US3376528A - Electromagnetic actuating device - Google Patents

Description

April 2, 1968 J. c. MACY 3,376,528

LECTROMAGNETI C ACTUATING DEVI CE Filed Sept. 10, 1965 '7 Sheets-Sheet 1 FlG.l

INVENTOR. JAMES C. MAC) A. TTORNEYS.

April 1968 J. Q. Ma 3,376,528

ELECTROMAGNETIC ACTUATING DEVICE Filed Sept. 10, 1965 '7 Sheets-Sheet 2 INVENTOR. JAMES C. MACY 4 T TORNEYS.

April 2, 1968 J. c.- MACY 3,376,528

ELECTROMAGNETI C ACTUATING DEVI CE Filed Sept. 10, 1965 7 Sheets-Sheet 3 F I63 A F'IGISB INVENTOR. JAMES c. MA c") V ATTORNEYS.

April 68 J. c. MACYV 3,376,528

ELECTROMAGNETIC ACTUATING DEVICE Filed Sept. 10, 1965 7 Sheets-Sheet 1 INVENTOR. JAMES C. MA Y A T TORNE Y5.

April 2, 1968 J. c. MACY 3,376,528

ELECTROMAGNETIC ACTUATING DEVICE Filed Sept. 10, 1965 '7 Sheets-Sheet 5 INVENTOR. JAMES C. MAC Y A TTORNEYS.

FIGS

J. C. MACY ELECTROMAGNETIC ACTUATING DEVICE April 2, 1968 7 Sheets-Sheet 6 Filed Sept. '10, 1965 INVENTOR. JAMES c. MACY W k M Z Arm/ 55% A ril 2, 1968 J. c. MACY '7 Sheets-Sheet 7 Filed Sept. 10, 1965 w wmmwWMwW a mhvvnv Q .FIC3.7'

INVENTOR. JAMES c. acy

3,376,528 ELECTROMAGNETIC ACTUATING DEVICE James C. Macy, Westfield, N..l., assignor, by mesne assignments, to Thrust, Incorporated, New York, N.Y., a

corporation of New Jersey Filed Sept. 10, 1965, Ser. No. 486,454 22 Claims. (Cl. 335-296) ABSTRACT OF THE DISCLOSURE This invention relates to electromagnetic actuating devices, and more particularly, to a novel type of electromagnetic actuating device capable of providing a powerful mechanical force over a relatively long traverse. This novel electromagnetic device is referred to as a contractuator.

The conventional electromagnetic relay includes an electrical coil, a movable magnetic armature, and a magnetic structure for completing the flux path around the coil. Devices of this general structure can provide a relatively powerful mechanical force as the armature is attracted in respouse to energization of the coil. However, the magnitude of the force is inversely proportional to the square of the working air gap length associated with the armature. Thus, any attempt at increasing the length of the stroke brings about a decrease in the force at the beginning of the stroke which is approximately proportional to the square of the distance to be traversed. Electrical devices of this general structure are, therefore, inherently short stroke devices.

By comparison, the conventional plunger-type solenoid is inherently a long stroke device and usually includes an iron plunger adapted to pass through the center of an electromagnetic coil. The mechanical force is created by the interaction between the magnetic flux of the plunger and the current passing through the energizing coil. The force created by the solenoid is relatively weak and suffers from the further disadvantage of being strongest at the middle of the stroke and weakest at the ends where maximum force is often required.

Electromagnetic devices can be classified either with the inherently short powerful stroke devices, or with the relatively long weak stroke devices. Because of the inherent characteristics of the prior electromagnetic devices it has not been possible to achieve a powerful force over a relatively long traverse without resorting to various boosting or supplementing techniques.

An object of this invention is to provide an electromagnetic actuator capable of converting electrical energy into a relatively powerful force over a long traverse.

Another object of the invention is to provide an electromagnetic actuator which can achieve optimum performance for given size, weight, geometrical configuration and electrical power conditions.

Another object is to provide an electromagnetic actuator in which the created force can be controlled as desired throughout the stroke.

Still another object is to provide an electromagnetic actuator which can be readily mass produced and in which standardized components can be assembled to satisfy varying operational requirements.

ire States Patent Yet another object is to provide an electromagnetic device which can easily be integrated as an operational portion of a system.

The electromagnetic device constructed in accordance with this invention converts electrical energy directly into mechanical force and is capable of providing a powerful mechanical force without limitation on the length of the stroke or traverse. The force is created by the magnetic attraction exerted upon a plurality of interconnected magnetic members. The working gap is broken into small increments so that a substantial force can be created without suffering the normal effects associated with a long stroke.

The invention is described in greater detail with reference to the following specification which sets forth several illustrative embodiments. The drawings are part of the specification wherein:

FIGURE 1 is a view of the actuating device in accordance with one embodiment of the invention with portions broken away for clarity of illustration;

FIGURE 2 is an exploded perspective assembly drawing showing details of some of the components for the actuating device in FIGURE 1;

FIGURES 3A and 3B are schematic diagrams illustrating two arrangements for interconnection of the electromagnetic coils for the unit in FIGURE 1.

FIGURE 4 is a partial cross sectional view illustrating another embodiment of the invention;

FIGURE 5 is a cross-sectional view of another actuating device in accordance with this invention wherein a single energizing coil is employed;

FIGURE 6 is a view of a helical actuating device in accordance with this invention with portions broken away for clarity of illustration;

FIGURE 7 similarly illustrates another helical actuating device; and

FIGURE 8 is a cross-sectional View showing the structure of the helix in FIGURE 7.

The mechanical force created by an electromechanical device can be expressed by the following formula:

where P is the pull or force, k is a constant, F is the magnetomotive force, a is the permeability of the working gap, A is the area of the pole face, and x is the length of the working gap. It should be noted that the created force is inversely proportional to the length of the working gap. In addition, the magnetomotive force is inversely related to the gap length and therefore the force falls ofl rapidly as the working gap length increases.

In the illustrative embodiments, structures are described which provide a powerful force over a relatively long traverse which would normally require a corresponding long air gap. With the structure in accordance with the invention, the air gap is broken into relatively small increments to eliminate the problems associated with the long air gap. The structurealso makes possible a relatively high magnetomotive force by minimizing the flux losses and by permitting use of materials in the working gap having a permeability greater than air.

The actuating device in accordance with one embodiment of the invention is illustrated in FIGURES 1 and 2, and includes a plurality of disc-shaped members 10, for convenience referred to as coil discs, each including an inner ring 11 and an outer ring 12. A concentrically wound electrical coil 14 is located between the inner and outer rings of the disc which form core pieces for the coil. A shoulder is machined in the upper and lower edges of rings 11 and 12 for positioning a pair of nonmagnetic washers 15 above and below coil 14. Coil 14 can therefore be completely enclosed to avoid exposure to the surrounding fluid.

Inner core piece 11 includes a groove 16 extending from its inner surface to thereby provide an upper annular flange 17 and a lower annular flange 18. An associated coupling disc 20 is externally machined about its periphery to cooperate with the groove and flanges on "the inner surface of core piece 11. The thickness of the coupling disc is the same as the thickness of coil disc 10. A circumferential groove 21 is machined surrounding disc 20 to provide upper and lower peripheral flanges 22 and 23. The coupling discs 20 are loosely mounted on a guide shaft 26 passing through the center thereof with a coupling disc located between each adjacent pair of coil discs. The flanges on the inner core pieces of adjacent coil discs rest within the groove 21 of the associated coupling disc and the flanges of adjacent coupling discs rest within the groove 16 of the associated inner core piece. This interlocked groove and flange arrangement permits the coil discs to move together until they touch one another, or to separate until the flanges engage. Thus, the coupling discs limit the separation between coil discs and, hence, determine the maximum working gap which can exist between adjacent coils and core pieces. Guide shaft 26 maintains the coil discs in alignment parallel to one another and insures that the movement of the coil discs in linear and parallel to the axis of the shaft.

The end cap 30 is preferably machined from a single piece of iron and includes an annular recess dimensioned to accommodate a circumferentially wound electrical coil 31 and a nonmagnetic washer 32. A coupling member 34 is pivotably mounted in the center of end cap 30 in a manner which prevents any longitudinal movement relative to the 'end cap. The coupling member is constructed so that the portion extended above the end cap (as viewed in FIGURE 2) is essentially the same as the portion of a. coupling disc extending from one of the coil discs. More specifically, coupling member 34 includes a peripheral flange 35 adapted to rest within the groove 16 of the adjacent coil disc, and a groove 36 adapted to cooperate with the flange of the adjacent coil disc- The other end of the coupling member is shaped as required for attachment to external equipment.

The upper and lower end caps for the actuating device are essentially the same and are attached to the upper and lower coil discs in the same fashion as is illustrated in FIGURE 1. The coupling members 34 each include a central recess 37 therein adapted to loosely hold guide rod 26. The length of the guide rod and the length of the cooperating recesses are selected so that the rod cannot fall out when the actuating device is in the fully extended condition but at the same time permit the actuating device to contract until the coil discs touch one another.

Although the flanges 22 and 23 on the coupling disc, and flanges 17 and 18 in the coil disc, are shown as complete circumferential flanges they could be formed as flange segments covering somewhat less than half of the circumference. Such an arrangement would have assembly advantages since the coupling disc could then be inserted within a coil disc and thereafter turned so that the flange segments become juxtaposed.

A flexible bellows-like cover structure 40 is mounted surrounding the unit to form a dust cover and thereby keep dirt and dust out of the air gaps between the coil discs or, if a fluid other than air is placed between the discs, cover 40 prevents this fluid from escaping. The outer rings of the coil discs are provided with grooves 41. Retaining rings 42 surround the bellows cover structure and cooperate with grooves 41 to maintain the cover in its proper position.

Retaining rings 42, coupling discs 20, coupling members 34 and guide rod 26 are preferably constructed from nonmagnetic materials such as aluminum or brass. The inner and outer core pieces 11 and 12, and the end caps 30, are constructed from a magnetic material such as iron and can be laminated if desired to reduce eddy current losses.

The electrical leads from the coils 14 and 31 can be brought out through suitable holes in the outer core pieces and end caps. If the coils are interconnected so that current flows through each coil in the same direction, for example, the clockwise direction as shown in FIGURE 3A, the coil discs will be attracted to one another and an overall contracting force is provided as indicated by the arrows. On the other hand, if the coils are interconnected in an alternate fashion as shown in FIG- URE 3B where current passes through one coil in a clockwise direction and through adjacent coils in a counterclockwise direction, the coil discs tend to repel one another and an overall expanding force is provided as indicated by the arrows. With suitable external switching, the same unit can be used to provide either a contracting or expanding force. The electrical coils can be energized simultaneously or sequentially depending on the type of motion desired.

The maximum linear traverse of the actuating unit, that is, the maximum distance that one end coupling member moves relative to the other when the coils are energized, is equal to the sum of the incremental working gaps 45 existing in the fully extended condition. If it is desired to increase the travel distance, this is easily accomplished by adding additional coil discs and coupling discs intermediate end caps 30. Therefore, the travel distance can be increased without decreasing the created force since the force is a function of the working magnetic gap associated with the individual coils. This working gap in turn is limited by the coupling discs so that a working magnetic contact is maintained between the core pieces of adjacent coil discs.

Another embodiment of the invention is illustrated in FIGURE 4 wherein the inner rings or inner core pieces 11' of the coil discs are loosely mounted directly upon guide rod 26. The maximum gap 47 beween adjacent coil discs is limited by inwardly flanged nonmagnetic coupling rings 46 which surround the adjacent flanges 48 of the adjacent coil discs. The coupling rings thus replace the coupling discs as well as the surrounding dust cover shown in FIGURE 1.

Another embodiment of the invention, utilizing a single electric coil, is shown in FIGURE 5. The coil 50 is a cylindrical, concentrically wound, coil encased in a suitable nonmagnetic material providing relatively smooth exterior surfaces. One end of coil 50 is accommodated within a suitably dimensioned resistively coated annular recess 51 in an end cap 52. In recess 51, between the end coil 50 and the bottom of the recess, is a flat coil spring 53 constructed from an electrically conductive material. The coil spring maintain electrical contact between an external lead 54 and one of the electrical leads 55 emerging from coil 50. The other end of coil 50 is similarly accommodated in an annular recess 61 within an end cap 62. The other lead 65 of coil 50 is coupled to an external lead 64 via a coil spring 63 located within recess 61. End caps 52 and 62 include extensions 57 and 67, respectively, adapted for attachment to external equipment.

Within the central opening of coil 50 there are a plurality of internally grooved and flanged discs 76 which are coupled to one another and to the end caps by means of externally grooved and flanged coupling discs 77. The coupling discs serve to limit the length of the working gap in essentially the same manner as previously described with respect to FIGURES 1 and 2. A plurality of externally flanged and grooved discs 78 surround coil 50 and are coupled to one another and to the end caps by means of internally flanged coupling rings 79. The coupling rings operate to limit the length of the air gap between discs 78 in essentially the same manner as previously described with respect to FIGURE 4.

End caps 52 and 62, and discs 76 and 78, are preferably constructed from a magnetic material such as iron whereas coupling discs 77 and coupling rings 79 are constructed from a nonmagnetic material.

When coil 50 is energized, a magnetic flux is created which passes through discs 76 and 78 and end caps 52 and 62 creating a force tending to pull the end caps together to thereby eliminate the working magnetic gaps 80 existing between the discs.

In the previously described embodiments of the invention the magnetic core structure has included individual segments disposed to break the working magnetic gap into smaller increments. In the embodiment of the invention shown in FIGURE 6 the segments of the magnetic structure are part of the same helical core structure. More specifically, the unit in FIGURE 6 includes a helical conductor 90 surrounded by electric insulation 91. Conductor 90 may be a single copper bar, or may be stranded including a plurality of separate conductors. The ends of conductor 90 are attached to suitable leads 89 brought out through the associated end caps 92 and 93, the leads being connectable to a suitable source of electric power.

The magnetic core structure associated with the helical conductor includes an inner helix 94 and an outer helix 95, which are both concentric with respect to the helix of conductor 90. The ends of the core helices are securely fastened to end caps 92 and 93 as by welding.

A cylindrical guide rod 98 passes through the center of inner core helix 94. The ends of the guide rod are flanged outwardly and these flanges are accommodated within a suitable recess 100 within end caps 92 and 93. The recess includes a shoulder 99 adapted to cooperate wth the flange 101 on the end of the guide rod. The cooperation between the flanges at the end of the guide rod and the shoulders of the end caps serve to limit the maximum distance between the end caps, and hence, the axial length of the working gap between adjacent core segments. Furthermore, the guide rod maintains the structure in linear alignment. In some cases the helical structure between the end caps will be relatively rigid and therefore the guide rod can be eliminated. However, in most cases the structure will be designed for maximum flexibility. For example, the cores 94 and 95 may consist of iron particles in a flexible medium and the conductor 90 may consist of a multiple structure of relatively fine Wires.

,When current passes through conductor 90, magnetic flux is created surrounding the conductor and in turn creates magnetic poles along the radial surfaces of the associated core helices. For example, the right side of the outer core helix 95 a viewed in FIGURE 6 may become a continuous helical north pole while the left side becomes a continuous helical south pole. Under these circumstances, the right side of the inner helix would become a south pole and the left side would become a north pole. It should be noted that the adjacent surfaces (opposite sides of the working gap) of a core helix are of opposite magnetic polarity and therefore tend to attract one another. As a result, end caps 92 and 93 are drawn toward one another and the gap between adjacent segments of the helices are eliminated.

Another helical embodiment of the invention is shown in FIGURES 7 and 8 including two separate helices displaced by l80 from one another and each disposed between the end caps. With two separate helices it is possible to create contracting or expanding forces as desired. Each of the helices 114) and 111 includes a flat conductive strip 112 in the center sandwiched between a pair of flat strips 113 and 114 of electrically insulating material as can best be seen in the cross-sectional view in FIG- UR'E 8. The insulating strips are wider than the conductive strip and secure a pair of magnetic strips 115 and 116 in positions located radially inside and outside, respectively, of the conductive strip. The helical structure is completed by a pair of strips 117 and 118 wrapped around the outer edges of strips 115 and 116 respectively. Strips and 117 form the inner core helix and strips 116 and 118 form the outer core helix. The individual strips can be made from layers of metal foil, and hence, the entire structure can be relatively flexible. Also, it should be noted that the structure shown in FIGURES 7 and 8 can be produced continuously from. an automatic machine and cut to length as required.

Two such helical structures 110 and 111 are secured between the end caps 120 and 121. The helical structures are identical but displaced from one another so the segments of one helix nest between the segments of the other helix. The conductors, at the ends of the helical structure, are attached to leads 122 brought out through the end caps. The helical structures are physically secured to the end caps by any suitable technique such as weldmg.

If the conductors are so energized via leads 122 that current flows through both helical structures in the same direction, e.g., clockwise, then a contracting force is created drawing the end caps together. However, if current flows through the helices in opposite directions, e.g., clockwise in one and counterclockwise in the other, then an expanding force is created pushing the end caps apart.

FIGURE 8 shows the cross section of one helical structure 111, and similar adjacent cross section of the other helix 110. With current flow through the helical structures in opposite directions, the magnetic flux will be as shown in dotted lines and the magnetic poles created on the faces of the magnetic core pieces will be as indicated by the letters Naud S. In each case the opposite magnetic poles appear on opposing faces and therefore they tend to repel one another. As a result, an expanding force is created as indicated by the arrows.

In some installations operation can be optimized by further reducing the reluctance of the magnetic path. Accordingly, a fluid or elastic material having ferromagnetic properties could be used to till the working gap such as shown in FIGURE 4 where a fluid with ferromagnetic properties fills the gaps 47. This would have the eflect of decreasing the magnetic flux losses in addition to decreasing the reluctance of the magentic path.

While only a few illustrative embodiments of the invention have been described in detail it should be obvious that there are numerous arrangements and variations within the scope of this invention. The invention is more clearly defined in the appended claims.

What is claimed is:

1. In a magnetic actuating device, the combination of a pair of magnetic end members capable of linear movelment along a common axis;

a magnetic structure disposed between said end members and including a plurality of magnetic segments surrounding said common axis for dividing any existing gap between said end members into a plurality of approximately equal increments; and

electromagnetic means operatively associated with said magnetic structure for creating a magnetic flux when energized, which magnetic flux passes through said magnetic segments to cause relative movement of said end members along said common axis.

2. A magnetic actuating device in accordance with claim 1 wherein said electromagnetic means comprises a plurality of electrical coils each surrounding said common axis and lying in a plane perpendicular to said common axis; andsaid magnetic segments are core pieces each attached to and concentric with a diiferent one of said electrical coils.

3. A magnetic actuating device in accordance with claim 2 wherein a pair of core pieces are associated with each of said electrical coils, one of said associated core pieces being disposed inside said coil and the other being disposed outside said coil.

4. A magnetic actuating device in accordance with claim 1 wherein said electromagnetic means comprises a single cylindrical electrical coil surrounding said common axis and disposed between said end members, and

said magnetic segments are discs lying in planes perpendicular to said common axis. 5. A magnetic actuating device in accordance with claim 4 wherein some of said discs are disposed within said electrical coil and some are disposed surrounding said coil.

6. A magnetic actuating device in accordance with claim 1 wherein said electromagnetic means is an electrical coil in the form of a helix surrounding said common axis and said plurality of magnetic segments are part of a helical core piece disposed concentrically of said electrical coil.

7. A magnetic actuating device in accordance with claim 6 wherein said magnetic structure includes a pair of helical core pieces concentric to said electrical coil, one of said core pieces being located inside said coil and the other being located outside said coil.

8. In a magnetic actuating device, the combination of a magnetic structure including a plurality of relatively movable magnetic segments disposed to provide a plurality of gaps between adjacent magnetic segments; guide means coupled to said magnetic segments for maintaining the same in alignment and for restricting the movement thereof to a desired path;

movement limiting means coupled between said magnetic segments to limit said gaps therebetween and thereby maintain a working magnetic contact between said magnetic segments;

electromagnetic coil means operatively associated with said magnetic segments for creating a magnetic flux when energized, which magnetic flux passes through said magnetic segments and said gaps therebetween to cause relative movement of said magnetic segments; and

a plurality of discs having a common axis and wherein said electromagnetic coil means comprises a plurality of separate coils each surrounding said common axis and each located in a separate one of said discs, and

said magnetic structure comprises magnetic segments in the form of a pair of concentric core pieces in each of said discs, the inner one of said core pieces lying inside said coil and the outer of said core pieces lying outside said coil.

9. A magnetic actuating device in accordance with claim 8 wherein said guide means comprises a cylindrical rod passing through the center of said discs.

10. A magnetic actuating device in accordance with claim 8 wherein said movement limiting means comprises a plurality of externally flanged coupling discs, and

said inner core pieces are flanged about a central opening and adapted for cooperation with said flanged coupling discs.

11. A magnetic actuating claim 8 wherein said movement limiting means comprises a internally flanged coupling rings and said outer core pieces are externally flanged and adapted to cooperate with the flanges of said coupling ring.

12. In a magnetic actuating device, the combination of a magnetic structure including a plurality of relatively movable magnetic segments disposed to provide a plurality of gaps between adjacent magnetic segments;

guide means coupled to said magnetic segments for maintaining the same in alignment and for restricting the movement thereof to a desired path;

device in accordance with plurality of movement limiting means coupled between said magnetic segments to limit said gaps therebetween and thereby maintain a working magnetic contact between said magnetic segments;

electromagnetic coil means operatively associated with said magnetic segments for creating a magnetic flux when energized, which magnetic flux passes through said magnetic segments and said gaps therebetween to cause relative movement of said magnetic segments; and wherein said electromagnetic coil means is a cylindrical coil having a central axis; and

said magnetic structure comprises a plurality of discs lying in parallel planes and movable along said axis, said discs being disposed both inside and outside said coil.

13. A magnetic actuating device in accordance with claim 12 wherein those of said discs within said cylindrical coil are flanged about a central opening and said movement limiting means comprises externally flanged coupling discs adapted to cooperate with the flanges of said discs.

14. A magnetic actuating claim 12 wherein those of said discs outside said coil are externally flanged, and

said movement limiting means comprises internally flanged coupling rings adapted to cooperate with said external flanges.

15. In a magnetic actuating device, the combination of a magnetic structure including a plurality of relatively movable magnetic segments disposed to provide a plurality of gaps between adjacent magnetic segments; guide means coupled to said magnetic segments for maintaining the same in alignment and for restricting the movement thereof to a desired path;

movement limiting means coupled between said magnetic segments to limit said gaps therebetween and thereby maintain a working magnetic contact between said magnetic segments;

electromagnetic co-il means operatively associated with said magnetic segments for creating a magnetic flux when energized, which magnetic flux passes through said magnetic segments and said gaps therebetween to cause relative movement of said magnetic segments; and wherein said electromagnetic coil means is in helix and said magnetic structure is in the form of a pair of concentric helices lying inside and outside the helix of said coil.

16. A magnetic actuating device in accordance with claim 15 further comprising. a pair of end members coupled to the ends of said helices and wherein said guide means and said movement limiting means comprise a cylindrical member passing through the common center of said helices and coupled to said end members to limit expansion of said helices.

17. In a magnetic actuating device, the combination of a plurality of electromagnetic coils each having a common axis;

a magnetic core member attached to and concentric with each of said coils;

guide means for maintaining said coils and concentric cores in parallel planes with a plurality of gaps therebetween and for permitting movement along said common axis;

separation limiting means coupled between said cores to limit the gaps therebetween; and

said electromagnetic coils when energized being operative to create a magnetic flux which passes through at least some of said core members and gaps to cause relative movement of said core pieces.

device in accordance with the form of a 18. Apparatus in accordance with claim 17 wherein each concentric core comprises a pair of core pieces and wherein the associated coil is between said core pieces.

19. Apparatus in accordance with claim 18 wherein the core pieces associated with the intermediate coils are separated and wherein the core pieces associated with the outermost of said coils are part of an integral magnetic structure.

20. In a magnetic actuating device, the combination of a cylindrical electromagnetic coil;

a pair of magnetic end members disposed at opposite ends of said coil;

a plurality of magnetic discs each disposed within said coil and flanged about a central opening;

a plurality of nonmagnetic externally flanged coupling discs coupled between said inside magnetic discs and said magnetic end members to limit the gap therebetween;

a plurality of externally flanged magnetic discs disposed outside said coil; and

a plurality of internally flanged nonmagnetic coupling rings coupled between said outside magnetic discs and said magnetic members to limit the gap therebetween.

21. In a magnetic actuating device, the combination an electromagnetic coil means in the form of a helix having a central axis;

a pair of magnetic end members disposed at the ends of said coil and adapted for movement along said axis;

an inner helical core piece disposed within said helical coil and attached to said end members; and

an outer helical core piece disposed surrounding said helical coil and attached to said end members.

22. In a magnetic actuating device, the combination of a pair of end members;

a pair of similar helical structures each secured between said end members, each of said helical structures including a central helical conductor and a pair of magnetic helical core pieces secured inside and outside, respectively, of said conductor.

References Cited UNITED STATES PATENTS 25 BERNARD A. GILHEANY, Primary Examiner.

GEORGE HARRIS, Examiner.

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