US 2213820 A
Description (OCR text may contain errors)
Sept. 3, 1940. R MAXSON 2,213,820
HIGH FREQUENCY APPARATUS FOR THERAPEUTIC AND SURGICAL USES Filed July 16, 1937 l v 1Z9 73 .57 58 #5 t 1 7 72 6 1Z2 @0 56 7 3 Y 35 I 36! 71 a? 357 l i l I l I 125i 133 il z 151 v 7 fla 220221:
j2j zozzadww z Ci l Patented Sept. 3', 1940 PATENT OFFICE HIGH FREQUENCY APPARATUS FOR- THERAPEUTIC AND SURGICAL USES Rolland H. Maxson, Milton, Wis., assignor to The Burdick Corporation, Milton, Wis., a corporation of Delaware Application July 16, 1937, Serial No. 153,929
My invention relates to high frequency apparatus for-therapeutic and surgical uses.
It has been generally recognized for some time that high frequency electrical currents in various wave length' ranges seem to be especially adaptable for certain particular uses by the doctor and the surgeon. Thus, for example, currents of short wave lengths, of which about fifteen meters may be taken as typical, can be effectively applied by spaced electrodes which are insulated from the patient to provide, with the tissue, the effect of an electrical condenser. "The current so applied, usually by means of a pair of spaced insulated electrodes in the form of flat pads or encircling cuifs, produces heat which is more or less concentrated in the skin and subcutaneous tissue between-the two electrodes and is relatively shallow of penetration. Currents of somewhat longer wave 7 lengths, of which about twenty-five meters may be taken as typical, can
be effectively applied (preferably by means of a flexible conducting insulated cable capable of being formed into fiat (pancake) or spiral coils of one or more turns) so as to create a magnetic field and thereby induce secondary (eddy) currents which tend to concentrate in the conductive tissue,such as muscle, blood vessels, nerves, and visceral organs. A suitable flexible cable may be provided by applying a tubular copper braid about a core of rope and then covering the braid with flexible rubber. The current so applied.by' encircling apart of the patients anatomy with one or more turns of the cable or by placing the pancake coils thereof fiatwise against the patientproduces heat in the deeper tissues and is effective in creating so-called-artificial fever. Again, a current of still longer wave lengthsay, about seventy metersis admirable for electrosurgical technique, such as cutting and coagulation, and for the production of heat caused by current flowing between electrodes-especially when applied -intern'ally in direct conducting contact with the skin or mucous membrane.
For the production of currents of such wide range of wave lengths (frequency) as, for example, between fifteen and seventy meters, and
to operateto best advantage in'sucha variety of applications as by electrostatic field, by magnetic field, and by direct contact conduction, a number of factors which are inconsistent with or opposed to each other must be taken into account.- Thus, the electrostatic field (condenser) application is best served by a current of high voltage and relatively low amperage. And since the heat depends upon the strength of the elec- (cl. 12s 422) trostatic field, it follows that the circuit must have a high ratio of inductance to capacity in order that a high voltage may be delivered at the patients electrodes. On the other hand, just the opposite characteristics are best for the magnetic field application. Because the magnetic field strength is a function of the cable ampere turns, it follows that for this application the amperage should be high and the voltage relatively .low.
It will be obvious, therefore, that'for these two applications (which, for convenience may be termed short wave diathermy) entirely different inductance-capacity ratios are necessary in a circuit effectively to provide the requisite high frequency oscillating current. In other words, a circuit for delivering current at high voltage and low amperage must have a small capacity and large inductance, whereas a circuit for delivering a current of low voltage and high amperage must be characterized by a large capacity and a low inductance.
Furthermore, the longer wave length (lower frequency) current (say, seventy meters) which is best adapted for surgical, coagulatory and direct-contact-conduction (long wave diathermy) applications cannot be effectively produced from circuits, which are effective for pro: ducing either of the shorter Wave (higher frequency) diathermy currents. The cutting current is very hard to control. When this application is attempted by current produced from short wave diathermy current circuits severe flashing and arcing between the active electrode and the tissue takes place, and the power ob,- tained is too limited for many operations where it is desirable to employ electro-surgical technique.
The problem which this inventionsolves is selectively to provide any one of three such widely different high frequency currents by means of a single unit (as contrasted to three separate units, each producing its own variety) ,the current in each of the several ranges being as effective as when produced individually, and wherein none of the circuits is a compromise-witha circuit for producing current in any other frequency range.
Another object is to provide a three-purpose apparatus which is readily effective for any selected one of its various applications.
Another object is to provide a three-purpose apparatus which is readilyefiective for any se- .lected one of its various applications.
Another object is to provide an apparatus which will'selectively produce the current desired for any one of its three-purpose applications from a single source of alternating current of the adapted to provide, from a single alternating curcable current of 425 watts at the terminals for;
usual frequency and voltage and will require but two thermionic tubes.
Another object is to provide an apparatus affording adequate protection for the patient against the inadvertent application of current of. line (commercial source) frequency.
Another "object is to provide an apparatus which is'simple, reliable and effective for the intended purposes.
Other objects and advantages will hereinafter appear.
The accompanying drawing is a schematic diagram of my preferred circuit. 1
More specifically, the circuit arrangement illustrated and now to be described is particularly rent source of the customary frequencyv and voltage and with one pair of thermionic oscillator tubes, the following: (1) a fifteeen-meter condenser or pad current of 425 watts at the terminals of a patients circuit, (2). a twenty-five meter the cable, and (3) a seventy-meter current for electro-surgery coagulation and long wave diathermy.
Referring to the drawing, the power line or source is connected through a plug receptacle I0.
One terminal of receptacle i is connected by a conductor II to one end of a primary I2 ofa filament transformer 13, used to heat the filaments of the thermionic tubes to be later described and to which it is connected by conductors I3, and by a conductor M to one end of the primary l5 of -a tube-plate' transformer IS. The other terminal of the receptacle is connected through a main-switch l1 and a rheostat I8 to the other side of the primary l2 of the filament transformer. This terminal of the line receptacle is also connected through a'hand-actuated oscillator input switch I9, a pair of snap switches 20 and 2|, an adjustable transformer contact 22 and any selected one of a plurality of taps 23 to 21 to the opposite side of the primary 15 of the plate transformer I6. If desired, a foot-operated switch, serving the same purpose as hand switch [9, may be connected in parallel with the hand switch through a suitable plug receptacle 19. The taps 23 to 21\are to permit adjustment of the primary winding of the plate transformer for variations of line voltage of 1.05, 110, 115, 120 and 125 volts, respectively (which will afford proper accommodation for such voltage variations as are found in ordinary commercial power lines), and
, the ratio of transformation is such'that the secondary delivers 5,000 volts-2,500 volts for each half section 28 and 29 of the secondary. The
ratio of the primary and secondary windings of filament transformer 13 is such that the secondary will deliver current at a voltage ratio of ten-to-oneto that-impressed on the primary.
, Rheostat I8 is variable (say; from 0 to 35 ohms), and should be adjusted to keep the voltage impressed upon the tube filaments between and 10.5 volts,the voltage range within which it has been found that so-called thoriated tubes (the kind I- prefer to use) operate to best advantage.
The two halves 28 and 29 of plate transformer "5 are in two circuits (one for each half wave of the source current) which supply low frequency power current and include the electrodes of two thermionic tubes 35 and 36, respectively. The
low frequency power current circuit for tube 35 includes a radio frequency choke coil 31 Whichis connected between the outside terminal of secondary 28 and the plate 35p of tube 35 by conductors 39 and 40 and which serves to exclude radio frequency current from the plate transformer winding 28. This circuit also includes the filament 35f of tube 35,-which filament is connected across the secondary winding of filament transformer l3 through conductors 13', as indicated. It also contains two cathode bias resistances 41' and 42 and preferably an indicating current-meter 43. Connections are made through the conductors 44, 45; 46, 41, 48, 49 and 50, back to the mid point between secondary sections 28 and 29. Resistance 4| may be of 250 ohm, 100 watt capacity while variable resistance 42 is preferably in the form of a 0-4000 ohm rheostat. Meter 43 may be a 0-400 D. C. milliammeter.
The grid 35g of tube 35 is connected to conductor 43 through a choke coil 56 and a grid leak resistance 51. From conductor 49 the grid leak to ground includes the condenser 58, conductors 59 and 45 and condenser 60. Good results can be obtained by making resistance '51 of 10,000 ohm,
100 watt capacity. Choke coil 56 serves to keep the radio frequency grid voltage in proper phase relation to the plate voltage and to balance the grid to ground. Resistance 51 is a grid leak to furnish grid bias.
The low frequency power circuit for the other half wave includes the choke coil 61, the plate 36p of tube 36, and the conductors 69 and 10by which nal of'transformer secondary 29 and to plate 36p,
respectively. This circuit also includes the filament 36f of tube 36, (which likewise is connected across the secondary winding of filament transformer 13 through conductors l3, as indicated), the two cathode biasing resistances 4i and 42,
meter 43 and the intermediate conductors 1|, 45,
46,41, 48, 49 and 50. The grid 36g of tube 36.is connected to ground through a choke coil 12, a grid leak resistance 13, and through condensers 58 and 60, asinthe case of the grid of tube 35.
The high frequency (generally termed radio coils 31 and 61 cause the tubes to be self-rectifying and to operate inwhat is generally termed full wave so that both alternating current half cycles or complete waves are taken from the plate transformer. This results in a high power factor on the source line. The tubes operate in socalled push-pull into the radio frequency oscillator.
Selection of tank circuits is effected by a doublebladed radio frequency switch, the movable contacts 80 and 8| of which are simultaneously operated. Blade 80, which is permanently connected to conductor 16, may be moved to engage either of two fixed contacts 82 and 83, while blade 8|, which is permanently connected to conductor 19, is moved at the same time to engage either of two I fixed contacts 84 and 85, respectively. When this Condensers and 18 the choke coil is connected to the outside termirent to flow. These condensers and the choke radio frequency switch is in the position shown with blades 80"and 8| engaging fixed contacts 83 and 84, respectively, the oscillator tubes are, by
conductors 86, 81 and 88, connected to a tank circuit which includes the fifteen-meter tank inductance 90 and the fifteen-meter tank capacitance condenser 9 I. When the blades of the radio frequency switch are transferred so that they engage the contacts 82 and .85, respectively, the oscillator tubes are associated, by conductors 92, 93, 94 and 88, with a tank circuit which includes a seventy-meter tank inductance 96 and a seventy-meter tank capacitance condenser 91.
It is desirable to insure that the flow of plate current to tubes and 36 is interrupted before theblades 80 and BI break circuit with their fixed contacts and that the plate current does not again begin to flow until the blades have engaged one or the other of their two fixed contacts, in order-that there may be no radio frequency current upon the switches when they are opened and closed and while they are open. This result may be accomplished by mounting the snap switches 20 and 2| upon a shaft common to blades 80 and 8| and making the blades 80 and 8| fan or segment-shaped so that they remain in engagement and out of engagement with their fixed contacts for intervals long enough to enable the snap switches to operate to closed position or to open position to complete or break the plate primary winding circuit) before the blades 80 and 8| engage or leave their fixed contacts. The dottedline in the circuit diagram indicates that blades 80 and 8| and snap switches 20 and 2| are operated together by a single handle.
In order that the thermionictubes may function to cause the productionvof the necessary high frequency oscillations from the 60-cycle supply, a feed back radio frequency excitation for the grids is provided from the one or the other previously described 1 tank circuits, depending upon which one of the tank circuits is connected to the output side of ,the oscillator .tacts 8285.
tubes by switches 80-8283 and 8I84 85. To provide this feed back excitation, the grid 35g of tube 35 is connected to theblade 8| of the .radio frequency switch through conductor I40 and a variable condenser MI; and the grid 36g of tube 36 is connected to the blade 80 of. the radio frequency switch through a conductor I42 and a variable condenser I43. As a result of these connections a feed back to the" been made, however, no further adjustment will ordinarily be necessary. e
The low voltage condenser I45 is bridged across the filaments 35f.and 361. This condenser, together with condensers 58 and 60, should-be of about .002 microfarad capacity each. They serve to maintain the radio frequency balanced to ground at the electrical center of .the network.
The fifteen-meter tank inductance 90 acts upon a pair of secondary windings I00 and'IOI which have their outer terminals connected by conductors I02 and I03, respectively, to the opposite sections of a split-stator type of condenser I04, each section of condenser I04 having a capacity of about I00 micro-microfarads land with.the rotor thereof grounded. The. purpose of this adjustable condenser is to tune the fifteen-meter output circuit to resonance with the corresponding tank circuit.
The other terminals of these secondaries 0| and I00 are connected through conductors I05 and I00 and impedance matching inductances I01 and I08, respectively, to a pair-of fifteenmeter paticnts terminals H0 and III. If desired, two additional fifteen-meter or pad or cufi patients terminals H2 and H3 may be connected to the output circuit through inductances H4 and H5, respectively. These induct-- an'ceg I01, I98, 4 and 5 are for the purpose of adding inductance to the fifteen-meter output circuit to obtain the proper relation between in-v ductance-and capacity in that circuit and thereby obtain the proper resonance and impedance match conditions. All of them should be outside of the magnetic fields of inductance windings 90, I00 and IOI so as not to pick up energy therefrom. The reason for the extra set of pa- .tients terminals H2 and H3 and inductances H4 and H5 is to afford a greater voltage range for the fifteen-meter output circuit. In other Words, they permit the patients circuit, with smaller pads and cuffs or a greater spacing of condenser electrodes, to be tuned to resonance.
The ends of inductance 96 of the seventymeter tank circuit are connected by conductors I20 and I2| and inductances I22 and I23, re spectively, to the patients cable terminals I24 and I25. When an inductance cable of about twelve feet (which length I have found is practical for general purposes) is connected to terminals I24 and I25, the cable is shunted around inductance 96 and, inasmuch as the inductance of the cable is low compared to that of inductance 96, practically all of the tank current flows through the cable. Thus, when the radio frequency switches are moved to the position where movable contacts 80 and 8| engage fixed contacts 83 and 85; respectively, and the treatment cable terminals are connected to the patients terminals I24 and I25, ='the cable virtually becomes the tank inductance under such conditions and the resonant wavelength becomes meters, depending upon variations in cable ind ductance .dueto differences in the number of cable turns deemed most desirable for particular treatment conditions, but I have found that normally such variations do not exceed the range of 22 to 28 meters. The inductances I22 and I23 tend to. minimize over-coupling of .the cable resistance to the oscillator when the cable is uncoiled or straightened out and, consequently, its inductance is relatively low. v 0 From the inductance 96 taps may be taken to provideseVenty-meter current to patients terminals for coagulation, for cutting (eleotro gery) and for long fwave diathermy. Thus, from a central tap the conductor I30 may lead through a condenser I3I to an inactiveor return terminal I32 to which may be connected the inactive elcctrodeof the patients circuits for the several seventy-meter modalities. From another tap a conductor I33 may lead through a condenser I34 to a coagulation electrode I35,
to which may be connected the active electrode for coagulationtechnique. Condensers I3-I and I34 maybe of low voltage rating of about .002 microfarad capacity. They provide blocking capacity which allows radio frequency current to pass, but further insulate the patient against the possibility of receiving current of the low' frequency (60 cycle) of the source. From another vtap a conductor I35 may lead through a'variable condenser I31 to an electro-surgery or cutting circuit patients terminal I38. This condenser,
which may be of 150 micromicrofarads caacross the filament circuits. A 0-to-13 volt A. C. instrument may be used. It may have its indi-' cating dial calibrated to show to which tap of transformer primary I5 the adjustable contact 22 should be connected. Thus, when the resistance I8 is adjusted to its all out or zero position,
the meter I46 indicates a voltage directly proportional to the source or line-'voltage. The meter 43 serves as a resonance indicator for the fifteen-meter (pad) circuit and since the platecurrent passing through it is proportional to the power output, it also serves as a meter or indicator of the power output. I
The total plate current which ;fiows through tubes 35 and 36, through resistance 4I and rheostat 42 causes a voltage drop in proportion to the plate current. When either tank circuit is loaded, the grid bias offered by resistances 5! and 13 falls off; but the plate current increases, causing 'a greater voltage drop over resistances 4| and 42, which adds more grid bias, tending to increase plate efliciency and minimize danger of overload. In addition, rheostat 42 may function to control the oscillator output by controlling the input.
That is, 'as the effective (adjusted) resistance of rheostat 42 is increased, the radio frequency plate current from-the tubes fiows into the connected tank circuit over a shorter period of the radio frequency cycle, causing less power to be delivered to the tankcircuit.
The electronic tubes, transformers,inductances, condensers, resistances and switch contacts may be mounted in a suitable cabinet which will also mount the meters. The operating handles for the switches" and for the variable resistances and condensers should preferably project through the walls of the cabinet so as to be operable from the outside, and the various patient terminals, preferably of the plug receptacle type, should be readily accessible from the outside so that the conductorsfor the various patient circuits may" be quickly and easily connectedand disconnected.
In operation the apparatus is connected to the power, supply line through .plug receptacle I0. With rheostat I8 adjusted to its all out orzero' position and. with main switch I! closed, and oscillator switch I9 open or in 01f position, the
line voltage can be determined (or if so calibrated,
I the proper tap of transformer I6 can be directly determined) by meter I46. The movable contact 22 of transformer I6 is moved the proper tap 23 to 21 to give the proper secon ary voltage for i the particular line voltage available. Ordinarily this adjustment needs to be madeonly once for a given power circuit. I
Bheostat I8 is now adjusted so that the filaments of ,-'tubes 35 and 36 have from 10 to 10 volts impressed upon them, as indicated by meter I46.
If electro-static (or so called pad or cuif) treatment is to be given, the radio frequency switch is moved so that its movable contacts 80 andSI engage fixed contacts 83 and 84 respectively. With the radio frequency switch in' this position and with switches I1 and I9 closed, the oscillator is actively associated with the terminals or plug receptacles III), III, H2 and H3, and the previously described feed-back circuits are connected to the 15 meter tank circuit. Whether the patient electrode (pad 'or cufi) circuits should be connected to terminals III] and III or to terminals H2 and H3 depends, as previously explained, upon the desired size and spacing of the electrodes used in the treatment. The power supplied can be controlled by rheostat 42 and a condition of resonance can be secured by adjusting condenser I04 until ammeter 43 shows maximum reading for any given position of rheostat any of the '70 meter modalities are to be employed, the radio frequency switch is moved to its other position with contacts 80 and 8| engaging contacts 82 and 85 respectively.
In this condition the oscillator is connected to the meter tank circuit (inductance 96 and capacitance 91) or to a 25 meter tank circuit (the cable and capacitance 91) depending upon whether the cable is not or is plugged into terminals I24 and I25. In either event, the grid exciting feed-back circuits are associated with the corresponding tank circuit.
The current and consequently'the power delivered to the cable or to the patients terminals tapped into inductance 96 (if the cable is not connected to its patients terminals) may be varied by adjusting rheostat 42. If' the surgery or dissection'-electrode I3! is to be employed, the character and amount of current delivered thereto can be controlled by rheostat 42 and variable condenser I31.
Having thus illustrated and explained the nature and an embodiment of my invention, what I claim and desire to secure by United States Letters Patent is as follows:
1. An electro-therapeutic and surgical apparatus comprising a pair of oscillator tube circuits for push-pull association with and operation from a source of alternatingcurrent, each oscillator tube circuit including a thermionic tube having filament, plate and grid; a plurality of tank circuits of different electrical characteristics; a pair of switches for selectively connecting both oscil-- lator tube circuits to either tank circuit; a pair .of feed-back circuits, one for each oscillator tube minals associated with each tank circuit.
2. An electro-surgical and therapeutic apparatus comprising a pair of oscillator-tube circuits for connection to and operation from a source of alternating current, each oscillator tube. circuit containing a thermionic tube having a filament, plate and grid; a plurality of tank circuits having different electrical characteristics so as to be resonant at different frequencies; a radio frequency switch for selectively associating either of the two tank circuits with the oscillator tube circuits; a feed-back circuit for the grid of each If either the electromagnetic field (cable) or tube, said radio frequency switch also serving to associate the feed-back circuits with whichever tank circuit is associated with the oscillator; a
switch which is operated with the radio frequency switch to open the plate input circuits of the tubes previous to the opening of the connections between a tank circuit and the plate output circuits; and terminals for patients circuits associated with each tank circuit.
3. An electro-therapeutic and surgical apparatus comprising a pair of thermionic tubes, each having a filament, plate and grid; a low frequency power current circuit for each tube, each power circuit delivering to one of the tubes alternate half cycles of current induced from an alternating current source; a plurality of tank circuits having different electrical characteristics "so as to be resonant to high frequency currents of different wave lengths; a pair of switches, each having a movable contact permanently connected to the plate of one of the tubes and fixed contacts connected to the tank circuits so that transferring the movable contacts of the switches from one set to the other of their fixed contacts changes the tank circuit connected to the tubes; a feed-back circuit for the grid of each tube connected to the movable contact of the associated switch so that the tube grids are under the infiuence of whichever tank circuit is connected to the tubes; and patients circuit terminals associated with each tank circuit.'
4. An electro-therapeutic apparatus comprising a plurality of tank circuits which are resonant to high frequency currents of different frequencies and with the aid of which currents of diiferent frequencies for various therapeutic uses may be obtained; a pair of thermionic tubes, each having a filament, plate and grid; means for simultaneously and selectively associating the high frequency side of both tubes with any one of the tank circuits, a pair of low frequency power circuits for associating the low frequency side of both tubes with a source of alternating current; and a pair of feed-back circuits, one for the grid of each tube, said circuits being connected by the aforesaid means to whichever tank circuit is associated with the tubes.
5. An electro-therapeutie apparatus comprising a pair of thermionic tubes, each having -a filament, plate and grid; 2, pair of low frequency power input circuits for inductively associating the low frequency side of the tubes with a low frequency source of alternating current; a plurality of tank circuits resonant to high frequency currents of different frequencies and with the aid of which currents of various frequencies for various therapeutic uses may be obtained, one of said tank circuits having a high ratio of inductance to capacity and the other a low ratio of inductance to capacity; a pair of feedback circuits, 5 one for the grid of each tube; means for simultaneously and selectively associating the high frequency sides of the. tubes with any one of the tank circuits, said means at the same time connecting the feedback circuits to the selected tank circuit; and patients circuit terminals associated with each of the tank circuits.
6. An electro-therapeutic apparatus comprising a pair of electronic tubes, each having a filament, plate and grid; a low frequency power input circuit for each tube to associate the same with a low frequency power supply line; -a high frequency circuit including the filaments and plates of the tubes and any one of a plurality of tank circuits, one of the tank circuits including an inductive patients cable, a feed-back circuit for providing high frequency excitation for the grid of each tube; and switches for selectively including any one of the tank circuits in' the high frequency-circuit and at the same time connecting the grid feed-back circuits to the selected tank circuit.
'7. An electro-therapeutic apparatus comprising a pair of electronic tubes, each having a filament, plate and grid; a low frequency power input circuit connected to thefilament and plate of each tube for each tube to supply the tubes with alternate half waves induced from a low frequency power supply line; a plurality of tank circuits resonant to high frequency currents of different frequencies and with the aid of which currents of various frequencies for various therapeutic us'es may be obtained, one of said tank cir-. cuits having a high ratio of inductance to capacity and the other a low ratio of inductance to capacity; a pair of simultaneously actuated radio frequency switches for selectively completing any one of several high frequency output circuits by associating the filaments and plates of the tubes with the different tank circuits; a feed-back circuit for the grid of each tube connecting the grid to a switch, whereby said switches associate the grids with whichever tank circuit is associated with the filaments and plates thereof; and patients circuit terminals associated with each tank circuit.
8. An electro-therapeutic and surgical apparatus comprising an oscillator tube circuit for association with and to be operated from/an alternating current source, said circuit including a thermionic tube including a control electrode, a plurality of tank circuits having different electrical characteristics, a switch for selectively con necting said tank circuits to said oscillator tube circuit, and a feed back circuit adapted to be connected between the switch and the-control electrode regardless of which'tank circuit is connected to the oscillator circuit, one of said tank circuits including a condenser and a relatively high inductance and another of said tank circuits including a set of patients cables having a relatively low inductance adapted to be connected across said condenser and said relatively high inductance.
9. An electro-therapeutic and surgical apparatus comprising a pair of thermionic tubes, each having a filament, plate and grid, a low frequency power input circuit for each tube including a connection from the power source to the plate and a connection from the power source to the filament, radio frequency chokes inserted in each of the plate connections, a plurality of tank circuits resonant to high frequency currents of different frequencies and with the aid of which currents of various frequencies for various therapeutic uses may be obtained, one of said tank circuits densersfrom each switch blade to the grid of the tube to which the otherblade is connected, fixed contacts adapted to be engaged by said blades and connected to said tank circuits, and patients circuit terminals associated with each tankcircuit.
, 10,-An electro-therapeutic and surgical appa ratus comprising a pair of thermionic tubes, each having a filament, plate and grid, a low frequency power input circuit for each tube including a connection from the power source to the plate, a
pair of switches, each having a movable blade and simultaneously operable between two positions, each of said blades being connected to the plate of one of said tubes through a condenserblocking the flow of low frequency current to the blade, a feed back circuit from each switch blade to the grid of the tube to which the other blade is connected, a first tank circuit resonant at a wave length of about 15' meters adapted to be connected to said switches when the latter are in one position, a second tank circuit including a condenser and a relatively high inductance resonant at a wave length of about 70 meters adapted to be connected to said switches when the latter are in their other position, and a pair of terminals connected to said last mentioned inductance adapted to receive a low inductance patients cable thereby to transform said second tank circuit to one resonant at a wave length of I about 25 meters.
ROLLAND H. MAXSON.