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 ahandle 12. All portions of the neuromuscular stimulator circuit that must be accessible and visible for proper operation are mounted on afront panel 14, which is secured to the casing byscrews 16. A chassis (not shown) is secured to the rear ofpanel 14 and adapted to slide into and out ofthecasing 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 twobatteries 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 afuse 22. Thefuse 22, which may have a current capacity of 3 amps., is enclosed 1n a casing 24 (Fig. 1) and may be readily replaced by removingcasing 24 fromfront panel 14. Also in series with the power supply is aswitch 26 which is operative to turn the electrical stimulator circuit on or off. It will be noted in Fig. l that thecontrol switch 26 is mounted for convenient operation on the front panel nA lead 28 connectscontrol switch 26 with the center tap ofprimary coil 30 of a step-up transformer, generally designated 32.Lead 34 connects one end tap of theprimary coil 30 with acontact 36, while anotherlead 38 connects the other end tap of theprimary coil 30 withcontact 40. Awire 42 makes an electrical connection between the right-hand side offuse 22 and a vibrator leaf 44. Anactuating coil 46 is disposed beneath the vibrator leaf 44 and connected between the center tap ofprimary 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 theinstant switch 26 is closed current will ow frombatteries 18 and 20 throughfuse 22, lead 42, leaf 44, contact 48, actuatingcoil 46, and throughswitch 26 back to the batteries. At the same time current will flow from vibrator leaf 44 throughcontact 40, lead 38, the lower half ofprimary coil 30, and switch 26 back to the batteries. A magnetic field will be created around actuatingcoil 46 due to current flow therethrough, which magnetic field draws vibrator leaf 44 downwardly to break the electrical connections between leaf 44 andcontacts 40 and 48. This movement of leaf 44 causes it to touchcontact 36. At this instant current will then be caused to ow from leaf 44 throughcontact 36, lead 34, the upper half ofprimary coil 30, lead 28 and switch 26 back to the battery. When the leaf 44 was removed from engagement withcontact 40, current ceased to ow throughlead 38 and the lower half ofprimary 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 actuatingcoil 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 withcontacts 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 ofprimary coil 30 and then current was caused to flow downwardly through the upper half ofprimary 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 oftransformer 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, thelamp 56 will glow. In Fig. 1, it will be seen that theneon glow lamp 56 is exposed on the face ofpanel 14 and, therefore, readily visible -to an operator of the neuromuscular stimulator to indicate when the stimulator is in condition for use.

Acapacitor 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 ofcapacitor 58 will depend upon the inductance values of thetransformer 32.

The lower tap of secondary coil 52 is connected through a lead 62 to acontact 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 alead 70. The upper tap of secondary coil 52 is connected to one terminal of a single-pole,doublethrow switch 72 byleads 66 and 74, while the lower tap of secondary coil 52 is connected to one terminal of a single-pole, double-throw switch 76 throughleads 62 and 78.

Single-pole, double-throw switches 72 and 76 and switch 68 are ganged switches. Avoltage 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 ofelement 80 and anammeter 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 againstelement 80. Sliding contact 86 is connected to anelectrode terminal 88 by alead 90, while theammeter 84 is connected to asecond electrode terminal 92 by a lead 94.

In Fig. 1 it will be noted thatammeter 84 andelectrode terminals 88 and 92 are readily visible and accessible from thefront panel 14.Knob 96, in Fig. 1, controls the position of contact 86 alongvoltage dropping element 80, whilelever 98 is the master control for the ganged switches 68, 72 and 76.

Whenlever 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 thevoltage dropping element 80 across the end taps of secondary coil 52.

Leads 97 and 99 are connected toterminals 88 and 92, respectively, and lead into an insulatedprobe handle 100. Within thehandle 100, leads 97 and 99 are connected toprobes 102 and 104. From this construction it will be noted that any current passing betweenprobes 102 and 104 will be indicated byammeter 84.

When, as previously explained, the magnetic eld surroundingprimary coil 30 is changing, voltages will be induced in the secondary coil 52. Since the tuningcapacitor 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 andcontacts 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.

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

With the circuit in condition for A.C. output, one of theprobes 102 is touched to an exposed nerve while theother probe 104 is touched to the body in the vicinity of the exposed nerve. Theknob 96 may then be rotated to move the contact 86 alongelement 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 byprobes 102 and 104 with the magnitude of this current being indicated byammeter 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, theswitcharms 68, 72 and 76 are thrown into their uppermost position, by movinglever 98 of Fig. 1 to its lowermost or D.C. position. Switch-arm 68 is connected through a lead 106 to aniron core choke 108 and alter capacitor 110. Asecond filter capacitor 112 is connected to the other end ofchoke 108. Thefilter capacitors 110 and 112 are connected together by a lead 114. Thus choke 108 andcapacitors 110 and 112 are connected into a pi filter circuit. The side ofchoke 108 connected tocapacitor 112 is also connected by a lead 116 to a terminal adjacent switch-arm 76 while the point of common connection betweencapacitors 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 throughlead 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 throughleads 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 throughlead 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 throughlead 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 andcontacts 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 ofchoke 108 andcapacitors 110 and 112 smooth out this pulsating direct current and thus non-pulsating direct current will appear betweenleads 116 and 118. Since switch-arms 72 and 76 are in their uppermost position, this non-pulsating direct current will also appear acrossvoltage 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 alongelement 80 and read byammeter 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 atglow lamp 56 will indicate whether the circuit is in condition for operation. By touching together probes 102 and 104 and noting that themeter 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 theprobes 102 and 104 is even simpler. If themilliammeter 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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