US3243542A - Electrical switch having magnetic holding means - Google Patents

Description

March 29, 1966 MAXWELL 3,243,542

ELECTRICAL SWITCH HAVING MAGNETIC HOLDING MEANS Filed Nov. 21, 1963 V 2 Sheets-Sheet 1 AT TORNEYS INVENTOR. PALMER M. MAXWELL March 29; 1966 M. MAXWELL 3,243,542

I ELECTRICAL SWITCH HAVING MAGNETIC HOLDING MEANS Filed Nov. 2-1, 1963 2 Sheets-Sheet 2 28' 16 H l 20 A 11 M 42a Q v Fm. 8

Fm 7 I INVENTOR.

PALMER M. MAXWELL ATTORNEYS United States Patent 3,243,542 ELECTRICAL SWITCH HAVING MAGNETIC HGLDING MEANS Palmer M. Maxwell, Wymherly Point Drive,

. Savannah, Ga. Filed Nov. 21, 1963, Ser. No. 325,350 9 Claims. (Cl. 200-87) This invention relates to electrical switches and more particularly to an electrical switch utilizing permanent magnets as means for holding the switching mechanism in selected positions and having a simplified construction which makes it relatively inexpensive to manufacture and extremely durable in use.

' Previous electrical switches have generally been characterized by a large number of moving parts and by the use of relatively expensive materials. In addition, previous electrical switches have usually not been easily adaptable from one type of switching action to another or from single switch to multiple or gang switch arrangements. As a result, previous electrical switches have generally been relatively expensive to manufacture with the variety of switching actions and in the variety of switch arrangements required by present day requirements for electrical switches. 7

Moreover, the usable life of previous electrical switches has generally been relatively short and they have not been readily repairable. This is because of the large number of moving parts and because the contacts used in these switches frequently become damaged and cannot be easily replaced.

The electrical switch disclosed herein substantially eliminates these and other problems and limitations associated with previous switches. It has a switching mechanism which moves as a unit and which is the only moving part of the switch. This permits the electrical switch to be of simple and durable construction and results in the usable life of the electrical switch being limited only by the usable life of the contacts. The contacts are easily replaced and as a result the usable life of the electrical switch is substantially unlimited.

In addition, the electrical switch disclosed herein is easily changed from one type of switching action to another and is readily adaptable to both single switch and multiple or gang switch arrangements. This reduces the initial cost of the electrical switch since common parts can be used regardless of the switching action or the number of switches in the arrangement. This adaptability also permits the electrical switch to be adapted to the requirements of a particular installation at the time of its use.

These improvements in electrical switches are provided by an electrical switch having a switching mechanism which comprises a movable permanent magnet and a plurality of movable contacts which are rotated within a housing by motion of a handle. Thehousing is a boxlike structure providing means for mounting the switching mechanism for rotatable motion, means for mounting a plurality of stationary contacts where they will be engaged by the movable contacts upon motion of the movable contacts, and means for positioning a plurality of stationary permanent magnets adjacent the movable permanent magnet. l

The movable magnet and the stationary magnets serve to hold the switching mechanism in various positions and the switching action of the electrical switch is varied by simply changing the relationship between the movable magnet and the stationary magnets. The stationary contacts are slidably inserted into slots in the housing so as to permit them to be easily and quickly replaced when damaged orworn. r

Nice

. These and other features and advantages of thepresent invention will be more apparent from consideration of the following specification taken in conjunction with the accompanying drawings in which like characters of reference denote corresponding parts through-out and in which:

FIG. 1 is an exploded fragmentary perspective view of a single switch embodiment of the present invention.

FIG. 2 is a fragmentary enlarged perspective view showing one of the stationary contacts and that portion of the housing in which the stationary contact is inserted.

FIG. 3 is a perspective view showing a second embodiment of the invention in which a plurality of electrical switches are arranged in a multiple or gang switch arrangement.

FIG. 4 is a perspective view of one of the common end plates used in the second embodiment of the invention shown in FIG. 3.

FIG. 5 is a side elevation view of an end plate and stationary magnet for a third embodiment of the present invention.

FIG. 6 is a side elevation view of an end plate and stationary magnet for a fourth embodiment of the present invention.

FIG. 7 is a side elevation view showing the positions of the movable and stationary contacts when the switching mechanism is in a neutral position between the sides of the housing.

FIG. 8 is a schematic view showing the movable magnet and the stationary magnets in the second embodiment of the present invention shown in FIG. 3 when the switches in the embodiment are adapted to a plurality of switching actions.

These figures and the following detailed description dis,- close a preferred specific embodiment of the invention, but the invention is not limited to the details disclosed herein since it may be embodied in other equivalent forms Referring now more particularly to FIG. 1, the present electrical switch comprises a box-like housing 10 made of plastic or other similar easily moldable material, a switching mechanism 11 rotatably positioned within the housing 10, and a pair of end plates 12 attached to the housing 10 for closing the ends of the housing -10 and for sup.- porting a plurality of stationary permanent magnets ada jacent to the switching mechanism 11.

The housing 10 has a base 15 integral with a pair of sides '16 which extend parallel to eachfother from the base 15 to a mounting block 17. The base 15 and the mounting block 17 are parallel to each other and the mounting block 17 is formed into two segmentsl7a and 17b which extend toward each other from the "sides 16. The extending ends of the segments 17a and 17b have concave surfaces 19 which together define a cylindrical space between the segments 17a and 17b. A switching cylinder 20 of the switching mechanism 11 is rotatably positioned within this cylindrical space between the segments 17a and 17b.

The switching cylinder 20 is integral with a handle 21 extending outwardly from the circumference of a switching cylinder 20 along the entire length of the switching cylinder 20. The length of the switching cylinder 20 is substantially that of the housing 10 and as a result .a large bearing surface is provided between the rotatable switching cylinder 20 and the mounting block 17 of the housing 10. I

The switching cylinder 20 has a slot 22 extending radially into it from diametrically opposite the handle 21 The slot 22 has an outer portion 24 continuous with an inner portion 25. The inner portion 25 of the sl0t 22 has parallel sides and it will be seen that the inner portion 25 of the slot 22 receives other parts of the switching mechanism 11. The outer portion 24 of the slot 22 has sides sloping away from each other so that the outer portion 24 of the slot 22 is widest 'at the circumferential surface of the switching cylinder 20 and it will be seen that the outer portion 24 serves as a movement limiting means for the other parts of the switching mechanism 11 inserted into :the inner portion 25 of the slot 22.

The outer portion 24 and the inner portion 25 lot the slot 22 extend :the entire length of the switching cylinder 20 and the di stance between the sides of the inner portion 25 of the slot 22 is just sufficient to receive a movablernagnet 26 sandwiched between two spring plates 28. The movable magnet 26 is a flat rectangular solid of permanently 'ma'gnetizable material and is positioned in the slot 22 by inserting one of its four edges into the inner portion 215 of the "slot 22. This places edges 55 and 56 of the movable magnet 26 at opposite ends of the slot 22 and parallel to each other.

The spring plates 28 are thin rectangular plates of resilient electrically conductive material bifurcated at one end to form two arms 28. The arms 28 extend 'outwater of the switching cylinder 20 when the opposite ends of the spring plates 28 are inserted in the inner 'portion 25 o'f tiie slot 22 on opposite sides of the movable magnet 26. 'As the spring plates 28 extend from the inner portion 25 of the slot 22, each spring plate 28 is "bent awa y frorn' tlie movable magnet '26 as it passes from the inner portion 25 of the slot 22. This serves to displace the arms 28' of the spring plates 28 in opposite directions "from the movable magnet 26 to the extent permitted by the-sloping sides of the outer portion 24 of the slot '22. At t'heexte'nding ends of the arms '28 are movable contacts 29. h

In eachsid'e -16 of the housing are two rectangular notches 30. The notches 30in each side 16 are parallel to the "base 15 and extend inwardly from opposite edges 27 of'tlie side 16. A'plurality'ofstationary contacts 31 are positioned within the notches 30 and the notches 30 are positioned in the sides 1'6so that the movable conta cts"2 9 engage and disengage the stationary contacts 31 as the switching cylinder 20 is rotated "by the handle 21.

Eachofthestationary contacts 31fhas a generally rectang'ular shape and is made of material suitable for use as anelectric'al contact. Recesses 32 in opposing :sides of each'of the stationary contacts 31 form a segment 32 of eddced width in each stationary contact 31 which has substantially the same width 'as'a notch 30 and the stationary contacts 31 arepositioned within the sides 16 by inserting '"the segments'sz' into the notches '30. I End plates 12 close the ends of the housing 10 and of the rifc 'itches30 "in the sides 16 of the housing 10 and with the end plates'12 in position, the stationary contacts 31 are 'fixedly positioned in the housing 10. However,

this arrangement for positioningthe stationary contacts 31 in the housing 10 permits the stationary contacts 31 to "be readily changed whcnworn or otherwise damaged s'iinpl'y removing an'e'ndplate 12 and slidably changing fthe stationary ccntact'3'1 in a notch 30. ,The movable contacts 29'areequally'easy to change by simply pulling afspririg p'late 28 'fromthe inner portion 25 of the slot 22 andinsertin'g anew spring plate 28.

I, .The end plates 12 are rectangular solids of easily moldlectrically hon-conductive material such as plastic. Thejend' plates 12 are attached to the edges 27 of the hpusing' 1 by inserting sc ews 35 through the end plates 12 andirito the "housing 10, In closing the ends of the housing 10, t he end plates 12 serve with the base and with the switching cylinder positioned within the mounting block 17 to seal the electrical switch disclosed herein against moisture or other foreign matter'entering the electrical switch and causing failure of the electrical switch. Theen'd plates 12 also serve to prevent slidahle motionofthe switching cylinder 20 along its axis ofrotation betwen the segments 17aand 17b of the mounting block 17. Two rectangular recesses 36 are formed in'the inner surface of one of the end -plates 12;

and two rectangular recesses 37 are formed in the inner surface of the other end plate 12. 14 is positioned within each of the rectangular recesses 36 and 37.

The stationary magnets 14 are rectangles of permanently magnetizable material similar to that used for the movable magnet 26 and when the end plates 12 are attached to the sides 27 of the housing 10, the stationary magnets 14 are maintained in position within the recesses 36 and 37 by the mounting block 17. The orientation of the stationary magnets 14 and of the movable magnet 26 from a magnetic polarity standpoint determines the switching action of the electrical switch disclosed herein and will be understood when the operation of the electrical switching is described below.

It will be seen from FIG. 3 and FIG. 4 that the electrical switch disclosed herein is easily and conveniently embodied in a multiple or gang switch arrangement by utilizing an end plate 112 between adjacent housings 10. The end plates 112 differ from. the-end plates 12 described above in that the recesses 36 and 37 are replaced by channels 38 which extend completely through an end plate 112. The stationary magnets 14 are positioned within and completely 'fill the channels 3 8 and serve as the stationary magnets '14 for two adjacent electrical switches. Thus, the end plates 112 permit a multiple or gang "switch arrangement to be provided with each end plate 112 serving adjacent electrical switches and eliminate the requirement for an end plate 12 for 'each electrical switch in a multiple or gang switch cmbodi ment of the invention.

In FIG. 5, anend plate '41 'for a third'embodime'nt of the electrical switch disclosed herein is shown. The end plate 41 has a single U-s'haped recess 42 into which a stationary magnet 40 is inserted. The stationary magnet 40 is U-shaped and combines into a single structure the two stationary magnets '14 of the first embodiment of the invention described above. In FIG. 6, an end .plate 51 for .a fourthernbodiment of the electrical'switch is shown. The end plate 51 has an H-shaped recess "52 into which a stationary magnet 50 is inserted. The stationary magnet '59 is I l-shaped and combines into a single structure the two stationary magnets '14 required for each end plate 12. It has been found that when a stationary magnet 40 or a stationary magnet 50 is used in an embodimentof the invention, the electrical switch will operate in substantially the same manner as when two stationary magnets '14 are used if the axis of rotation of the switching cylinder 20 is at the midpoint of the bridge 53 of the stationary magnet 40 and of the bridge 53' of the stationary magnet 50. However, the combining into a single stationary magnet 40 or 50 of the two stationary magnets 14 simplifies the manufacture and assembly-of the electrical switch disclosed herein.

OPERATION Regardless of whether the electrical switch disclosed herein is embodied in a single switch or multiple or gang switch arrangement and regardless of whether stationary magnets 14, 40 or 50 are used. in an embodiment of the electrical switch disclosed herein, the "operation of the electrical switch is in terms ofthe multiple or gang switch arrangement schematically shown in FIG. 8 and utilizing stationary magnets 14. This is a gang switch arrangetion parallel to the axis of rotation of the switching cylinder 20 into which the movable magnet-26h inserted A stationary magnet and in the embodiment of the invention schematically shown in FIG. 8, the movable magnet 26a is polarized so that its edge 55 is south and its edge 56 is north, the movable magnet 26b is polarized so that its edge '55 is north and its edge 56 is south, and the movable magnet 260 is polarized so that its edge 55 is south and its edge 56 is north.

The stationary magnets 14 are polarized so that when positioned in recesses 36 and 37 or in channels 38, the stationary magnets 14 are polarized in directions parallel to the axis of rotation of the switching cylinders 20. As a result, the entire side 58 or 59 of a stationary magnet 14 will be polarized either north or south depending upon the manner in which the stationary magnet 14 is inserted into a recess 36 or 37 or a channel 38. In the-embodiment of the invention schematically shown in FIG. 8, the sides 58 of the stationary magnets 14 adjacent to the edge 56 of the movable magnet 26a are polarized south, the sides 59 of the stationary magnets 14 adjacent to the edge 55 ofthe movable magnet 26a are polarized north, the sides 58 of the stationary magnets 14 adjacent the edge 56 of the movable magnet 26b are polarized south, the sides 59 of the stationary magnets 14 adjacent the edge 55 of the movable magnet 26b are polarized north, the sides 58 of the stationary magnets 14 adjacent the edge 56 of the movable magnet 260 are polarized south,

and the sides 59 of the stationary magnets 14 adjacent the edge 55 of the movable magnet 26c are polarized north.

With the foregoing magnetic orientation of the movable magnets 26 and stationary magnets 14, the normal position of the movable magnet 26a of electrical switch A and of the movable magnet 260 of electrical switch C is a position inclined toward a side 16 of a housing 10. This is because the magnetic polarities of the edges 55 and 56 of the movable magnets 26a and 260 cause the edges 55 and 56 to seek the faces 58 and 59 of the stationary magnets 14 at opposite ends of the movable magnets 26a and 260. The result is that the movable magnets 26a and 260 tend to position themselves between opposing, rather than adjacent, pairs of stationary magnets 14. The opposing pair of stationary magnets 14 between which a movable magnet 26a or 26c tends to position itself is. dependent upon the position into which the movable magnet 26a or 26c is rotated by the handle 21.

When a movable magnet 26a or 26c is positioned between an opposing pair of stationary magnets 14, the movable contacts 29 on one side of the movable magnet 26a or 260 engage the stationary contacts 31 in a wall 16. Thus, the arrangement of permanent magnets 14 and movable magnets 26a and 26c shown schematically in FIG. 8 results in an electrical switch A and an electrical switch C which are normally in one of two operative positions and which may be changed from one operative position to another by simply exerting enough force on a handle 21 to overcome the magnetic forces tending to hold the electrical switch A or the electrical switch C in an operative position.

In the electrical switch B of the multiple or gang switch arrangement schematically shown in FIG. 8, the magnetic polarities of the faces 58 and 59 of the stationary magnets 14 are such that the edges 55 and 56 of the movable magnet 26b are magnetically repulsed by all of the stationary magnets 14. This results in the movable magnet 26b tending to position itself midway between adjacent stationary magnets 14 rather than between opposing stationary magnets 14. Thus, although the movable magnet 26b can be moved by a handle 21 so that the movable contacts 29 engage stationary contacts 31, the movable magnet 26b returns to and is normally in a position in which the movable contacts 29 do not engage stationary contacts 31. The electrical switch B will be understood to be a normally open switch and the switches 6 A and C will be understood to be norm-ally closed switches.

It will be obvious to those skilled in the art that many variations may be made in the embodiments chosen for the purpose of illustrating the present invention without departing from the scope thereof as defined by the appended claims.

What is'claimed as invention is:

1. An electrical switch comprising a housing having a side with two notches extending inwardly from an edge; two stationary contacts each having a segment of reduced width and having its segment of reduced width inserted into the notches in the side of the housing; a first end plate at an end of said side and having a recess in its inner surface; a second end plate at the opposite end of said side from the first end plate and having a recess in its inner surface; a switching cylinder positioned within said 'housing with a first end adjacent the first end plate and a second end adjacent the second end plate and rotatable about an axis of rotation parallel to the said sides, said switching cylinder having a slot extending inwardly; a rectangular plate having one edge fixedly inserted into the slot in the switching cylinder, said rectangular plate being permanently magnetized in a direction parallel to its inserted edge; a spring plate having one edge inserted into the slot in the switching cylinder and having two outwardly extending arms at its opposite edge; a movable electrical contact fixedly positioned at the extending end of each of the said arms of the spring plate and positioned to engage the said stationary contacts upon rotation of the switching cylinder about its axis of rotation; and a first block fixedly positioned in the recess of the first end plate and permanently magnetized in the same direction as the movable magnet is magnetized; and a second block fixedly positioned in a recess of the second end plate and also permanently magnetized in the same direction as the movable magnet.

2. An electrical switch comprising a housing having a first side with a notch extending inwardly from a first edge and a notch extending inwardly from a second edge and a second side with a notch extending inwardly from a first edge and a notch extending inwardly from a second edge; a plurality of stationary electrical contacts, each of said stationary contacts having a segment of reduced width and each of the said stationary contacts having said segment of reduced width inserted into one of the plurality of notches in the said sides of the housing; a first end plate extending between the-first edge of the first side of the housing and the first edge of the second side of the housing and having a pair of recesses in its inner surface, one of which is located adjacent the first side and the other is located adjacent the second side wherein a non-recessed area occurs between the pair of recesses; a second end plate extending between the second edge of the first side of the housing and the second edge of the second side of the housing and having a pair of recesses in its inner surface, one of which is located adjacent the first side and the other is located adjacent the second side wherein a non-recessed area occurs between the pair of recesses; a switching cylinder positioned within said housing with a first end adjacent the first end plate and a second end adjacent the second end plate and rotatable about an axis of rotation parallel to the said sides of the housing, said switching cylinder having a slot extending radially inward between its first end and second end; a rectangular plate having one edge fixedly inserted into the slot in the switching cylinder, said rectangular plate being permanently magnetized in a direction parallel to its inserted edge; a first spring plat-e having one edge inserted into the slot in the switching cylinder and having its opposite edge formed into a plurality of outwardly extending arms; a second spring plate having one edge inserted into the slot in the switching cylinder on the opposite side of the rectangular plate from the first spring plate and having its opposite edge formed into a plurality ofout-ward-ly extending arms; a plurality of movable electrical contacts fixedly positioned at the extending ends of the arms of the first 'spring'plate and of the second spring plate and positioned to engage the stationary contacts upon rotation of the switching cylinder about its axis of rotation; a pair of first blocks fixedly positioned in the corresponding recesses of the first end plate and magnetized in the same direction as the rnovable magnet is magnetized whereby the non-recessed area between the recesses becomes a non-magnetized gap between the blocks, said gap being of substantially the same width as the adjacent polarized face of the movable magnet, and a pair of second blocks fixedly positioned in the corresponding recesses of the second end plate and magnetized in the same direction as the movable magnet is magnetized whereby the non-recessed area between the recesses becomes a non-magnetized gap between the blocks, said gap being of substantially the same width as the adjacent polarized face of the movable magnet.

' 3. Anelectrical switch comprising:

(A) a pair of parallel spaced-apart sides, each side having a set of stationary electrical contacts secured thereto,

(B) an actuator located between the sides and pivotally mounted on an axis parallel to the sides, the actuator comprising;

(1) apivotal support,

(2) a magnet mounted on the pivotal support parallel to and extending radially outward from the pivotal axis, the magnet being magnetized in a direction parallel to the pivotal axis there by providing two opposite faces of different polarity,

(3) and a pair of electrical contactors mounted on the pivotal support parallel to the pivotal axis and adjacent each of the faces of the magnet which are parallel to the magnetization, each electrical contactor having a plurality of arms extending radially outward from the pivotal axis and corresponding in number to the stationary electrical contacts secured to the sides,

(C) a first wall extending perpendicularly between the sides and communicating with one edge of each, the

first wall lying adjacent a first polarized face of the actuator magnet,

(D) and a pair of magnetic members secured to the first wall adjacent its end portions which communicate with the edges of the sides, the magnetic members being separated from each other by a non-magnetic area having substantially the same width as the adjacent first polarized face of the actuator magnet, and each magnetic member being magnetized in a direction perpendicular to the first wall. v

4. An electrical switch according to claim 3 wherein;

(A) a second wall is provided extending perpendicularly between the edges of the sides opposite to the first wall, the second wall lying adjacent a second polarized face of the actuator magnet;

(B) and a pair of magnetic members secured to the second wall adjacent its end portions which communicate with the edges of the sides, the magnetic members being separated from each other by a nonmagnetic area having substantially the same width as the adjacent second polarized face of the actuator magnet, and each magnetic member being magnetized in a direction perpendicular to the second wall.

5. An electrical switch according to claim 3 wherein the actuator is pivotally mounted at a point midway along its length and the magnetic members, which are secured to the first wall, are interconnected in an area adjacent the pivotal mounting of the actuator to form a-single H-shaped magnetic member.

6. An electrical switch according to claim 3 wherein the actuator is pivotally mounted at one of its ends and the magnetic members, which are secured to the first wall, are interconnected in an area adjacent the pivotal mounting of the actuator to form a single U-shapedmagnetic member. i 7. An electrical switch according to claim 3 wherein the direction of magnetization of the pair of magnetic members secured to the first wall is opposite to the directionof magnetization of the actuator magnet.

8. An electrical switch according to claim 3 wherein the direction of magnetization of the pair of magnetic members secured to the first Wall is the same as the direction of magnetization of the actuator magnet.

9. A gang-type electrical switch assembly comprising a plurality of switches as defined in claim 4 wherein the first wall of one switch lies in face-to-face relationship with the second wall of the adjacent switch.

References Cited by the Examiner UNITED STATES PATENTS 2,666,112 1/1954 Nelson 200-87 2,793,265 5/1957 Crissinger ZOO-67 2,853,576 9/1958 Tigerohiold ZOO-"67 BERNARD A. GILHEANY, Primary Examiner. I. J. BAKER, Assistant Examiner.

Claims (1)

1. AN ELECTRICAL SWITCH COMPRISING A HOUSING HAVING A SIDE WITH TWO NOTCHES EXTENDING INWARDLY FROM AN EDGE; TWO STATIONARY CONTACTS EACH HAVING A SEGMENT OF REDUCED WIDTH AND HAVING ITS SEGMENT OF REDUCED WIDTH INSERTED INTO THE NOTCHES IN THE SIDE OF THE HOUSING; A FIRST END PLATE AT AN END OF SAID SIDE AND HAVING A RECESS IN ITS INNER SURFACE; A SECOND END PLATE AT THE OPPOSITE END OF SAID SIDE FROM THE FIRST END PLATE AND HAVING A RECESS IN ITS INNER SURFACE; A SWITCHING CYLINDER POSITIONED WITHIN SAID HOUSING WITH A FIRST END ADJACENT THE FIRST END PLATE AND A SECOND END ADJACENT THE SECOND END PLATE AND ROTATABLE ABOUT AN AXIS OF ROTATION PARALLEL TO THE SAID SIDES, SAID SWITCHING CYLINDER HAVING A SLOT EXTENDING INWARDLY; A RECTANGULAR PLATE HAVING ONE EDGE FIXEDLY INSERTED INTO THE SLOT IN THE SWITCHING CYLINDER, SAID RECTANGULAR PLATE BEING PERMANENTLY MAGNETIZED IN A DIRECTION PARALLEL TO ITS INSERTED EDGE; A SPRING PLATE HAVING ONE EDGE INSERTED INTO THE SLOT IN THE SWITCHING CYLINDER AND HAVING TWO OUTWARDLY EXTENDING ARMS AT ITS OPPOSITE EDGE; A MOVABLE ELECTRICAL CONTACT FIXEDLY POSITIONED AT THE EXTENDING END OF EACH OF THE SAID ARMS OF THE SPRING PLATE AND POSITIONED TO ENGAGE THE SAID STATIONARY CONTACTS UPON ROTATION OF THE SWITCHING CYLINDER ABOUT ITS AXIS OF ROTATION; AND A FIRST BLOCK FIXEDLY POSITIONED IN THE RECESS OF THE FIRST END PLATE AND PERMANENTLY MAGNETIZED IN THE SAME DIRECTION AS THE MOVABLE MAGNET IS MAGNETIZED; AND A SECOND BLOCK FIXEDLY POSITIONED IN A RECESS OF THE SECOND END PLATE AND ALSO PERMANENTLY MAGNETIZED IN THE SAME DIRECTION AS THE MOVABLE MAGNET.

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