US2043484A - Variable frequency stroboscope - Google Patents

Description

June 9, 1936. B. MILLER VARIABLE FREQUENCY STROBOSCOPE Filed June 15, 1954 2 Sheets-Sheet 1 INVENTOR BENJAMIN MILLER ATTORNEY June 9, 1936. B. MILLER v 2,043,484

VARIABLE FREQUENCY STROBOS COPE Filed June 15, 1934 2 Sheets-Sheet 2 INVENTOR BENJAMIN MILLER ATTORN EY Patented June 9, 4 1936 VARIABLE FREQUENCY STROBOSGOPE Benjamin Miller, Richmond Hill, N. Y., assignor to Doherty Research Company, New York, N. Y., a corporation of Delaware I Application June 15, 1934, Serial No. 730,741

19 Claims.

This invention relates generally to electric impulse producing apparatus. It is particularly concerned'with an improved variable frequency stroboscope.

The present invention is a continuation in part of and improvement on that described in my copending application Serial No. 709,901, filed February 5, 1934. In my aforesaid copending application I have described a novel form of an electric flash producing apparatus including as elements a gaseous conductor stroboscope lamp of the type in which discharge across two main electrodes is initiated by an appropriate stimulus applied to a third electrode; together with a condensershunted across two main electrodes, and a circuit for charging the condenser. The flash producing apparatus described in my aforesaid copending application was particularly developed for examining stroboscopically the timing of a variable speed spark ignition engine and for accurately determining the speed of the engine; and for this purpose excitation of the lamp is effected by applying the ignition impulse of the engine to the third electrode of the lamp.

The term trigger tube will be herein em-' ployed to designate a gaseous conductor device in which conduction of current'through a gaseous medium between two primary electrodes'is initiated by applying the proper stimulus, as for example by changing the potential of a third electrode. A further description will be found in my aforesaid copending application.

A particular object of the present invention is to provide an improved variable frequency stroboscope which is adapted for general use in examining engines and other moving elements. Impulses suitable for initiating discharge of a condenser through a gas onvapor tube strobo-- scope lamp may be obtained by interrupting the current in the primary of a transformer such as v an automotive ignition coil, by including in the primary circuit a rotary circuit breaker, a storage battery, and drive for the circuit breaker. The rate of flashing is then controlled by the speed of the circuit breaker.

By having the element to be examined drive method may be improved by substituting for the circuit breaker a contactor in the grid circuit of a trigger tube, whereby a condenser is suddenly charged or discharged through the trigger tube and primary of the transformer. This requires less power, and is more generally applicable, but there remain cases in which the element to be examined cannot be made to provide the impulses, even by the photo electric cell method described in my copending application Serial No. 709,901. For such cases it is necessary to use an impulse generator whose rate can be controlled. One method is to use a circuit breaker or contactor driven by some means independent of the element to be examined, such as a motor; means must then be provided for varying the speed.

The use of an auxiliary moving element and of a variable speed drive mechanism involves complications which I desire to avoid, so that a primary object of the present invention is to provide an improved combination of a condenser discharge stroboscope with an impulse generator embodying no moving elements and which requires only the variation of an impedance to control the rate of flashing.

The impulse generator of the present invention consists essentially of a static inverter in which a condenser is suddenly charged or discharged through a gaseous conductor device at intervals controlled by varying an impedance in the circuit, and the sudden current flow through the gaseous conductor device is used to generate the impulse which is conducted to the exciting electrode of a stroboscope lamp. The invention comprehends two types of static inverter-the driven type and the self-excited type. I prefer to employ that variety of self-excited inverter known as a relaxation inverter when a wide range of flashing rates isto be covered, but I have provided a new type of self-excited inverter which is particularly suited for very precise frequency control, as will be more fully discussed below.

The driven static inverter comprises a variable frequency vacuum tube oscillator, a condenser, means for charging the condenser, and a trigger tube. The alternating current generated by the vacuum tube oscillator is impressed on the grid of the trigger tube, whereby current flows through the trigger tube, in pulses whose frequency is equal to the frequency for which the oscillator is set, to charge or to discharge the condenser.

Vacuum tube oscillators are so well known that it is not necessary to describe one; that type known as a multivibrator is suitable. From thedescription of the use of the various modifications of self-excited static inverters, those skilled in the art will readily understand how to modify the circuits shown to use a driven inverter. Since the self-excited inverter requires fewer parts and is simpler to build and operate, it is preferred, but the invention is limited only by the appended claims. I

Other objects of the invention are: to provide a stroboscopic tachometer of the type described in my copending application Serial No. 709,901 embodying a ratiometer adapted to give impulse rate readings which are independent of voltage variations; to provide a static inverter which employs a trigger tube, and which is adapted to furnish impulses at a controlled rate, which rate is relatively insensitive to supply voltage changes and substantially independent of variations in trigger tube characteristics; and to provide an impulse generator which is adapted for exciting a stroboscope and which is a static inverter employing a trigger tube in a circuit which allows a wide range of impulse rates to be covered by means of a simple control.

With the above and other objects and features in view. the invention resides in the variable frequency stroboscope which is hereinafter described and particularly defined in the accompanying claims.

80 The present invention will be hereinafter more particularly described by reference to the accompanying drawings, in which:

Fig. 1 is a wiring diagram of a condenser discharge trigger tube stroboscope having an exciting electrode connected to the secondary of a transformer, in which flashes of the tube are initiated by impulses impressed on the primary of the transformer by means of a static inverter embodying a gaseous discharge device connected in shunt circuit'with a condenser; together with a source of potential, and a variable impedance for varying the impulse rate.

Fig. 2 is a wiring diagramof a modified arrangement of the apparatus shown in Fig. 1

.in which the gaseous discharge element of the static inverter is connected in series circuit with the condenser and with a source of potential.

Fig. 3 is a wiring diagram of an improved form of electrical impulse generator comprising a condenser, a charging circuit for the condenser which is a slightly damped oscillating circuit including an electric check valve, an in ductance and a variable impedance, and a condenser discharge circuit: comprising the primary of a transformer and a trigger tube.

- 5 and anode respectively connected by leads I! and H with the terminalsof a condenser It. The. charging circuit for condenser l8 includes a source of electrical potential It connected to the respective terminals of the condenser. through leads l9 and 20. Preferred stroboscope circuits for use with the present invention are more particularly described in my aforementioned co- I pending application Serial No. 709,901.

The several circuits shown in Figs. 1, 2, 3, and 3 .4 aredesigned particularly to illustrate various modifications of the impulse generator adapted to initiate discharge or flashing of the stroboscope tubes Hi. In all of the figures the means employed includes a transformer 24 having its secondary connected by a lead 26 with an excit- 5 ing electrode 22 of the tube Ill. The impulse generator shown in Fig. 1 includes the primary of transformer 24 and a gaseous discharge tube 28 having its anode and cathode connected by leads in the discharge circuit of a condenser 10 30.; 32 is a source of direct current energy which is connected by leads through a variable impedance VR with the terminals of condenser 30. Current may flow from 32 to the condenser at a rate controlled by VR. As soonas the poten- 15 tial of the condenser builds up to the breakdown voltage of the tube 28, the condenser suddenly discharges throughthe tube and the priniary of the transformer, inducing a high voltage impulse in the secondary of the transformer 20 which is conveyed to the stroboscope l0 and initiates a flash thereof.

In the impulse generator shown in Fig. 2 the primary coil of the transformer 24' and the gaseous discharge tube 2| are connected in the 25 charging circuit of the condenser 30, and the variable impedance VB is shunted across the terminals of the condenser. Current is supplied from the energy source 32 whose potential is above the breakdown potential of tube 28. When 30 the circuit is made, current flows'to charge the condenser 30 until the potential across the electrodes of tube 2| falls below the potential at which it remains conductive. Discharge of the condenser then takes place at a rate controlled 35 through the tube and the transformer primary.

The impulse induced at this time in the second- 40 'ary'of the transformer is conveyed to the stroboscope lamp l0 and initiates a flash thereof.

In the impulse generator shown in Figs. 1 and 2 the gaseous conductor device has but two electrodes. It is possible to use a three electrode device, a trigger tube; this is to be preferred in some cases since the breakdown potential may be varied by varying the potential of the third electrode with respect to one of the main electrodes. The gaseous discharge device may have 50 a cathode heated by appropriate means, if desired; a hot cathode tube has in general a lower potential drop while current is flowing, which may be an advantage in some cases. The variable impedance VR may have a high or low 5 ratio of inductance to resistance. Alternatively, the capacity of condenser 30 may be changed to change the rate of flashing; it is possible to change all three types of impedance in the circuit, capacitative, inductive, and resistive, or so any two, or only one. It is alsopossible to use a variable impedance in thepath between the condenser 30 and the gaseous discharge device 28,

but this is less desirable, since the suddenness of the current flow through the primary of the 5 transformer would be lessened.

The frequency of the impulses produced by the static inverter shown in Figs. 1 and 2 is affected by the source potential and the breakdown potential of the gaseous discharge device. I have 70 therefore developed the impulse generator shown in Fig. 3, which consists essentially of a condenser 30 which is charged from energy source82 through an inductance L, a variable inductance impedance VR and through an electric check denser over the source.

' capacitance of 30.

valve V. Condenser 30 is shunted by a circuit comprising a trigger tube 34 and the primary of a spark coil transformer 24. The control electrode or grid 36 of tube 34 is connected to the cathode of check valve V through the resistor R38. The secondary of spark coil 24 is connected to the exciting electrode of the strobo scope Iii. The operation of the impulse generator shown in Fig. 3 is substantially as follows: When the apparatus is first connected to the energy source 32 the potential of condenser 30 is zero. As soon as current starts to flow the control electrode 36 of trigger tube 34 is' made negative to its cathode by the amount of drop 5 across the check valve V, which drop maybe about 10 to 15 volts if V is a hot cathode mercury vapor rectifier. I prefer to employ at 34 a positive grid trigger tube, that is, one whose grid must be made positive with respect to its cathode to initiate conduction with the highest anode-cathode potential to be used. However, a

negative grid tube may be used, together with an auxiliary bias, if necessary. It is sufiicient that the total negative bias including the drop across V, be great enoughto keep the tube in the non-conducting state with the highest potential to which condenser 30 will be charged. The inductance L and resistance VR are so chosen with respect to the capacitance of 30 that L, VR and 30 form an oscillating circuit, preferably one which is but slightlydamped. Therefore current continues to flow into condenser 30 until its potential exceeds the potential of source 32. The current is then decreasing, and eventually falls to zero. At this time the tendency is for the current to reverse, and for the condenser to discharge back into the source, but this reversal is not possible because of the check valve action of V. The excess voltage is therefore immediately transferred to the check valve, 'mak-- ing its cathode positive with respect to its anode by this amount. The check valve is by-passed by the circuit comprising R-38, the control electrode 36 of tube 34, and the cathode of the trigger tube. Current therefore may flow through this circuit in amount which is limited by the resistance of R"; but as soon as the control'electrode 36 becomes 'sufliciently positive with respect to its cathode, the trigger tube 34 breaks down, the condenser 30 discharges through the trigger tube and through the-primary ofthe spark coil 24, thereby inducing a high voltage impulse in .the secondary of the transformer which is conveyed to the exciting electrode of the stroboscope Ill. The inductance of the spark coil 24 causes the voltage of condenser 30 to become less than the cut off voltage of trigger tube 34, so that conduction ceases. The control electrode 36 then regains control and the process is repeated. The discharge through the trigger tube 34 takes place exactly at the instant of maximum condenser potential, regardless of the tube characteristics, since the potential of the control electrode 36 with respect to its cathode then changes instantly from negative by the amount of the drop through valve V when conducting, to positive by the amount of excess potential of the con- The time required to reach the maximum condenser potential is independent of the source potential, so that the number of impulses per second is governed only by theresistance VR inductance of L and the The number of flashes of stroboscope lamp l0 per second may be varied by changing VR, as shown in the diagram. It

is also possible to make condenser 30 variable.

Also, while it is preferred to use a stroboscope lamp II! which is separate and distinct from the trigger tube 34, it is possible to employ the light developed bythe trigger. tube 34 for stroboproduct of inductance and capacitance used, so. that for long time intervals-of the order of second or more-this product becomes inconveniently large. Also, only a comparatively narrow range of speeds can be covered with a simple variable impedance. To take care of the very low speeds, and to get avery wide range of speeds with a simple variable impedance, I have modified the circuit of Fig. 1 by using a vacuum tube to limit the current flows to condenser 30,

the variable impedance being used to control the current which the vacuum tube will pass. An illustration of a preferred modification of this type is shown in Fig, 4. r

There are two ways of controlling the current through the vacuum tube. One method employs a variable resistance in the cathode heating circuit. The electron emission is a function of the cathode temperature, so that by making the potential of the anode with respect to the cathode sufiiciently high, the current flow can be controlled by controlling the cathode temperature, which can be controlled by controlling the current flow to the cathode heating circuit. In practice this method is restricted to tungsten cathodes, since other commonly available cathodes may lie injured by the application of anode voltage sufiicient to attract all the electrons emitted. I prefer to use the second method, in which the current flow is limited to a value less than the total emission by a third electrode whose potential with respect to the cathode is controlled by means of a variable resistance. In order to make the current flow to the anode substantially independent of the anode potential, a. fourth electrode may be employed, this fourth electrode being maintained at a constant positive potential with respect to the cathode. Even better results are obtained by employing as well a fifth electrode, whose potential is also maintained con- I stant with respect to the cathode, usually at the same potential as the cathode. My preferred method is then to use a pentode, or triple grid tube, with one grid connected to the cathode, a second grid maintained at a constant potential positive with respect to the cathode, and the third grid connected to a variable resistance so that its potential with respect to the cathode can be controlled. I have found that a 58'type tube is quite suitable for this purpose; flashing rates may be varied continuously from 0 to any upper limit fixed by the dionization time of the trigger tubes employed. The application of the pentocle is shown in Fig. 4.

Fig. 4 is a wiring diagram of a preferred arrangement of variable frequency stroboscope tachometer. This stroboscope tachometer comprises two main circuits, one being a'stroboscope and R. P. M. indicator closely resembling the apparatus described in my aforementioned copending application Serial No. 709,901; and the other circuit comprising an impulse generator adapted for supplying excitation impulses to the stroboscope at a controlled rate.

Referring to Fig. 4, a transformer 42 is shown as energized from alternating current mains H, the transformer serving to charge the condenser 46 through rectifier valve 41 and through inductance L". Condenser I6 is charged from condenser 46 through inductance L and a check valve V. Stroboscope lamp l discharges condenser l6 when it is excited by an impulse impressed'on its exciting electrode 22. tween the stroboscope circuit illustrated in Fig. 4 and similar circuits illustrated in my aforementioned copending application Serial No. 709,901 lies in the substitution of a ratiometer RM in place of the ammeters described in my said copending application, for use as a rate meter. The ratiometer RM is calibrated to show the ratio of the currents flowing in its two coils. One of these coils is in series circuit with the stroboscope lamp i0, and the other in parallel circuit. The current in the coil which is inparallel with stroboscope I0 is proportional to the supply voltage, so that the instrument readings are independent of supply voltage variations.

The impulse generating apparatus for exciting the stroboscope lamp ID, as it is illustrated in Fig. 4, comprises a transformer 5| which is energized from A. C. mains 52 and which trans-' former supplies induced current which is rectified by a valve 53 and filtered by inductance L and a condenser 55 to a voltage divider comprising a resistor R", glow tubes I and T and a potentiometer R. The glow tubes T" and T serve to maintain constant the potential between their electrodes. The constant potential between the electrodes of glow tube T is impressed between the cathode and screen grid of pentode tube P, while any desired fraction of the constant potential between the electrodes of glow tube :I" is impressed between th'e cathode and the control grid of pentode P". g v

Another voltage divider which consists oi' resistor R potentiometer R." and glow tube '1, is supplied with direct current from any convenlent source such as the source from which the condenser 46 of the stroboscope circuit is charged. a

From the potential across R. condenser 30 is I charged through pentode P at a rate which is controlled by the potential between the cathode and control grid of pentode P and which rate is independent of the potential across R so long as the potential across 30 is substantially less than the potential across R. The potential across condenser 30 is impressed be tween the anode and cathode of trigger tube 84,

while any desired fraction 01 the constant popotentiometer R" and then discharges through c the primary of transiormer 24, thereby inducing in the secondary of transformer 24 a high voltage impulse which excites stroboscope lamp III to discharge condenser I6.

The chief difference beselected intervals, and means for'conveying to the It is possible to control the interval between flashes oi. stroboscope I0 by adjusting potentiometer R or by adjusting R", but I prefer to use R to control the flash rate, and to use R. only to adjust from time to time to compensate '5 for any changes in characteristics in trigger tube 64.

While the apparatus, for controlling impulse rates hereinbefore described is the preferred means foraccomplishing the objects of the pres- 10 ent invention, other possible means of control may be used including circuits adapted to change the capacity of condenser 30, and circuits adapted to change the screen grid potential of pentode 60.

Having thus described my invention, what is claimed as new is:

1. In combination with apparatus for producing electrical energy flashes of the type in which a condenser is periodically discharged through a gaseous conductor lamp,*means for initating discharge of the condenser at selected intervals consisting of a static inverter for producing electrical impulses at a controlled rate, said inverter comprising a gaseous discharge device connected in circuit with a source of potential and with meansfor varying the impulse ratecomprising a variable impedance, and means for conveying such im-,, a

'-means connected in series circuit with the energy source and with the condenser for charging the condenser to a potential higher than the potential of the energy source, said means comprising an electric check valve and an inductance forming with the condenser a slightly damped oscillating circuit, means for discharging the condenserat '40 apparatus the voltage impulse which occurs at the instant that current ceases to flow through said electric check valve.

3. Apparatus of the type defined in claim 1 in which the gaseous discharge device is a gaseous conductor device having at least three electrodes of a type in which the maximum potential which can be established between two main electrodes without appreciable current flow therebetween may be controlled by controlling the potential between one of said main electrodes and a third electrode.

4. Apparatus of the type defined in claim 1 in which the static inverter includes a second condenser arranged in series circuit with the gaseous discharge device and the source of potential.

5. Apparatus of the type defined in claim 1 in which the static inverter includes a second condenser shunted across the gaseous discharge device. a

6. In combination with apparatus for producing electrical energy flashes of the type in which a condenser is periodically discharged through a gaseous conductor lamp, means for initiating discharge of the condenser at selected intervals coning electrical energy flashes of the type in which a condenser is periodically discharged through a gaseous conductor lamp, means for initiating discharge of the condenser at selected intervals consisting of a static inverter for producing electrical impulses at a controlled rate, said inverter comprising a gaseous discharge device connected in circuit with a source of potential, a second condenser shunted across the gaseous discharge device, and means for varying the impulse rate comprising a variable impedance connected in the charging circuit of the second condenser, and means for conveying such impulses to the flash producing apparatus.

8. In combination with apparatus for producing electrical energy flashes of the type in which a condenser is periodically discharged through a gaseous conductor lamp, means for initiating discharge of the condenser at selected intervals consisting of a static inverter for producing electrical impulses at a controlled rate, said inverter comprising a gaseous discharge device connecting in circuit with a source of potential, a second condenser shunted across the gaseous discharge device, and means for varying the impulse rate comprising a vacuum tube connected in the charging circuit" of the second condenser together with means for controlling the current flowing through said vacuum tube, said means comprising a variable impedance.

9. In combination with apparatus for producing electrical energy flashes of the type in which a condenser is periodically discharged through a gaseous conductor lamp, means-for initiating discharge of the condenser at selected intervals consisting of a static inverter for producing electrical impulses at a controlled rate, said inverter comprising a gaseous discharge device connected in circuit with a source of potential, a second condenser shunted across the gaseous discharge device, and means for varying the impulse rate comprising a vacuum tube connected in the charging circuit of the second condenser, said vacuum tube having at least three electrodes and being of the type in which current flowing between the anode and the cathode may be controlled by varying the potential between the cathode and the third electrode, together with means for varying the potential betweenthe cathode and said third electrode.

10. Apparatus of the type defined in claim 9 in which said vacuum tube is of the type having at least four electrodes in which current flowing between the anode and the cathode may be controlled by varying the potential between the cathode and the third electrode, together with means for maintaining a substantially constant potential between the cathode and the fourth electrode.

11. Apparatus of the type defined in claim 9 in which said vacuum tube has at least five elec trodes and is of the type in which the current flowing between the anode and the cathode may be controlled by varying the potential between the cathode and a third electrode, together with means for maintaining a substantially constant potential between the cathode and the fourth electrode, and means for maintaining a substantially constant potential between the cathode and the fifth electrode.

12. Apparatus of the type defned in claim 9 in which the gaseous discharge device is a gaseous conductor device having at least three electrodes of the type in which the maximum potential which can be established between two main electrodes without appreciable current flow therebetween may be controlled by controlling the potential between one of said main electrodes and a third electrode; and in which said vacuum tube has at least five electrodes and is of the type in which current flowing between the anode and the cathode may be controlled by varying the potential between the cathode and a third elec trode, together with means for maintaining a substantially constant potential between the cathode and a fourth electrode, means for maintaining a substantially constant potential between the cathode and a fifth electrode, and means for 'controlling the potential between one of the main electrodes and a third electrode of said gaseous discharge device.

13. A static inverter comprising a condenser, a source of potential for charging the condenser, means connected in series circuit with the energy source and with the condenser for charging the condenser to a potential higher than the poten-' tial of the energy source, said means comprising an electric check valve and an inductance forming with the condenser an oscillating circuit, means for discharging the condenser comprising a gaseous discharge device having at least three electrodes and of the type in which the maximum potential which can be established between two main electrodes without appreciable current flow therebetween may be controlled by controlling the potential between one of said main electrodes and a third electrode; said condenser discharge means also including means for impressing between the third electrode and one of the main electrodes of said gaseous conductor device the voltage impulse which occurs at the instant current ceases to. flow through said electric check valve.

14. A static inverter of the type defined in claim 13, embodying as an additional elementa variable impedance for controlling the intervals between condenser discharges.

15. A static inverter of the type defined in claim 13 in which the cathode of the gaseous discharge device is connected to the anode of the electric check valve and to one side of the condenser, in which the anode of the gaseous dis-. charge device is connected to the other side of the condenser, and in which a third electrode of the gaseous discharge device is connected to the cathode of the electric check valve.

16. A static inverter of the type defined in claim 13 embodying as an additional element a variable impedance for controlling the intervals between condenser discharges, and also embodying an ammeter in the condenser charging circuit whereby the number of condenser discharges per unit of time is measured.

17. In combination with apparatus for producing electrical energy flashes of the type in which a condenser is periodically discharged through a gaseous conductor lamp, means for initiating discharge of the condenser at selected intervals consisting of a static inverter for producing electrical impulses at a controlled rate, said inverter-comprising a second condenser, a source of energy for charging the condenser, means connected in series circuit with the energy source and with the second condenser for charging the second condenser to a potential higher than the potential of the energy source, saidmeans comprising an electric check valve and an inductance forming with the second condenser a slightly damped oscillating circuit, means for discharging the second condenser comprising a gaseous of the type in which the maximum potential which can be established between two main electrodes without appreciable current flow therebetween may be controlled by. controlling the potential between one of said main electrodes and a third electrode, means for impressing between said third electrode and one of the main electrodes of said gaseous discharge device the voltage impulse which occurs at the instant current ceases to flow through said electric check valve, together with means for varying the impulse rate comprising a variable impedance included as an element in the inverter circuit, and means for conveying such impulses tothe flash-producing apparatus.

18. Electric flash producing apparatus for measuring rates'comprising a condenser, a source of energy for charging the condenser, means connected in series circuit with the energy source and with the condenser for charging the condenser to a predetermined potential higher than the potential of the energysource, said means comprising an electric check valve and an inductance forming with the condenser a slightly damped oscillating circuit, means for discharging the condenser periodically, the number of discharges per unit of time being proportional to the rate to be measured, and a ratiometer having one coil connected in series circuit with the condenser and having another coil connected in parallel circuit with the condenser.

19. A variable frequency stroboscope comprising a condenser, means for charging said condenser, a gaseous conductor lamp shunted across said condenser, and means for initiating discharge of said condenser through said gaseous conductor lamp at selected intervals comprising a relaxation inverter and a transformer, said relaxation inverter comprising a second condenser, a source of energy for charging said second condenser, a gaseous discharge device adapted to discharge said second condenser when its potential reaches a selected value lower than that of the energy source, and means forcontrolling the time required for said second condenser to be charged to said selected value comprising a five electrode vacuum tube and means for controlling the current flowing through said five electrode vacuum tube.

- BENJAMIN MILLER.

CERTIFICATE OF CORRECTION.

Patent'No. 2,043,484. June 9, 1936.

BENJAMIN MILLER.

It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows: Page 2, second column, line '74, strike out the word "inductance"; page 3, second column, line 9, for "inductance" read'impedance; 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 Patent Office.

Signed and sealed this 8th day of September, A. o. 1956.

Leslie Frazer (Seal) Acting Commissioner of Patents.

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