US2879404A - Means and method of controlling high tension electrical circuits - Google Patents

Description

March 24, 1959 T. H. ROGERS ETAL 2,879,404

MEANSTE1:IIgI METHOD OF CONTROLLING HIGH 0N ELECTRICAL CIRCUITS Filed May 2. 1956 5 Sheets-Sheet 1 PULSE FORMING AND TIMING CIRCUIT Fl .I

I 5 4 FILM CHANGER I I 56 I U I I F|G 5 OPTIONAL 52 I f glIfr PRIMARY POWER AND OONTROL cmcun's I 53/'I l /57 HIGH VOLTAGE POWER EQUIPMENT PULSE FORMING SUPPLY AND TIMING CIRCUIT IO 55 INVENTOR.

THOMAS H. ROGERS BY GORDON F. BAVOR ATTORNEYS March 24, 1959 T. H. ROGERS ETAL 2,879,404

MEANS AND METHOD OF CONTROLLING HIGH TENSION ELECTRICAL CIRCUITS Filed May 2, 1956 3 Sheets-Sheet 2 FIG. 2

sxasrme x RAY IGENERATOR CONTROL I UNIT PULSE MONITOR INVENTOR.

moms H. ROGERS BY GORDON F. BAVOR ATTDRNEYS March 24, 1959 TENSION ELECTRICAL CIRCUITS 3 Sheets-Sheet 3 Filed May 2, 1956 w 6E 5&3 523 mm +m 1 +0 I W ml II W 55855 w 3 5. 3 .l

INVENTOR.

THOMAS H. ROGERS GORDON F. BAVOR United States Patent 2 'MEANS AND METHOD OF CONTROLLING HIGH TENSION ELECTRICAL CIRCUITS Thomas Rogers, New Canaan, and Gordon F. Bavpr, Nor-walk, Conn., assignors to 'Machlett Laboratories, Incorporated, Springdale, -Co'nn., a corporation of Connecticut Application May 2, 1956, Serial'No. 582,252 '7 Claims. romeo-9's This invention relates to novel means and method 'of controlling high=tension electrical circuits and has particular reference to-a system and methodtor controlling theop'eration of a high voltage'eriergy-'responsive device whereby the devicemay be subje'c ted tohigh voltage for precise ultra-short periods of time. Still more specifically this invention refers to a system and method for con trolling the operation of a high voltage device suchas an X=raytube "or the like so as toprovide precisely timed ultra-short X-ray'exposures in a selected range such as from-one to five milliseconds.

In radiography the problems concernedfwith exposure duration are complicated where motion is involved in the subject matter and the problem of obtaining sharp, clearlimages becomes more and more complicated as the speed of motion increases. These problems are-solved by therprovision of more precise, shorter-interval timers and more Xi-rayprod'ucingpower in a fine focus X-r'ay tube.

.X-ray apparatushas become increasingly'more capable in theserespects. First, radiography at 4 second was evolved and later 43 second exposures became feasible. Fairly recently, however, it became desirable in work with heart radiography, particularly angiocardiography, to obtain. still shorter exposures in rapid succession, such as severalper second.

Conventional high-powered generators havesometimes been used for this purpose and in these instances the basic circuit of the generator involved the opening and closing of a switch in the primary circuit of a high-tension transformer. This required carefulconsideration, in the development of timing and contacting systems, of the proper phasing of the instancofcontact make and break witht'he sine-wave of the power supply. Such considerations generally limited possible timeintervals to integral multiples of a one-cycle period of the AC. power supply. Recently developedsystemsperrnit using an odd number of half-cycles, with minimum durations of second.

Other known prior art indicates that efforts have been tension or secondarysideofthe circuit. This invention comprises improved and novel-means and method of-accomplishing the control=ofsuch exposures by switchingin the high-tension circuit.

It is a primaryobject of this invention to provide a system for controlling the operation of high voltage energy responsive devices such as X-ray tubes or the like whereby the devices may be'subjected to high voltage for precise ultra-short periods of time.

It is another object of this invention to provide a system for controlling the operation of an X-ray tube so as to provide precisely timed ultra-short exposures in a selected range such as'from one to five milliseconds.

Another object is to provide a system of the above characterwhichis adapted to existing diagnostic or other equipment.

-A further object is theprovisiori ofva simplified system of, the above character which requires low control-circuit l atented N lar. Zfl,

voltage and power requirements as well as relatively simple control circuitry.

A still further object is the provision of a system of the above character which embodies novel safety means for preventing damage to the high voltage device in the event of control voltage 'failure in the high voltage circuit.

Other objects and advantages of the invention will become apparent from the following descriptiontakenin connection with the accompanying drawings, in which Fig. 1 is a simplified schematic diagram of the circuit of the switching system;

Fig. 2 shows the circuit details involved in supplying the various potentials required for the operation of the switching tube, and alsoshows the safety circuit, thegrid return circuit, and the method of cutting the electronic switching system in or out;

Fig. 3 is a block diagram showing the various component parts of the system of the present invention as used with existing film changing equipment; and

Fig. 4 shows the'basic circuit of a pulse generator usable with this system.

In the drawings the system is shown as being used for controlling the operation of an X-ray tube but it is to be understood that other high voltage energy responsive devices can be substituted for the X-ray tube if desired.

The circuit of the presently described system is employed in conjunction with a rectified X-ray generator having electrical energy storage means such as capacitors across the output whereby the generator, which is energized 'inadvance of making the exposure, chargesth'e capacitors to its full open-circuit voltage, and the exposure is then made at that voltage. The power'for energizing the X-ray tube comes directly from the capacitors and, therefore, the need for synchronizing the exposure with the sine-wave power operating the transformer is eliminated. Also, in such a system, essentially square pulses are drawn fromthe capacitors.

A pulse forming and timing circuit generates a control signal in the form of an essentially square voltage pulse, the width of which can be adjusted over a suitable range. The signal is applied to the control grid of a switching tube through a pulse transformer which transmits square pulses five milliseconds or less in width without appreciable distortion. The pulse transformer also isolates the control circuit from the high-tension part of the system.

The control grid bias, screen grid potentials, and filament power for the switching tube are provided by an isolation transformer through suitable rectifiers and filters.

Embodied inih'e system also is asafety circuit which is provided to protect thee'quipment from damage in the event of loss of negative bias on the switch tube ineasecircuit which minimizes driving power requirements by.

bypassing thecurrent drawn by the controlgrid, during exposure.v periods "of the X-ray tube, around the bias su 1 ply without'the use of a shunting resistor.

- single oil-'filled tank for insulation reasons.

Withinthe tankis' also located an auiriliarymulti'polef high-tension switch, solenoid-operated by -re'rnote con and the X-ray tube by sheckproo'r r 1 3 trol, by which the electronic switching system, including the energy storage capacitors, can be cut in or out of the cicuit at will. Thus, the X-ray equipment can be used conventionally if desired.

Referring to the drawings, and more particularly to the schematic diagram of Fig. 1, it will be seen that a pulse-forming and timing circuit 10 is provided for generating a control signal in the form of anessentially square voltage pulse P which is applied to the control grid of a switching tube 11 through a pulse transformer 12. The pulse-forming and timing circuit 10, to be described later, allows adjustment of the pulse within a suitable range. The pulse transformer 12 transmits square pulses of controlled width such as about milliseconds or less without appreciable distortion and also serves to isolate the control circuit from the high tension part of the system. The primary of pulse transformer 12 is connected directly into the circuit 10. A negative bias for the control grid of the switching tube 11 is provided by a suitable D.C. power supply 13. This negative bias is applied to the control grid of the switching tube through the secondary of pulse transformer 12. Special circuitry is provided to apply this negative cut-off bias to the control grid of the switching tube 11, as will be described more fully hereinafter with respect to Fig. 2. Positive screen grid potential in tube 11 is provided by a second D.C. power supply 14.

A high voltage generator 15 such as a four-valve fullwave bridge rectifier type as illustrated in Fig. l is utilized to charge two energy storage capacitors 16 in a balanced circuit as the primary source of energy for the X-ray tube 17. The high voltage switching tube 11 is in series with the positive side of the high voltage generator circuit to the anode of the X-ray tube 17.

- The generator 15 is energized in advance of making an exposure with the X-ray tube 17 and charges the condensers 16 to its full open-circuit voltage, and then the exposure can be made at that voltage. In the absence of the capacitors 16, the power for energizing the X-ray tube 17 would come directly from generator 15, in which case the exposure voltage would be considerably below the open-circuit voltage of the generator 15 due to the regulation of the transformer therein, and the power line. The switching tube 11 and the insulation of the system also, in the absence of the capacitors 16, must withstand the full open-circuit voltage. Since the power for energizing the X ray tube 17 comes from the capacitors 16, in accordance with this invention, instead of directly from the generator 15, the need to synchronize the exposure with the sine-wave of the power'line is eliminated. With the present system the only time'it may be desirable to synchronize the exposure with the power line frequencies might be in the event that it is desired to obtain a high rate of repetitive exposures. X-ray exposure current wave shape, pulse P1, is essentially identical to the shape of the voltage pulse P applied to the high voltage switching tube, the amplitude of the current being limited by the emission-limited characteristics of the X- ray tube 17. Y

In Fig. 2, all switches, relays, and the like are shown in the positions they occupy when the system of the present invention is installed in, but is not electrically switched into, the existing X-ray equipment and no power is applied to the system, line switch 41 being open.

The control grid bias and screen grid potentials of switching tube 11 are provided by an isolation-transformer 18'with multiple secondaries, throughsuitable rectifiers 19-20 and filters. Transformer 18 also supplies filament power to all tubes in the high tension system except X-ray tube 17. The screen potential of switching tube 11 is supplied by rectifier 20 in a full-wave center-tap type of power supply embodying a simple filter capacitor 21 and bleeder resistor 22. The control grid. negative bias supply consists of rectifier 19 in a second full-wave center tap type. ofpower supply comprising-a.

filter capacitor 23, bleeder resistors 24 and 25, and a bias control tube 26. This bias control tube 26 is a tetrode which is operated at cut-off in its static condition and receives its grid bias and screen potentials from the voltage divider- bleeder resistors 24 and 25 which form part of the negative bias supply. A coupling capacitor 27 and control grid signal amplitude control potentiometer 28 are provided for driving tube 26 from the pulse transformer 12. The purpose of tube 26 is to provide a low impedance path for the control grid pulse current of the high voltage switching tube 11 and a high impedance path to the negative bias supply during the inter-pulse period, thereby reducing the average continuous current from the negative bias supply.

Control tube 26 minimizes driving power requirements by bypassing the current drawn by the control grid of the switching tube 11 around the bias supply during exposure periods.

Also shown in Fig. 2 is the safety circuit which is provided to protect the equipment from damage in the event of loss of negative bias on the switch tube 11 in case of power unit failure. This circuit consists of a voltage sensitive relay 29 and a current limiting resistance 30 across the negative bias supply, which relay 29 is actuated only when bias voltage is up to normal. Relay 29 closes the circuit to a signal light 31 which actuates a photocell 32 in an interlocked circuit, the chassis (not shown) which supports the photocell and associated circuitry being at ground potential.

The photocell circuit itself is basically of conventional design but possesses novelty in the present system by embodying a tranparent glass or plastic rod or transilluminator R which is positioned between the photocell 22 and the signal light 31. Thus, the signal light 31, which is part of the bias power supply, transmits light rays through the transilluminator R to operate the photocell 22. The transilluminator thus serves to electrically isolate the high tension circuitry from ground.

It will be understood that should the negative bias to switching tube 11 fail for reasons such as, for example, short circuiting of the filter capacitor 23, burning out of the filament in rectifier 19, power failure in transformer- 18, or for any other reason, this will cause relay 29 to open, thus turning off signal light 31 and stopping operation of photocell 32, thereby recovering negative bias on a thyratron tube 33 also in the circuit. Thyratron tube 33 operates on AC. and, during operation of the system, is in a self-rectifying half-wave condition. Thus, the negative bias on the grid of thyratron tube 33, with falling plate voltage from the applied sine wave plate potential, allows this tube to turn itself off. This drops out plate relay 34, the contacts of which are in series with the coil of interlock relay 35. Such deenergizing of relay 35 opens its contacts which are in series with the coil of a primary contactor 36, which is part of the existing diagnostic equipment with which the present system is used, which contactor 36 controls power to the high tension generator 15. This removes the high voltage which charges the pulse energy storage condensers or 34 opens.

In the absence of a safety device of this character, loss of negative bias and subsequent loss of control of the high voltage switching tube 11 would result in a continuing .runaway condition, even after the initial discharge of-the energy storage condensers 16, which might cause extensive damage to the system and particularly to the X-ray tube 17.

.Arelay 38 is provided to bypass the series safety cir-' .cuits embodying relay 35 when, in a completed installation, the presently described system is turned oif' and not being" used, so that the existing diagnostic unit into which the present system is connected may be operated in the normal manner.

This system includes high tension switching means for completely disconnecting the high voltage elements o f the switching tube circuitry so that conventional radrography techniques may be performed conveniently without requiring manual changes to be made after the system is installed in existing equipment. The high'ten- S1011 switching means consists of three ganged switches 39, 39a and 39b whichxare connected to an actuating mechanism which comprises two solenoids 40 and 40a. The switches 39, 39a and 39b in'Fig. 2 are shown in positions permitting conventional use of the X-ray equipment, switch 39 connecting the anode ofthe X-ray .tube 17 directly to the positive side of the high tension generator 15. It will be noted that at all times the cathode of the X-ray tube 17 is permanently connected to the negative side of the high tension-generator 15. v

"To make radiographs with this system, 'it is merely necessaryto close the line switch 41 which will cause energization of solenoid 40. Solenoid 40 thus will actuate switches 39, 39a and 39b, and thereby connect the present system into the existing X-ray equipment. Switch 39, inthis position, connects the anode of the X-ray tube 17 to the cathode of the high'voltage switching tube 11. Switches 39a and 39b now connect the high voltage sides of condensers 16 to the positive and negative sides respectively of the high voltage generator 15.

After suificient time has elapsed to permit the various tubes in the system to reach operating temperatures, radiographic exposures may be made in a manner to be described hereinafter in connection with Fig. 3.

After all desired exposures have been made with the present system, the system may be turned off with line switch 41, which removes power from the system and, throughsolenoid 40a and time delay 42 which bypasses" switch 41, the high tension switches 39, 39a and 39b are returned to their initial positions as shownin Fig. '2. The time delay 42 functions to remove power'from the solenoid 40a after a predetermined time interval. High tensionswitches 39a and 39b now connect the high voltage'side'of the condensers 16 to discharging current limiting resistors 43 and 43a respectively.

A micro-switch '44 is mechanically actuated by the high voltage switching means and serves asa high vo1tage switch position safety interlock so that relay 35, which controlshigh tension generator primary contactor 36, will not close unless the high voltage switches have been completely closed to-eleetrically insert this system in the existing equipment.

Signal lights are provided to indicate the proper or improper operation of or failure of power to thevarious circuits involved. Light 45 indicates that power is being supplied to the pulse-forming and timing circuit 10. Light 46 shows that power is being applied to power transformer 18 and the safety circuit described hereinbefore. Signal light 47 operates from microswitch 44 andindicates that the high tension. switches 39, 39a and 39b are in proper position for operation of this system,

while light 48 indicates, when lighted, that theseswitches arenot properly positioned for this purpose.

A pulse monitor circuit 50 isincorporated in the pulse-- forming and timing circuit '10 to indicate an exposure by the X-ray tube and to facilitate adjustment and calibrationof the X-ray tube filament circuit by indicating the peak X-ray tube current in the presently described pulsed operation. The unit making up the pulse monitor 50 is essentially a peak voltmeter which reads the voltage drop .across a current viewing'resistor 49 in the center tap ground circuit of the energy storage capacitors 16.

Referring to Fig. 3, it .is to be understood that the system of'the present invention is adaptable for use with various types of conventional film changing devices. The present system may be used to control the film changer or may be partially controlled by the film changer. For example, a film changer is .indicatedby numeral 51 in Fig.3 and is shown connected to the pulse'forming and timing circuit 10 through a manually operable selector switch 52 which allows selective manual or automatic control of'the system. Switch '52 is shown in position to permit manual operation of the system whereby X-ray exposures may be made without using the film changer 51, the system being operated'by manual closure of push button 53 to permitexposures to be'made. When switch 52 is moved'to the opposite position, this connects the film changer 51 into the system in such a manner that the film changer actually controls X-ray exposures to the extent that no exposures can be made, even when pushbutton 53 is operated, until the film changer 51 has completed its automatic cycle. It is particularly pointed out that an operator has constant control of the device during the actual exposure cycles by being required to actuate push button 53 before any-exposurescan be made either manually or automatically.

An optional recycling circuit 54 can be provided for use with certain film changers where it is needed or desired such as, for example, in instances where the film changer will not function without a recycling signal or is not of the free running type. Such a recyclingcircuit 54 thus can be used to start operation of the film changer 51 after completion of an X-ray exposure.

'Fig. 3 also shows that the pulse forming .and timing circut 10 receives power, whenline switch 41 is closed, through the power supply 55. The 'primary power and control circuits 56 also receive power when switch 41 is closed and control'the power to the high voltage equipment 57, as described in detail in connection with Fig. 2.

An example of one fundamental pulse forming and timing circuit capable of being used in this system is shown in Fig. 4. This circuit, indicated'by numeral 10 in Figs. 1, 2 and 3, embodies a pulse generator for forming the substantially square pulses which are transmitted to the pulse transformer 12, and is adapted to form pulses of controlled width and voltage. A pulse amplifier is alsoincluded to increase the power levelof the pulses developed by the generator, and a synchronizing trigger circuit is also embodied for initiating'the formation of pulses by the generator when the push butto 53'is manually operated.

The pulse generator may be of any suitable design and is shown in Fig. 4 as a driven monostable cathodecoupled multivibrator which is comprised, in this particular structure, of two triodes 58 and 59. This multivibrator is driven by a synchronizing amplifier tube 60 which is'connected in parallel with tube 58.

The pulse width is manually adjustable over the desired range by means of a potentiometer 63 which varies the time constant of the timing control circuit'of the multivibrator, which -circuit includes-a capacitor 64 as well as the potentiometer 63.

The pulses developed by the pulse generator are amplified by a vacuum tube amplifier 61 to a predetermined power level controlled by means such as a grid driving controlpotentiometer 62. The plate circuit of the pulse amplifier 61 is connected to the primary of the pulse transformer 12.

'It is apparent from the foregoing description thata be understood, however, that although the individual.

' parts of the system have been shown and described specifically, many of these parts may be modified within the scope of this inventionsuch as, for example, bysubstituting a thyra'tron or other pulse forming circuit for the pulse forming and timing circuit 10, or by use of -7. other types of rectified high voltage generators other than generator 15.

It is also to be understood that although a major portion of the preceding description relates to a system for controlling an X-ray tube so as to provide precisely timed ultra-short X-ray exposures, the system can be used for other purposes such as, for example, to apply pulses shorter than or longer than one millisecond to energy responsive devices of the type used in radar transmitters, particularly klystrons designed for operation at relatively high voltage levels. In some instances, also, the energy storage capacitors may be eliminated whereupon the power for energizing the energy responsive device will be provided directly from the generator.

Accordingly, while the novel features of the invention have been shown and described and are pointed out in the annexed claims, it is to be understood that various omissions, substitutions and changes in the details shown and described may be made by those skilled in the art without departing from the spirit of the invention. Therefore, it is to be understoodthat all matter shown or described is to be interpreted as illustrative and not in a limitingsense.

We claim:

1. A system for operating a high tension circuit embodying an energy responsive device by means of a low tension control circuit, comprising a generator in the high tension circuit, electrical energy storage means across the output of the generator and adapted to be charged to the full open-circuit voltage of the generator, one side of the energy storage means being connected to the energy responsive device, a high voltage switching tube of the tetrode type connected between the other side of the storage means and the energy responsive device, means connected with the switching tube for providing control grid bias and screen grid potentials thereon, pulse forming and timing means in the low tension circuit for generating a control signal in the form of an essentially square voltage pulse, and a pulse transformer between the high and low tension circuits adapted to receive the pulse from the pulse forming and timing means and to apply the pulse to the grid of the switching tube to cause discharge of energy from the energy storage means to the energy responsive device.

2. A system for operating a high tension circuit embodying an energy responsive device by means of a of the generator, one side of the energy storagemeansbeing connected to the energy responsive device, a high voltage tetrode switching tube connected between the other side of the storage means and the energy responsive device, means connected with the switching tube for providing control grid bias and screen grid potentials thereon, a pulse forming and timing means in the low tension circuit for generating a control signal, a pulse transformer between the high and low tension circuits adaptedv to receive the control signal from the pulse forming and timing means and to apply the signal to the grid of the switching tube to cause discharge of energy from the energy storage means to the energy responsive device, and a safety circuit connected to the means for providing control grid bias to. the switching tube comprising means for automatically discontinuing operation of the high voltage generator. 7

3. A system for operating a high tension circuit embodying an energy responsive device by means of a low tension control circuit, comprising a high voltage generator in the high tension circuit, electrical energy storage means across the output of the generator and'adapted to.

be charged to the full open-circuit voltage of the generator, one sideof the energy storage means being connected to the energy responsive device, a high voltage tetrode switching tube connected between theother side of the storage means and the energy responsive device, means connected with the switching tube for'providing control grid bias and screen grid potentials thereonya pulse forming and timing means in the low tension circuit for generating a control signal, a pulse transformer between the high and low tension circuits adapted to receive the control signal from the pulse forming and timing means and to apply the signal to the grid of the switching tube to cause discharge of energy from theenergy storage means to the energy responsive device, and asafety circuit connected to the means for providing'control grid bias tothe switching tube comprising a voltage-1 sensitive relay across the means for providing control grid bias and adapted to be closed when said means is functioning, a lamp connected with the relay so as to, function when the relay is closed, a photocell in the low tension circuit, transilluminating means between the lamp and photocell for directing light from the lamp to the photocell, and means operable when the photocell is inoperable to break the circuit to the generator.

4. A system for operating a high tension circuit embodying an X-ray tube by means of a low tension control 4 circuit, comprising a generator in the high tension circuit,

- voltage tetrode switching tube connected between the other side of the storage means and the X-ray tube, pulse forming and timing means in the low tension circuit for generating a control signal in the form of an essentially squarevoltage pulse, and a pulse transformer between the high and low tension circuits adapted to receive the pulse from the pulse forming and timing means and toapply the pulse to the grid of the switching tube to cause, discharge of energy from the energy storage means to the X-ray tube.

5. A system for operating a high tension circuit em-' bodying an X-ray tube by means of a low tension con-- trol circuit, comprising a generator in the high tension circuit, electrical energy storage means across the output. of the generator and adapted to be charged to. the full} open-circuit voltage of the generator, one side of the potentials thereon, a pulse forming and timing means in energy storage means being connected to the X-ray tube, j a'high voltage switching tube of the tetrode type con-, nected between the other side of the storage means and the X-ray tube, means connected with the switching tube, for providing control grid bias and screen grid potentials thereon, pulse forming and timing means in the lowtension circuit for generating a control signal in the form of an essentially square voltage pulse, and a pulse transformer between the high and low tension circuits adapted, to receive the pulse from the pulse forming and timing, means and to apply the pulse to the grid of the switching tube to cause discharge of energy from the energy stor-, age means to the X-ray tube.

6. A system for operating a high tension circuit cmi bodying an X-ray tube by means of a low tension con-.

trol circuit, comprising a high voltage generator in the. high tension circuit, electrical energy storage means. across the output of the generator and adapted to be charged to the full open-circuit voltage of the genera-. tor, one side of the energy storage means being con nected to the X-ray tube, a high voltage tetrode switching tube connected between the other side of the storage means and X-ray tube, means connected with the switching tube for providing control grid bias and screen grid' forming and timing means and to apply the signal. to thegrid of the switching tube to cause discharge of energy 9 from the energy storage means to the X-ray tube, and a safety circuit connected to the means for providing control grid bias to the switching tube comprising means for automatically discontinuing operation of the high voltage generator.

7. A system for operating a high tension circuit embodying an X-ray tube by means of a low tension control circuit, comprising a high voltage generator in the high tension circuit, electrical energy storage means across the output of the generator and adapted to be charged to the full open-circuit voltage of the generator, one side of the energy storage means being connected to the X-ray tube, a high voltage tetrode switching tube connected between the other side of the storage means and the X-ray tube, means connected with the switching tube for providing control grid bias and screen grid potentials thereon, a pulse forming and timing means in the low tension circuit for generating a control signal, a pulse transformer between the high and low tension circuits adapted to receive the control signal from the pulse forming and timing means and to apply the signal to the grid of the switching tube to cause discharge of energy from the energy storage means to the X-ray tube, and a safety circuit connected to the means for providing control grid bias to the switching tube comprising a voltage-sensitive relay across the means for providing control grid bias and adapted to be closed when said means is functioning, a lamp connected with the relay so as to function when the relay is closed, a photocell in the low tension circuit, transilluminating means between the lamp and photocell for directing light from the lamp to the photocell, and means operable when the photocell is inoperable to break the circuit to the generator.

Morrison Dec. 7, 1937 Quittner July 5, 1949

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