2 corresponding with that of arm 3 and thus the lamps l are connected directly across lines L U to provide for maximum brilliancy thereof.

Upon movement of switch 8 to its dotted line position contact arm 4 of sending impedance 4 will be rendered effective. Contact arm 4 is in a position midway between the extremities of sending impedance 4, indicating ultimate energization of lamps l at fifty per cent of full voltage. When contact arm is rendered active there will be a difference in the voltages across coils 6 6* of relay 6 due to the difference in the positions of contact arms 4* and 5 and contactor 6 of said relay will consequently be attracted in a direction to engage stationary contact 6' pending movement of contact arm 5" to eiIect balancing of the voltages across the coils. Contact arms 5" and 2 will be moved by motor M in a downward direction, and the control circuit will be balanced when arm 5' reaches a position with respect to receiving impedance 5 which corresponds with that occupied by contact arm 4 with respect to sending impedance .4, Motor M will then be de-energized by reason of contactor 6 assuming its neutral position. Contact arm 2' will then be in a position with respect to autotransformer dimmer 2 to impress fifty per cent tively simple means I have found it possible in a a preset or indicating illumination control system to obtain accurate voltage control of a lamp circuit in accordance with indicated values substantially irrespective of the value of the lamp load.

Fig. 2 illustrates a more complete form of illumination control system, and is presented to disclose more clearly the manner in which the invention is ordinarily employed. The system of Fig. 2 is essentially like that of Fig. 1, difiering mainly therefrom in the provision of refinements and additions to the control means to afiord the desired flexibility of control and operation of the autotransformer dimmers.

Two groups of lamps lo, M are shown by way of example, and it may be assumed that the lamps of the two groups are of'the same color, one group representing stage footlights, for instance, and the other group representing border lights. It is to be understood" that additional groups of lamps of the aforementioned and different colors would ordinarily be employed to provide a complete theater or stage lighting system, and that the control means for such additional groups would be arranged in any suitable manner to provide for Joint or selective control of all or certain of said groups of lamps.

The voltage supplied to the groups of lamp IO, N is controlled by the variable autotransformer dimmers l2, l8, respectively. The latter are connected across lines L L, which represent a suitable alternating current source. Associated with autotransformer dimmers l2, l3 are the movable contact arms I2, I3 respectively; arm i2 being connected to lamps I and arm H being connected to lamps l I.

Referring to the control means for autotransformer dimmer l2, contact arm I2 is adapted to be adjusted responsively to unbalance of a balancing control circuit by a reversible split field motor M through the medium of suitable reduction gearingll. The balancing control circuit comprises the sending autotransformers l5, l6, II, a receiving autotransformer l8 and a balanced relay l9. Each of the sending autotransformers l5. l8, H has associated therewith a movable contact arm l5", l6, l1, respectively. Said contact arms are adapted to be connected selectively. to one end or the other. of a fader autotransformer through the medium of pairs of manually operable switches I, I5", I8 |6 and I'l respectively. A movable. contact arm 20 is associated with fader autotransformer 20 and is connected to one of the coils of relay 9, the other coil thereof being connected to the movable contact arm l8 associated with receiving autotransformer l8. Contact arm I8 is mechanically connected with and electrically insulated from contact arm I2 of autotransforiner dimmer |2 in the manner hereinbefore described.

The fader autotransformer 20 serves in a measure the same purpose as switch 8 in the system of Fig. 1, except that a more gradual transition of ,the intensity of illumination of lamps I0 is e1- fected between the values selected by the contact arms l5, l8, l1 by moving contact arm 20'; or,

if desired, the degree of illumination of said lamps may be maintained at any intermediate value by adjusting contact arm 20 to points intermediate the ends of fader autotransformer 2B.

The sending autotransformers l5, I6, I! are supplied with current at a suitably reduced voltage from the secondary winding 30 of a transformer through the medium of one or both of.

the scene master autotransformer 28 and the grand master autotransformer 29. The primary winding 3|! of transformer 30 is connected across lines U, L. The sending autotransformers i5, IS, H are connected at one end to one or the other of contact arms 28*, 29, associated respectively with master autotransformers 28, 29, through the medium of double throw switches it, it, i7, respectively. Contact arms 28, 29 are shown in positions with respect to their associated autotransformers to provide for connection of the sending 'autotransformers l5, l6, l1

directly across the terminals of the secondary winding 3|! of transformer 30. The scene master autotransformer 28 is adapted to be connected directly across the terminals of the secondary winding 30 or through the contact arm 29*, selectively, by means of a double throw switch 3|. The grand master autotransformer 29 and also the receiving autotransformer I8 are adapted to be connected across the terminals of said secondary winding. Thu by means of the master autotransformers 28, 29 proportional and cascaded potentiometer control of the balancing control circuits for the several groups of lamps in a lighting system may be effected, either jointly or severally. A push button switch 9 is preferably provided to open and close the circuit from the secondary winding of transformer 30 to the balancing control circuit; i. e., the master. sending, fader and receiving autotransformers. and the balanced relay.

' The system further includes a line switch 2|, continuity switches 22, 23, a normally closed switch 4|, and an electromagnetically Operated switch 25, the contacts of which are closed upon energization of its winding to connect autotransformer dimmer l2 across lines L L The winding of switch 25 is energized'upon closure of the aforementioned switches 2|, 22, 23 and M. Cone tinuity switch 22 is shown as a double throw color master switch which is ordinarily provided with an additional stationary contact, as shown, which may be connected to a grand master switch in a well known manner to provide for Joint continuity control of all or certain of the differently colored groups of lamps employed in a complete theater lighting system. Switch 4| is adapted to be opened as by contact arm l2, as shown, when the latter is in its extreme dim position. Thus the winding of switch 25 is deenergized when lamps I!) are black-out. the contacts of said switch being opened to deenergize the lamps l0 and autotransformer dimmer l2.

The operation of the control system, as here- I insofar described, is as follows 3-.

Upon closure of line switch 2| and push button switch- 9 the sending autotransformers l5, l8, II are connected in parallel across the terminals of secondary winding 30' of transformer 30, the circuit therefor extending from the left hand terminal of winding 30" through push button switch 9, by conductors l2 and 43 through the right hand contact and the contact arm of switch 3!, by contact arm 28 and conductor 44 through the left hand contacts and the contact arms of switches l, I6 I19, through the autotransformers l5, I6, i1, and by conductor 48 and line L to the right hand terminal oi secondary winding 30". with the arrangement shown and just described the illustrated positions of contact arms l5, [6, ll will provide for supply to lamps III of zero, fifty per cent and maximum voltages, respectively. Thus assuming closure of switch II (another of the switches. say 11, having previously been closed), it will be apparent from the positions of fader contact arm 20', the contactor of relay l9 and the receiver contact arm I8 that the control circuit is balanced at a point to provide for zero voltage and consequent blackout of lamps I0. Dimmer contact arm l2, being movable simultaneously with receiver arm l8, as aforedescribed, is in the extreme dim position, as shown, and is in engagement with an extension on switch 4| to open the same, for the purpose hereinbefore mentioned.

To vary the voltage of lamps II! from that indicated by sender arm l5 (zero) to that indicated by sender arm l6 (fifty per cent) the aforementioned switch I! is then opened and switch l6 is closed to connect arm I6 to the upper end of fader autotransformer 20, switch iS having been closed previously, as mentioned. Fader arm 20 is then moved to its upper extreme position to provide a predetermined voltage differential across the opposed coils of relay I9. The resultant response of relay I9 effects operation of motor M in a direction to move receiver arm l8 and dimmer arm i2 simultaneously to is similar to that aforedescribed. Thus dimmer arm l3- associated with autotransformer dimmer I3 is adapted to beadiusted by a reversible'split field motor M through the medium of suitable reduction gearing 34 in response to unbalance of a balancing control circuit which comprises the sending autotransformers 35, 38, 31, a, receiving autotransformer 38, a balanced relay 39, and a chanlcally connected. By this arrangement the balance the control circuit and to supply fifty per cent voltage to the lamps, respectively. It is assumed that the continuity switches 22, 23 are closed. It is apparent that switch 4| will be closed immediately upon movement of dimmer arm l2 from its extreme dim position.

In a similar manner, to vary the voltage of lamps (0 from fifty per cent to that indicated by sender arm l1 (maximum) switch i5 is opened and switch ll is closed to connect arm ll, to the lower end of fader autotransformer 20. Movement of fader arm 20' to its lower extreme position will effect response of relay l9, as aforedescribed, whereupon motor M will operate in a direction to move receiver arm l8 and dimmer arm I 2 simultaneously to balance the control circuit and to supply maximum voltage to the lamps, respectively.

It will be apparent from the foregoing that operation of the proper switches (li H", I 6, l6, li HF) and movement of fader arm 20 alternately to.its opposite extreme positions will provide for variation of the lamp voltage and consequently the illumination intensity from one to other of the values indicated by the sender arms l5, l6, H.

The fields F and F of motor M are adapted to be energized throughthe medium of the electromagnetically operable switches 32, 33, respectively, the closing windings of said switches being adapted for energization, selectively, by balanced relay i9.

illumination of a plurality of groups of lamps may be varied from one preset intensity toanother by manipulation of a minimum number of control devices. Also the contact arm 40' associated with fader autotransformer 40 is mechanically connected with contact arm 20, an arrangement whereby the transition from one to another of the intensities preset for the individual groups of lamps in a lighting system may be effected jointly.

The sending autotransformers 35, 3B, 31 are adapted to be connected across the terminals of secondary winding 30 of transformer 30 through the medium of double throw switches 35 36 31, respectively, and through the master autotransformers 28, 29 in a manner similar to that hitherto described.

Proportional dimming of the several groups of lamps may be effected by moving contact arm 18 of the scene master autotransformer 28 to the position shown in dotted lines, for instance. Cascading control of the several groups of lamps is effected through the medium of the grand master autotransformer 29 by moving contact arm 29"- to the position shown in dotted lines, for instance. Switch 3| would be moved to its dotted line position to obtain such cascaded control.

the voltage supplied through the autotrans-- former proper and is used to compensate for the A small normally engaged. electromagnetically voltage drop in the line to insure full brilliancy of the lamps when contactor I3 is in the extreme position opposite to that illustrated.

While I prefer to employ variable autotransformers for the'sending, receiving, fader and master potentiometer impedances, as shown, it is to be understood that other kinds of impedances may be employed, such as potentiometer resistors.

It will be apparent from the foregoing that by utilizing avariable autotransformer which is controlled responsively to the setting of an indicating control device the voltage and degree of illumination of a group of lamps will relatively accurately correspond withthevalue indicated by such device substantially irrespective oi changes in the lamp load. The desired character of illumination control is thus provided with relatively simple and economical apparatus. Furthermore the improved regulating system hereinbefore described obviates the necessity for a direct current supply or for rectifiers and provides conveniently and efllciently operable means for obtaining maximum flexibility of control.

What I claim as new and desire to secure by Letters Patent is as follows:

1. In an illumination control system, in combination, a group of electric lamps, a constant voltage source of current supply therefor, means for varying the voltage of said lamps and for maintaining the voltage thereof substantially constant at a desired value, said means comprising a variable potentiometer type impedance device connected across said source and adapted to impress a variable voltage upon said group of lamps, means for regulating the output voltage of said device comprising a plurality of sending impedances each having a contact arm associated therewith and adapted for adjustment to provide for preselection of a value of control voltage, means associated with said sending impedances to provide a balancing control circuit, said last mentioned means including a variable receiving impedance, a fader impedance, means associated with said sending impedance contact arms for connecting any one of the same to either end of said fader impedance and for conby the other of said active contact arms, means associated with said sending impedances and said fadcr impedance to provide a balancing control circuit, said last mentioned means including a variable receiving impedance and means including power operated means operable upon unbalancing of said control circuit due to variation of the control voltage to adjust said receiving impedance, whereby said control circult is balanced, and simultaneously to adjust said dimming device, whereby the voltage of said lamps is adjusted to and maintained at a value having a predetermined relationship to the control voltage value, and means including an electromagnetically operable brake releasable automatically ,upon balancing of said circuit to insure stopping of said motor.

3. In an illumination control system, in combination, a plurality oi groups or. electric lamps, a constant voltage source of alternating current supply therefor, a variable autotransformer dimmer f r each of said groups of lamps, means for necting any other of said contact arms toethe other end of said fader impedance, a manually operable contact arm associated with said iader impedance for traversing the same, whereby the control voltage is varied from the value preselected by one of said active sending impedance contact arms to the value preselected by the other of said active contact arms, means including a reversible electric motor operable automatically in response to variation of the control voltage to adjust said receiving impedance. whereby said control circuit is balanced, and simultaneously to adjust the output voltage of said device directly in accordance with and to a degree definitely corresponding tothe value of said control voltage, whereby the voltage of said group of lampsis adjusted to and maintained at a value having a predetermined relationship to the control voltage value.

2. In an illumination controlsystem, in combination, a group of electric lamps, a source of alternating current supply. therefor, a variable autotransformer dimming device connected across said source for controlling the voltage of said lamps, regulating means for said dimming device comprising a plurality of sending impedances connected across said source, each of said impedanoes having an associated movable contact arm to provide for preselection 01 a value of control voltage, a fader impedance, means associated with said contact arms to provide for connection of any one of the same to either end of said .tader impedance and to provide for connection of any other of said contact arms to the other end of said fader impedance, a manually operable contact arm associated with said iader impedance for traversing the same, whereby the control voltage is varied gradually from the value preselected by one or said active sending impedance contact arms to the value preselected regula ing the output voltage or each of said dimmers comprising a plurality of sending pctentiometer impedances connected to a suitable source of current supply, each of said impede ances having a movable contact arm to provide for preselection of a value of control voltage, a fader impedance, means including switches associated with each of said contact arms, respectively, said switches being operable selectively to provide for connecting any one of said contact arms to either end of said iader impedance and to provid e/ior connecting any other of said contact-arms to the other end of said fader impedance, means to provide for joint operation of certain of the switches associated with the respective sets of contact arms, a movable contact arm associated with each of said fader impedances, whereby the control voltage is varied gradually from the value preselected by one of said active sending impedance contactarms to the value preselected by the other of said active contact arms, means to provide for joint operation ofof lamps controlled thereby is adjusted to' and maintained at a value having a predetermined definite relationship to the value of control voltage;

4. In an illumination control system, in combination, a lamp circuit, a constant voltage source of alternating current supply therefor, means to control the degree of energization of said lamp circuit, said means comprising a variable autotransformer dimming device connected across said source, means for regulating the output volt- Y age of said device comprising a plurality of variable autotransformers connected in parallelmatically to said control voltage including means 3 operable to adjust the output voltage of said device directly in accordance with and to a degree definitely corresponding to the value of said control voltage, whereby the degree of energization of said lamp circuit is adjusted to and maintained at a given constant value for a given preselected control voltage.

5. In an illumination control system, in combination, a plurality of independently controllable lamp circuits, a constant voltage source of alternating current supply therefor, a variable autotransformer dimmer associated with each of said lamp circuits for controlling the degree of energization thereof, means for regulating the output voltages of said dimmers comprising individual variable sending autotransformers to be energized from said source, associated means providing for individual, group and master control of the electrical conditions of said sending autotransformers, said means comprising variable autotransformers so disposed and connected as to provide for maintaining a predetermined deflnite relation between the electrical conditions of the several sending autotransformers during joint control of such conditions of the latter, and means associated with each of said regulating means and responsive automatically to variations of the control voltages provided by the latter including means operable to adjust the output voltages of said dimmers directlyin accordance with and to degrees definitely corresponding to the values of said control voltages, whereby the degree of energization of each of said lamp to said lamps is adjusted to and maintained at a given constant value for a given preselected control voltage, and means associated with said dimmer to compensate for voltage drop between said source and said lamps, whereby to insure maximum intensity of illumination of said lamps upon adjustment of said dimmer to provide the same, said means comprising additional turns of the winding of said autotransiormer dimmer connected between one side of said source and said lamps.

8. In an illumination control system, in combinatioma group of electric lamps, a source 01' alternating current supply therefor, a variable autotransiormer dimming device connected across said source for controlling the voltage of said lamps, regulating means for said dimming device comprising a plurality of sending impedcircuits is adjusted to and maintained at a given illumination thereof, said means comprising a variable autotransformer dimmer connected across said source, means for regulating the output voltage of said dimmer comprising means for supplying a variable control voltage, means responsive automatically to said control voltage including means operable to adjust the output voltage of said dimmer directly in accordance with and to a degree definitely corresponding to the value of said control voltage, whereby the voltage supplied to said lamps is adjusted to and maintained at a given constant value for a given preselected control voltage, and switch means associated with said dimmer and operable automatically upon adjustment of said dimmer to provide for zero intensity of illumination of said lamps to effect complete deenergization of the latter.

7. In an illumination control system, in combination, a group of electric lamps, a source of alternating current supply therefor, means interposed between said source and said lamps to control the degree of energization of the latter between values providing for substantially zero and maximum intensities of illumination thereof, said means comprising a variable autotransiormer dimmer connected across said source, regulating means for said dimmer comprising means for supplying a variable control voltage, means responsive automatically to said control voltage including means operable to adjust saidv dimmer directly in accordance with the value of said control voltage, whereby the voltage supplied ances of the variable autotransiormer type connected across said source, each of said impedances having an associated movable contact arm to provide for preselection of a definite value of I said lamps is adjusted to and maintained at a value having a predetermined definite relationship to the preselected control voltage value, and means including an electromagnetically operable brake releasable automatically upon balancing of said control circuit to insure substantially immediate stopping of said motor after operation of the latter in either direction.

9. In an illumination control system, in combination. a group of electric lamps, a source of alternating current supply therefor, means for adjusting the voltage of said group of lamps to and maintaining the same substantially constant at a desired .value, said meanscomprising avariable autotransformer dimming device connected across said source, regulating means for said device including an impedance of the variable autotransformer type for supplying a variable control voltage, means associated with said sending impedance to provide a balancing control circuit, said last mentioned means including a receiving impedance of thevariable autotransformer type, means including a reversible electric motor operable automatically upon unbalancing of said control circuit, as an incident to adjustment of said sending impedance, to adjust said receiving impedance, whereby said control circuit is balanced, and simultaneously to adjust said dimming device, whereby the voltage of said lamps-is adjusted to and maintained at a value having a predetermined definite relationship to the control voltage value, and means including an electromagnetically operable brake releasable automatically upon balancing of said control circuit to insure substantially immediate stopping of said motor after operation of the latter in.

either direction.

RICHARD B. HUNTER.

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