US2704064A - Neurosurgical stimulator - Google Patents

Description

March l5, 1955 2,704,064

J. A. FIZZELL ET AL NEUROSURGICAL STIMULATOR Filed Sept. l0, 1952 E VI..

United States Patent() NEUROSURGICAL STIMULATOR James A. Fizzell and James G. Golseth, Pasadena, and Robert W. Kay, Altadena, Calif., assignors to The Meditron Company, a corporation of California Application September 10, 1952, Serial No. 308,868

6 Claims. (Cl. 12S-2.1)

Our invention relates to neouromuscular stimulators used by surgeons when operating on areas of the body wherein nervous and muscular tissue may be exposed for direct electrical stimulation.

In many fields of surgery, it has been found advantageous to use electrical Stimulators to apply electrical current to body tissues, to determine which of them are electrically excitable tissues. Illustrations of such elds are traumatic surgery, tumor surgery, plastic surgery, neurosurgery, and orthopedic surgery.

The practical use of direct electrical stimulation to the nerves is well illustrated by certain tumor removal operations. Tumors often form in the parotid gland and the tumorous material surrounds branches of the seventh nerve (cranial). This seventh nerve is the nerve which controls the facial muscles. When a surgeon dissects this tumorous material, it is almost impossible to visually distinguish between the tumorous tissues and the nerve tissues which pass therethrough. It has been found that the only practical way to distinguish these tissues is by usi-ng an electrical stimulator. The surgeon applies an electrical current to an area of the tumor and notes whether there is a reaction of a facial muscle. If such a reaction occurs, the electrode is touching the tumorous tissue in the close vicinity of a nerve, or actually contacting the nerve. If such a facial muscle reaction does not take place, the electrode is then touching tumorous material which may be excised. Thus, by careful probing, the surgeon may excise tumorous material without severing any of the branches of the seventh nerve. The severance of these branches of the seventh nerve would cause a facial paralysis and hence must be avoided.

Another eld of surgery which illustrates the desirability of directly stimulating exposed nerves is the eld wherein damaged nerves are exposed for corrective surgery. Pressure may be exerted on the nerves by the formation of scar tissue thereabout, or from any other source, such as a bone bearing against a nerve. Another common condition is that the nerve may have a neuroma in continuity (certain cells in the nerve multiply rapidly and cause an abnormal growth on the nerve). These conditions greatly impair the function of the nerve.

One of the most common of these conditions today is that wherein the formation of scar tissue about a nerve impairs its function. A great number of men received injuries on the battlefield during World War II for which neither time nor equipment permitted all of the attention that might have been needed. These injuries in many instances were permitted to heal so that scar tissue formed about the nerves and subsequently created pressure thereon with a consequent loss of complete muscular control over muscles innervated by the aifected nerves. Surgery must be used to relieve this condition.

` The surgeon exposes the nerve and scar tissue a-nd can then proceed to check the continuity of the nerve by stimulating it proximally to the scar tissue. The results of these tests enable the surgeon judiciously to remove the damaging scar tissue. Such tests, of course, also would indicate whether the nerves are dead, and also probing of the scar tissue with the electrodes will enable the surgeon to excise the scar tissue without severing a nerve. 1 -The conditions mentioned above which create pressure upon a nerve are quite different from those met in traumatic surgery. In traumatic injuries, areas of the body may be torn open and nerves and tendons severed. The surgeon must know which tissues are nerves and which are tendons in order to properly suture them. Since 2,704,064 Patented Mar. 15, 1955 ICS tendons and nerves look alike, the only sure way to differentiate is to stimulate the tissue and note whether a muscular contraction follows. If a nerve is stimulated, a muscular contraction will follow while the stimulation of a tendon will create no such muscular response.

In a neuromuscular stimulator both alternating and direct currents are useful. The alternating current produces a sustained or tetanic contraction of a muscle when it ows either through that muscle or through its intact nerve supply or through its area of representation in the cerebral cortex. For comparison, the direct current, when used at low values, produces only a slight twitch of the muscle at the time when the circuit is closed. Some surgeons nd that direct current is more efficacious in mapping out epileptogenic scars in the cerebral cortex;

The electrical stimulation of a nerve is due to the current passing therethrough. Hence, a stimulator for use by surgeons must be such that the operator knows whether current is being fed to a nerve and the existence of a circuit fault must be readily apparent in order to prevent misinterpretation.

The body contains various types of electrically conductive uids, and the surgeon must know when these conductive fluids, rather than the nerve, are passing the current. The oridinary procedure is to raise the nerve from the vicinity of these conducting fluids and apply the electrodes, but the possibility of conduction through the iluids may nevertheless exist and thereby cause a short circuit path between the electrodes.

Nerves like all other conductors have a current capacity, and if this current capacity is exceeded, the nerve may be burnt and thereby permanently injured. In testing nerves through the skin, the likelihood of such burning is greatly reduced, but when the nerves are directly stimulated, care must be taken to keep the current value below the capacity of the nerve. The surgeon must, therefore, be fully aware of what current is passing between the electrodes in order to prevent the inadvertent burning of a nerve.

Experimentation in stimulating nerves with alternating current has revealed that a square wave form of current is the most efficient type of alternating current. The reason for this is immaterial to the instant disclosure, but it is significant that our stimulator is adapted to meet this requirement and provides an approximate square wave of current between the probes.

Accordingly, an object of our invention is to provide a new and improved neuromuscular stimulator adapted to enable either direct or percutaneous-stimulation of nervous and muscular tissues.

Another object is to provide a neuromuscular stimu lator wherein both square wave alternating current and direct current are available at the probes.

Another object is to provide a neuromuscular stimulator which is simple to operate, apprises the operator of the exact condition under which the circuit is operating and removes the danger of causing nerve injury.

Other objects are to provide a neuromuscular stimulator which is portable, which is operable from its own power supply, and which meets all of the peculiar requirements created due to the nature of its intended use.

These and other objects of our invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a perspective view of a portable neuromuscular stimulator showing particularly the controls and probes; and c Fig. 2 is a schematic diagram lllustrating the circuit of the neuromuscular stimulator.

A preferred embodiment of our portable neuromuscular stimulator is illustrated in the drawings, and as seen in Fig. 1, the apparatus is housed in an open front casing, generally designated 10, provided with a handle 12. All portions of the neuromuscular stimulator circuit that must be accessible and visible for proper operation are mounted on a front panel 14, which is secured to the casing by screws 16. A chassis (not shown) is secured to the rear of panel 14 and adapted to slide into and out ofthe casing 10. This provides ready accessibility for servicing and replacing the components of the stimulators electrical circuit.

arcanes In the schematic diagram of the stimulator shown in Fig. 2, the source of power consists of two batteries 18 and 20 connected in parallel. These batteries may, f or example, be ll/z-volt dry cells. Connected in series with the power supply is a fuse 22. The fuse 22, which may have a current capacity of 3 amps., is enclosed 1n a casing 24 (Fig. 1) and may be readily replaced by removing casing 24 from front panel 14. Also in series with the power supply is a switch 26 which is operative to turn the electrical stimulator circuit on or off. It will be noted in Fig. l that the control switch 26 is mounted for convenient operation on the front panel nA lead 28 connects control switch 26 with the center tap of primary coil 30 of a step-up transformer, generally designated 32. Lead 34 connects one end tap of the primary coil 30 with a contact 36, while another lead 38 connects the other end tap of the primary coil 30 with contact 40. A wire 42 makes an electrical connection between the right-hand side of fuse 22 and a vibrator leaf 44. An actuating coil 46 is disposed beneath the vibrator leaf 44 and connected between the center tap of primary coil 30 and a contact 48 disposed above vibrator leaf 44.

The vibrator leaf 44 is mechanically xed at its lefthand end as indicated at 50, but is free to move upwardly or downwardly over its right-hand portion. The leaf 44, however, is normally urged in an upward direction either by its own resilience or by a mechanical biasing means such as spring 51.

From the above description it will be seen that at the instant switch 26 is closed current will ow from batteries 18 and 20 through fuse 22, lead 42, leaf 44, contact 48, actuating coil 46, and through switch 26 back to the batteries. At the same time current will flow from vibrator leaf 44 through contact 40, lead 38, the lower half of primary coil 30, and switch 26 back to the batteries. A magnetic field will be created around actuating coil 46 due to current flow therethrough, which magnetic field draws vibrator leaf 44 downwardly to break the electrical connections between leaf 44 and contacts 40 and 48. This movement of leaf 44 causes it to touch contact 36. At this instant current will then be caused to ow from leaf 44 through contact 36, lead 34, the upper half of primary coil 30, lead 28 and switch 26 back to the battery. When the leaf 44 was removed from engagement with contact 40, current ceased to ow through lead 38 and the lower half of primary coil 30. Electrical connection between leaf 44 and contact 48 was also broken when the leaf 44 was pulled downwardly, hence, current no longer flows through actuating coil 46 and the forces holding the leaf in its downward position cease. This causes the leaf to spring back to the position shown in Fig. 2 and into engagement with contacts 40 and 48.

The circuit is then in condition to initiate another cycle such as that described above. It will be noted that current was first caused to flow upwardly through the lower half of primary coil 30 and then current was caused to flow downwardly through the upper half of primary coil 30. This reversal of current flow causes the magnetic field of the transformer to build up first in one direotion, decrease to zero, and then build up in the other direction. The center tapped secondary coil 52 of transformer 32 is linked with this changing magnetic eld and, therefore, alternating voltage is induced in the secondary coil. The frequency of this voltage will be mainly clpendent upon the speed of movement of vibrator leaf A voltage dropping resistor 54 which may, for example, have a resistance of 1/2 megohm, and a neon lamp S6 are connected in series across the outside taps of secondary coil 52. Thus, whenever a voltage is induced in the secondary coil 52, the lamp 56 will glow. In Fig. 1, it will be seen that the neon glow lamp 56 is exposed on the face of panel 14 and, therefore, readily visible -to an operator of the neuromuscular stimulator to indicate when the stimulator is in condition for use.

A capacitor 58 is also connected across the outside taps of secondary coil 52. This capacitor may, for example, be of .0l microfarad with a peak voltage rating of 1600 volts. Capactior 58 is used -to tune the secondary transformer circuit so that the time constant of the circuit agrees with that of vibrator leaf 44. Therefore, it should be appreciated that the capacity of capacitor 58 will depend upon the inductance values of the transformer 32.

The lower tap of secondary coil 52 is connected through a lead 62 to a contact 60, which is disposed above leaf 44, while the upper tap of secondary 52 is connected by a lead 66 to a contact 64, which is disposed below leaf 44. The center tap of secondary coil 52 is connected to one side of a single-pole, single-throw, switch 68 by a lead 70. The upper tap of secondary coil 52 is connected to one terminal of a single-pole, doublethrow switch 72 by leads 66 and 74, while the lower tap of secondary coil 52 is connected to one terminal of a single-pole, double-throw switch 76 through leads 62 and 78.

Single-pole, double- throw switches 72 and 76 and switch 68 are ganged switches. A voltage dropping element 80 is connected between the switch legs of singlepole, double- throw switches 72 and 76. A lead 82 is connected between one side of element 80 and an ammeter 84, which may be a universal meter having a scale range of from 0 to 10 milliamperes either alternating or direct current, and a sliding contact 86 is arranged to bear against element 80. Sliding contact 86 is connected to an electrode terminal 88 by a lead 90, while the ammeter 84 is connected to a second electrode terminal 92 by a lead 94.

In Fig. 1 it will be noted that ammeter 84 and electrode terminals 88 and 92 are readily visible and accessible from the front panel 14. Knob 96, in Fig. 1, controls the position of contact 86 along voltage dropping element 80, while lever 98 is the master control for the ganged switches 68, 72 and 76.

When lever 98 of Fig. l is thrown to its uppermost or A.C. output position, switch-arm 76 contacts the terminal connected to lead 78 and at the same time switch-arm 72 contacts the terminal connected to lead 74, thereby connecting the voltage dropping element 80 across the end taps of secondary coil 52.

Leads 97 and 99 are connected to terminals 88 and 92, respectively, and lead into an insulated probe handle 100. Within the handle 100, leads 97 and 99 are connected to probes 102 and 104. From this construction it will be noted that any current passing between probes 102 and 104 will be indicated by ammeter 84.

When, as previously explained, the magnetic eld surrounding primary coil 30 is changing, voltages will be induced in the secondary coil 52. Since the tuning capacitor 58 is of a value so as to tune the secondary transformer coil circuit, a substantially resistive load will be reflected to the primary. Due to the adjustment of the time constant of the transformer circuit and to the quick action of the making and breaking of contact between the vibrator leaf 44 and contacts 36, 40 and 48, alternating voltage having a substantially square wave form will be produced in the secondary transformer circuit. This illustrates one of the novel features of our invention since it has been found by experiment that a square wave form of current is the most efficient type of an electrical stimulus.

The lamp 56 will be lit when voltage is induced in the secondary coil and thereby indicate that the electrical stimulator is properly functioning. When lever 98 is in the AJ-C. output position, the voltage dropping element 80 is connected to the end taps of secondary coil 52 through leads 78, 62, 74 and 66, and switch- arms 72 and 76 and the square wave current will flow through element 80.

With the circuit in condition for A.C. output, one of the probes 102 is touched to an exposed nerve while the other probe 104 is touched to the body in the vicinity of the exposed nerve. The knob 96 may then be rotated to move the contact 86 along element 80 to control the amount of current flowing between the probes. Hence, it will be seen that a square wave form of current will be applied between the body and a nerve by probes 102 and 104 with the magnitude of this current being indicated by ammeter 84.

The above description has mainly dealt with the portions of the circuit which are utilized when an alternating current is to be used as the stimulating current. When direct current is to be used as the stimulus, the switcharms 68, 72 and 76 are thrown into their uppermost position, by moving lever 98 of Fig. 1 to its lowermost or D.C. position. Switch-arm 68 is connected through a lead 106 to an iron core choke 108 and a lter capacitor 110. A second filter capacitor 112 is connected to the other end of choke 108. The filter capacitors 110 and 112 are connected together by a lead 114. Thus choke 108 and capacitors 110 and 112 are connected into a pi filter circuit. The side of choke 108 connected to capacitor 112 is also connected by a lead 116 to a terminal adjacent switch-arm 76 while the point of common connection between capacitors 110 and 112 is connected by a lead 118 to a terminal adjacent switch-arm 72. As previously mentioned, contact 60 is connected to the lower end of coil 52, contact 64 is connected to the upper end of coil 52, and the center tap of coil 52 is connected to a terminal adjacent switch-arm 68. With the switches in their D.C. output positions, it will be seen that the center tap of coil 52 is connected to one input side of the pi filter circuit through lead 70, switcharm 68, and lead 106, while the other input side of the pi filter circuit, namely, lead 114, is connected to the vibrator leaf 44 through leads 120 and 42.

When the vibrator leaf 44 is in its uppermost position, it will be noted that lead 114, of the pi filter circuit, is connected to the lower end tap of secondary coil 52 through lead 62, contact 60, leaf 44, lead 42 and lead 120, while, when the leaf 44 is in its lower position, the lead 114 will be connected to the upper end tap of coil 52 through lead 120, lead 42, leaf 44, contact 64, and lead 6 The center tap of transformer coil 52 remains connected to the other side of the pi filter circuit at all times that switch-arm 68 is in its upper position, regardless of the position of the vibrator leaf 44. It should be apparent that the leaf 44 and contacts 60 and 64 act as a synchronous rectifier and cause pulsating direct current to be applied to the input of the pi filter circuit. The filtering action of choke 108 and capacitors 110 and 112 smooth out this pulsating direct current and thus non-pulsating direct current will appear between leads 116 and 118. Since switch- arms 72 and 76 are in their uppermost position, this non-pulsating direct current will also appear across voltage dropping element 80. Thus, with one of the probes contacting the skin immediately above a subcutaneous nerve and the other probe contacting the skin at some other portion, the presence or responsiveness of the nerve may be tested by passing direct current between the probes, the value of which may be adjusted by moving contact 86 along element 80 and read by ammeter 84.

As previously mentioned, the surgeon using our stimulator during an operation must know the actual condition of the stimulator circuit in order to prevent a misinterpretation of the results. As a practical matter, the most serious situation is where, upon application of the probes to a nerve, a deliection of ammeter S6 occurs and a muscular response does not follow. The surgeon must be sure that the fault lies in the nerve itself and is not due to a fault of the stimulator circuit, shortcircuiting effects of body fiuids, and the like.

One of the novel features of our neuromuscular stimulator is that the circuit is so arranged that by a minimum of quick tests the operator may assure himself as to the reliability of his findings. A glance at glow lamp 56 will indicate whether the circuit is in condition for operation. By touching together probes 102 and 104 and noting that the meter 84 deilects, the operator is assured that the circuit is in condition to pass stimulating current between probes A102 and 104.

The test to determine whether body fluids or some other conductive substance is shorting the probes 102 and 104 is even simpler. If the milliammeter 84 indicates the flow of current when the probes are not touching anything, then there is some unwanted conductive path between the probes or elsewhere in the electrode circuit. Of course, such a faulty set of electrodes should not be used until the conductive path has been cleared. In most cases in the operating room, this will be done with a piece of gauze or a sponge which will wipe away the blood or saline that is lodged between the probes. If necessary, however, the electrodes can be completely rebuilt in very short time or can be completely replaced with another set in a few seconds.

Having now described our neuromuscular stimulator in full detail, it should be apparent that the novel device meets all of the various requirements necessary in a stimulator for use by surgeons when stimulating nervous and muscular tissue either directly or percutaneously. is to be understood that our invention is not limited to the details shown and described, but may assume various forms, modifications, and equivalents coming within the scope of the appended claims. Y

We claim:

1. In a neuromuscular stimulator having probes arranged for stimulation of nerves, a direct current power supply, a center-tapped primary transformer coil, actuating means including a vibrator leaf connected to alternately pass direct current from said power supply in opposite directions through first one-half of said primary coil and then through the other half of said primary coil, a center-tapped secondary transformer coil, a filter circuit, a voltage dropping element, an ammeter connected in series between said element and probes, a movable contact engaging said element and in series with said probes, rectifying contacts connected to the ends of said secondary coil and arranged to alternately contact said leaf, said leaf being connected to said filter circuit, and switch means arranged in one position to connect said element across the ends of said secondary coil to provide an alternating voltage at said probes and in another position to connect said filter circuit to the center-tap of said secondary coil and to said element to provide unidirectional voltage at said probes.

2. In a neuromuscular stimulator having probes arranged for stimulation of either nervous or muscular tissue, a direct current power supply, a center-tapped primary transformer coil, actuating means including a vibrator leaf connected to alternately pass direct current from said power supply in opposite directions first through one-half of said primary coil and then through the other half of said primary coil, a center-tapped secondary transformer coil, a capacitor connected across the ends of said secondary coil to adjust the time constant of the transformer, a lter circuit, a voltage dropping element, a current measuring device connected in series between said element and probes, a movable contact engaging said element and in series with said probes, rectifying contacts connected to the ends of said secondary coil and arranged to alternately contact said leaf, said leaf being connected to said filter circuit, and switch means arranged in one position to connect said element across the ends of said secondary coil to provide alternating current at said probes and in another position to connect said filter circuit to the center-tap of said secondary coil and to said element to provide direct current at said probes.

3. In a neuromuscular stimulator having probes arranged for the stimulation of nerves, a direct current power supply, a primary transformer coil having primary center and end taps, said primary center tap being connected to one side of said power supply, a vibrator leaf connected to the other side of said power supply, contact means adjacent said vibrator leaf and connected to said primary end taps, a vibrator leaf actuating coil, said actuating coil being operatively associated with said vibrator leaf and intermittently moving said vibrator leaf in a given direction to cause current to flow through said primary coil between said primary center tap and one of said primary end taps, said vibrator leaf being movable in the opposite direction to cause said current to ow intermittently through said primary coil between said primary center tap and the other of said primary end taps, a secondary transformer coil having secondary center and end taps, a filter circuit, synchronous rectifier contacts adjacent said leaf and connected to said secondary end taps, a voltage dropping element, a current measuring device connected to said element in series with said probes, a movable contact connected to said element in series with said probes, and switch means arranged in one position to connect said element across said secondary end taps to provide alternating voltage between said probes and arranged in another position to connect said filter circuit and said synchronous contacts between said secondary coil and said element to provide unidirectional voltage between said probes.

4. In a neuromuscular stimulator having probes adapted to provide electrical stimulus to nerves and muscles, a power supply, a vibrator leaf connected to one side of said power supply, a primary transformer coil having primary center and end taps, said primary center tap being connected to the other side of said power supply, a rst contact mounted on one side of said leaf and connected to one of said primary end taps, a second contact mounted on the other side of said leaf and connected to the other 7 of said primary end taps, a third contact mounted on said one side of said leaf, an actuating coil mounted on' said other side of said leaf and connected between said third contact and said primary center tap, said actuating coil being constructed and arranged to move said leaf out of contact with said first and third contacts and into contact with said second contact in response to a magnetic field created about the actuating coil by current flow therethrough, means biasing said leaf into contact with said third contact when current flow ceases within said actuating coil due to the separation of said third contact and said leaf, a secondary transformer coil having end taps, a condenser connected between said secondary end taps to tune said secondary coil, a voltage dropping element connected across said secondary coil, a current measuring device connected between said probes and said element, and a movable contact adapted to engage said element and connected in series between said probes and said element, whereby, alternating current of a nearly square wave form will ow between the probes when the latter have conducting tissue therebetween, which current ow will be indicated by said current measuring device and the magnitude varied by moving said movable Contact relative to said voltage dropping element.

5. In a neuromuscular stimulator having probes adapted to provide electrical stimulus to nerves, a power supply, a vibrator leaf connected to one side of said power supply, a primary transformer coil having primary center and end taps, said primary center tap being connected to the other side of said power supply, a first contact mounted on one side of said leaf and connected to one of said primary end taps, a second contact mounted on the other side of said leaf and connected to the other of said primary end taps, a third contact mounted on said one side of said leaf, an actuating coil mounted on said other side of said leaf and connected between said third contact and said primary center tap, said actuating coil being constructed and arranged to move said leaf out of contact with said first and third contacts and into contact with said first and third contacts and into contact with said second contact in response to a magnetic field created about the actuating coil by current flow therethrough, means biasing said leaf into contact with said third contact when current ow ceases within said actuating coil due to the separation of said third contact and said leaf, a secondary transformer coil having secondary center and end taps, a filter circuit having input and output sides. said input side being connected to said secondary center tap, a first rectifying contact connected to one of said secondary end taps and disposed on said one side of said vibrator leaf, a second rectifying contact connected to the other of said secondary end taps and disposed on said other side of said vibrator leaf, a lead connecting said vibrator leaf to said input side of said filter circuit, a voltage dropping element connected across said output side of said filter circuit, an ammeter connected in series between said probes and said element, and a movable contact engaging said element and connected in series between said probes and element, whereby said rectifying contacts will alternately engage said vibrator leaf to apply pulsating direct current to said input side of said filter circuit, which pulsating direct current will be smoothed out and applied across said element, and between said probes said ammeter reading the direct current ow between said probes, which direct current ow may be varied in amplitude by moving said moving contact.

6. In a neuromuscular stimulator having probes adapted to provide electrical stimulus to nerves and muscles, a power supply, a vibrator leaf connected to one side of said power supply, a primary transformer coil having primary center and end taps, said primary center tap being connected to the other side of said power supply, a first contact mounted on one side of said leaf and connected to one of said primary end taps, a second contact mounted on the other side of said leaf and connected to the other of said primary end taps, a third contact mounted on said one side of said leaf, an actuating coil mounted on said other side of said leaf and connected between said third contact and said primary center tap, said actuating coil being constructed and arranged to move said leaf out of contact with said first and third contacts and into contact with said second contact in response to a magnetic field created about the actuating coil by current flow therethrough, means biasing said leaf into contact with said third contact when current ow ceases within said actuating coil due to the separation of said third contact and said leaf, a secondary transformer coil having secondary center and end taps, a tuning condenser connected between said secondary end taps to tune said secondary transformer coil and thereby adjust the time constant of the transformer to agree with that of the vibrator, a filter circuit having input and output leads, a first rectifying contact connected to one of said secondary end taps and disposed on said one side of said vibrator leaf, a second rectifying contact connected to the other of said secondary end taps and disposed on said other side of said vibrator leaf, a lead connecting said leaf to one of said input leads of said filter circuit, a first switch having a first terminal connected to said secondary center-tap and a switch leg attached to the other input lead of said filter circuit, a voltage dropping element, a current measuring device connected to said element in series with said probes, a movable contact adapted to engage said element and arranged in series with said probes, a second switch having a first terminal connected to one of said output leads of'said lter circuit and a second terminal connected to said one secondary end tap and a switch leg connected to said element, a third switch having a first terminal connected to the other of said output leads of said filter circuit and a second terminal connected to said other secondary end tap and a switch leg connected to said element, and means for simultaneously moving said switch legs to contact said first terminals associated therewith to cause direct current to flow between said probes and for simultaneously moving said switch legs to contact said second terminals associated therewith to cause alternating current to ow between said probes, said current measuring device indicating the current flow between said probes and the amplitude of the current flow between the probes being controlled by moving said movable contact relative to said element.

References Cited in the file of this patent UNITED STATES PATENTS 2,564,279 Reynolds Aug. 14, 1951

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