US3192340A

US3192340A - Center pivoted armature electro-magnetic switching mechanism

Info

Publication number: US3192340A
Application number: US265408A
Authority: US
Inventors: Aime J Grenier
Original assignee: Texas Instruments Inc
Current assignee: Texas Instruments Inc
Priority date: 1963-03-15
Filing date: 1963-03-15
Publication date: 1965-06-29
Anticipated expiration: 1982-06-29

Description

June 29, 1965 I A. J. GRENIER 3,192,340

CENTER 'PIVOTED ARMATURE ELECTROMAGNETIC SWITCHING MECHANISM Filed March 15, 1963 5 Sheets-Sheet 2 sq f FIG. 7 4! $5 L. 58 1/05- I m I I 06' 6 [08 IL a a x- V2 IA/ raw-am,

' Amid. a: ,yww 4775'.

June 29, A GRENIER CENTER PIVOTED ARMATURE ELECTROMAGNETIC SWITCHING MECHANISM Filed March 15, 1963 5 Sheets-Sheet 3 ANS INVE'IVTOR, I 4/! 1/, ER

United States Patent 3,192,340 CENTER PIVOTED ARMATURE ELECTRO- MAGNETIC SWITCHING MECHANISM Aime J. Grenier, North Attleboro, Mass., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed Mar. 15, 1963, Ser. No. 265,408 11 Claims. (Cl. 200-87) This invention relates to new and improved electrical switches and more particularly to a new and improved rela A inong the objects of the invention is the provision of a switch which is compact, simple in design, capable of operation over a relatively Wide range of operating parameters, and which is easily manufactured, assembled into finaloperative condition and calibrated.

Another object of the invention is the provision of a switch which is relatively insensitive to position orientation.

A further obg'ect of the invention is the provision of a switch which is substantially unaffected by vibration.

It is a further object of the invention to provide an improved switch which provides for contact wiping and which minimizes contact chatter and arcing.

Other objects will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the elements and combinations of elements, features of construction, and arrangements of parts which 'will be exemplified in the structures hereinafter described, and the scope of the application of which will be indicated in the following claims.

In the accompanying drawings, in which one of the various possible embodiments of' the invention is illustrated:

FIG. 1 is a top plan view of an electrical switch embodying the present invention;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a side elevational view thereof;

FIG. 4- is a bottom plan view thereof;

FIG. 5 is a top plan view of the switch shown in FIG. 1 with the cover removed and with the switch rotated 180 from the position shown in FIG. 1;

FIG. 6 is a sectional view taken on line 66 of FIGS. 5 and 7;

FIG. 7 is a view similar to FIG. 5 with the electromagnet subassembly removed for clarity of illustration;

FIG. 8 is a fragmentary view similar to FIG. 7 showing the switch parts in a contacts-closed position and Patented June 29, 1965 ice ing bracket 16 having

openings

17, 18, for receiving suitable mounting fasteners (not shown).

As best seen in FIGS. 1 and 2, cover 14 has a boss or projection 20 having an opening 21 extending throughout its length. Boss 20 also has an extension (not shown) which projects into the interior of base 12 and is received in a cavity 22 in base 12 which is best seen in FIGS. 5 and 7. Cavity 22 has an opening 24 in register with opening 21 in boss 20. Aligned with

openings

21 and 24 is an opening 26 in bracket 16. Surrounding opening 26 is a depressed or dimpled portion 27 which mates with a correspondingly shaped depression (not shown) in base 12 to facilitate alignment of the cover, base and bracket. A single fastening means, for example, in the form of a bolt or screw 28 is inserted through

openings

21 and 24 and threadably engaged in opening 26 in bracket 16 which tightly secures cover 14, base 12 and bracket 16 together. v

As best shown in FIGS. 1, 2, 5 and 7, switch 10 includes three electrically

conductive terminals

33, 34 and 35,

v having portions extending exteriorly of switch 10 rematerial such as S.A.E. No. 1010 cold rolled steel.

of illustration;

FIG. 9 is an end view of the electromagnet subassembly shown in FIG. 5;

FIG. 10 is an enlarged fragmentary view of a portion of the switch shown in FIG. 6;

FIG. 11 is a partly schematic and partly pictorial view of an exemplary circuit diagram with which the switch shown in FIGS. 110 may be used.

Similar reference characters indicate corresponding parts throughout the several views of the drawings.

Dimensions of certain of the parts asshown in the drawings may have been modified and/or exaggerated for the purposes of clarity of illustration.

Referring now to FIGS. 14,, an electrical switch embodying the instant invention takes the form of an electromagnetic relay generally indicated by the reference number 10, and includes a housing or casing comprising a base 12 and a cover 14 made of a suitable insulating material such as, for example, arc-resistant moldable phenolic resinous material.

Switch 10 may be provided with 7, suitable mounting means such as, forexample, a mountspectively through openings 30, 31 and 32 in cover 14.

Terminals

33, 34 and 35 are slidably received and tightly and frictionally held or nested in suitably formed slots in base 12, as best seen in FIGS. 5-7.

Referring to FIGS. 5, 6 and 9, switch 10 includes an electromagnet, generally designated by reference numeral 42 for actuating an armature assembly, generally designated by reference numeral 44. The electromagnet 42 includes a retaining spool 45 formed of a suitable flexible electrically insulating material such as, for example, nylon. Disposed about the spool in the exemplary manner shown in FIGS. 5 and 6 is an electromagnet winding 46 which may be formed of wire of desired diameter and with the desired number of turns to provide a switch of either the current or potential types. The winding is formed of a suitable electrically conductive material such as, for example, copper and forms the coil of the electromagnet 42.

The spool has an opening 48 (FIG. 9) of substantially rectangular'cross section extending in a lengthwise direction throughout the entire length of the spool. As best seen in FIG. 6, opening 48 receives the

leg portions

50, 52, of two L-shaped members having a substantially rectangular cross section and being formed of a suitable magnetic The

leg portions

50, 52, of the L-shaped members are received in opening 48 in a close overlapping relationship to form the core of electromagnet 42. Leg

portions

54, 56, of the L-shaped members extend away from the spool and core to form the poles of electromagnet 42 and are slidably and tightly received in suitably formed slots or guideways 58 provided by base 12. The spool and the L-shaped members have dimensions such that when cover 14 is placed on base 12, the cover abuts the top portion 53 of spool 45. Thus, when fastening means 28 is tightened, the flexible spool 45 is compressed slightly to maintain L-shaped

member leg portions

50, 52, in a tightly held overlapping relationship within opening 48 and to tightly maintain

leg portions

54, 56, within guideways 58 and to maintain the electromagnet assembly 42 in fixed assembled relation with the switch casing.

As best seen in FIGS. 5 and 9, end of winding 46 is electrically connected as by soldering or welding to terminal 35. A slot 122 is provided in the end portion 123 of spool 45 in which lead 120 is disposed to prevent loosening or unraveling of winding 46. The other end 124 of winding 46 extends through a slot (not shown) in end 125 of spool 45 and is electrically connected to terminal 33. An advantage of the electromagnet assembly is that it can be mass produced with

terminals

33 and 35 as a separate assembly which can be quickly and economically assembled into the switch.

As best seen in FIGS. 6-8, armature assembly 44 includes an armature 60 mounted for rotation about the longitudinal axis of a post 62 mounted in base 12. Armature 60 is formed of iron and has cantilever mounted thereon, a contact arm generally designated at numeral 63 which will be described in greater detail below. Arm 63 at its free end 105 mounts an electrical contact 64 for engagement with a stationary electrical contact 66 mounted on terminal 35 as shown in FIGS. 7 and 8. Energization of coil 46 of electromagnet 42 to a predetermined amount will cause rotation of armature 60 in a clockwise, contacts-closing direction (as seen in FIGS. 7, 8 and 11).

As best seen in FIGS. 6 and 10, the armature assembly 44 also includes a resilient spring means 68 in the form of a coil spring for biasing the armature for movement in a counterclockwise, contact-opening direction as seen in FIGS. 7 and 8. As best seen in FIG. 10, one end 72 of spring 68 is seated in an annularly extending notch or recess 73 in a flange 70 formed integrally with and at one end of post 62. Notch 73 renders flange 70 cup-shaped for seating end 72 of spring 68. Flange 70 also provides a slot 76 in the wall thereof which communicates with annular recess 73. End portion 74 of spring 68 extends through slot 76 and is bent as at 75 (see FIG. 7)

to anchor end portion 74 to the flange 70 and post 62.

The other end 80 of spring 68 is seated in annularly extending notch or recess 77 in a bushing 78 which is formed separately from and telescopically receives post 62 in an opening 79 therein (FIG. 10). Notch or recess 77 renders bushing 78 cup-shaped for seating end 80 of spring 68. Post 62 and cup-shaped bushing 78 are formed of an electrically insulating material such as, for example, nylon or tetrafiuoroethylene polymer. Nylon is a par ticularly desirable material for this purpose because of its good bearing properties which minimizes friction between bushing 78 which bears against and rides on armature 60.

An extension or end portion 82 of end 80 of spring 68 passes through a slot 84 in bushing 78 which communicates with notch 77. End portion 82 abuts a projection or pin 86 in armature 60. Rotation of post 62 in a counterclockwise direction (as viewed in FIG. 7) will stress spring 68 to provide a biasing force tending to rotate armature 60 in a counterclockwise, contactsopening direction. This biasing force can be varied (either decreased or increased) by rotating post 62 respectively in either a clockwise or counterclockwise direction as viewed in FIG. 7. This calibrates the switch 10 because it determines the strength of the electromagnetic force (produced by electromagnet 42) required to rotate armature 60 in a clockwise or contacts-closing direction (as viewed in FIG. 7) against the biasing force of spring 68.

Post 62 includes three diiferent diameter portions 62a, 62b and 620, as best seen in FIG. 10. Portions 62a, 62b and 620 are respectively received in apertures 79 (of bushing 78), 88 (provided by armature 60) and 98 (provided by base 12). The junction between portions 62a and 62b and between portions 62b and 62c provide annularly extending shoulders which respectively abut raised portions on armature 60 and base 12 as best seen in FIG. 10. Portion 620 also extends exteriorly of base 12 to permit calibration to be effected externally of the switch 10. In this regard a conventional spring fingered nut or fastener 92 which may be of the so-called Tinnerman type is applied aboutthe exterior end of portion 620. Fastener 92 is received in a suitably formed recess 93 in the exterior of base 12. Fastener 92 permits rotation of post 62 for calibration purposes. After the post has been rotated to the desired adjusted position fastener 92 exerts a pressure against post 62, to maintain the latter in the adjusted position. Thereafter the opening or re 4 cess 03 is infilled through an aperture 96 in bracket 16 with a thermosetting resinous material or potting compound 94 to permanently seal and maintain the post and fastener in fixed adjusted position.

As best seen in FIGS. 7 and 8, the contact arm 63 comprises an elongated spring member generally indicated at 100 which is cantilever mounted at one end 101 on armature 60 by fastening means, for example, in the form of a rivet 102 passing through bent tab portion 104 formed integrally with contact arm 63. Contact arm 63 is formed of a suitable electrically conductive spring material such as beryllium copper. Movable contact 64 is electrically connected and mounted on free end 105 of member 100. Contact arm 63 has a second integrally formed bent tab portion 106 extending transversely from end 105 of member 100 and overlying armature 60. Tab 106 provides window opening 108, through which the armature projection or pin 86 projects. Upon closing of

contacts

64 and 66, spring member 100 is stressed and urges or biases contact 64 against contact 66 which contributes to provide desired contact pressure. postioned to abut or engage tab 106 in the contact-open position as shown in FIG. 7 to act as a stop to prevent undesirable contact engagement which might otherwise result when switch 10 is subjected to vibration and shock.

Spring member 100 has a transverse bend 110 intermediate its

ends

101, 105 to form first and

second segments

112, 114. Segment 112 includes a projection 113 positioned for abutting engagement with armature 60. Pigtail 126 is electrically connected at one end as at 128 to an upstanding portion of tab 104 as best seen in FIG. 6 and is electrically connected at its other end to terminal 34 as best seen in FIG. 7.

Operation of relay 10 is as follows: When coil 46 is de-energized,

contacts

64, 66 are normally open because spring 68 biases armature 60 to a contacts-open position I as shown in FIG. 7 and in broken lines in FIG. 8. When coil 46 is energized sufficiently to create an electromagnetic force sufficient to overcome the bias of spring 68, armature assembly 42 will move in a contacts-closing direction. Upon initial engagement of

contacts

62 and 64, as armature 60 moves in the contacts-closing direction,

segments

112 and 114 of spring member 100 will pivot as a unit about cantilever mounted end 101 whereby substantially the full length of member 100 acts as a relatively weak or soft spring.

Segments

112 and 114 will continue to pivot as a unit about end 101 until bend 110 moves into engagement with armature 60. Thereafter on further movement of armature 60 in the contacts-closing direction, only segment 112 will pivot about bend 110 toward armature 60 until abutment 113 moves into engagement with armature 60. Continued movement of armature 60 in the contacts-closing direction after engagement of abutment 113 and armature 60 will result in pivotal movement of the portion of segment 112 intermediate end 105 and abutment 113 in a direction toward armature 60. Segment 112 in pivoting about bend 110 or abutment 113 acts as a relatively stiff member, it being understood that the shorter the length of the spring, the stiffer the spring member acts. In effect, spring 100 acts a triple rate spring advantageously acting as a relatively soft spring on initial contact engagement to minimize contact chatter and/or bounce and progressively acts as a stiifer spring advantageously to cushion the shock of the armature striking against the contact arm when the armature follows through in its rotating movement in the contacts-closing direction after initial contact engagement. After initial contact engagement, continued movement of armature 60 in the contacts-closing direction advantageously results in relative movement between engaged

contacts

64 and 66 which provides a contact Wiping action.

When coil 46 is de-energized, or when its energization falls below a predetermined amount or current value (if it is of the current type), armature 60 will move in a Pin 86 is.

counterclockwise, contacts-opening direction under the bias of spring 68 to return the parts from the solid line FIG. 8 contacts-closed position to the FIG. 7 contactsopen position.

FIG. 11 shows an exemplary circuit diagram using switch as a motor starting relay with a motor having a main or run winding R and a starting winding S One end of winding R is connected to terminal 33 by conductor 200. The other end of R has a common connection with one end of winding S and is connected by conductor 202 with L one side of a power source. The other end of winding S is connected by conductor 204 with terminal 34. Terminal 35 is electrically connected with the other side L of the power source by conductor 206. When the circuit is energized, current will initially flow from L to end 120 of coil 46, serially through coil 46 to end 124 thereof to terminal 33, through conductor 200, through winding R to conductor 2tl2 and L When power is first applied to the motor to run winding R initially draws a relatively large inrush of current. This current, initially drawn by the run winding R energizes elect-romagnet 42 to rotate armature 60 in a contactsclosing direction. Closing of

contacts

64 and 66 connects the motor start winding S into the circuit. Current then also flows from L through conductor 2% to terminal 35 to contact 66 to contact 64 through contact arm 63 to pigtail 128 to terminal 34 through conductor 2&4, to start Winding S through conductor 202 to L the other side of the power source. As the motor gathers speed, a back Voltage is generated in the run winding R with the result that the current passing through the run winding and coil 46 decreases to the extent that the biasing force exerted by spring 68 on armature 6t) exceeds the electromagnetic force exerted by electromagnet 42 and urges the armature 60 in a contacts-opening direction to quickly disconnect the start winding S Thereafter current will continue to flow only in the run winding circuit until the motor is de-energized.

It will be seen from the foregoing that this invention provides a switch having a number of advantages which include the advantage of being substantially non-position sensitive and relatively insensitive to the effects of vibration and shock; the unique multiple rate spring action afforded by the contact arm which inhibits contact chatter as described above; a construction which is simple to assemble wherein final assembly is achieved by means of the single fastener 28 which holds the cover, base, mounting bracket, and all internal parts in place, with the sole exception of the armature mounting post 62 which is held by nut 92 and potting 94; the advantage of contact wiping action for maintaining clean contacts, reducing contact resistance and inhibiting contact sticking; and

the advantage of simple calibration which may be effected externally of the switch casing.

In view of the above it will be seen that the several objectsof the invention are achieved and other advantageous results are obtained.

It is to be understood that the invention is not limited in its application to the details of construction and arrangement of parts illustrated in the accompanying drawings, since the invention is capable of other embodiments and of being practiced or carried out in various ways. Also, it is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.

As many changes could be made in the above constructions without departing from the scope of the in vention, it is intended that all matter contained in the above descripion or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense, and it is also intended that the appended claims shall cover all such equivalent variations as come within the true spirit and scope of the invention.

I claim:

1. A switch comprising a housing; an armature in said housing; means mounting said armature for rotary movement in each of two opposite directions; said means comprising a post mounted in said housing and having a portion thereof disposed in an aperture in said armature; said switch including magnetic means associated with said armature for causing movement of the latter in one of said directions; a pair of electrical contacts mounted in said housing, one of said contacts being movable into and out of engagement with the other of said contacts in response to movement of said armature; resilient means comprising a spring, one end of which is connected to said post and the other end of which abuts a portion of said armature for biasing said armature for movement in the other of said directions; said post being movable relative to said other end of said spring to an adjusted posi tion for adjustably varying the bias exerted against said armature; and means for maintaining said post in said adjusted position.

2. The switch as set forth in claim 1 and wherein said post includes a cup-shaped flange which seats one end of said spring and is connected with a portion thereof, and a cup-shaped bushing member disposed intermediate said flange and armature, which seats the other end of said spring, said bushing member having an opening through which a portion of said other end of said spring extends for abutting engagement with said armature portion.

3. The switch as set forth in claim 2 and wherein said post and bushing member are formed of anelectrically insulating material such as a tetrafiuoroethylene polymer or the like.

4. The switch as set forth in claim 1 and wherein said magnetic means comprises an electromagnetic coil and said post includes a portion extending exteriorly of said housing whereby movement of said post to adjusted position can be effected exteriorly of said housing.

5. A switch comprising a housing; a movably mounted armature; magnetic means associated with said armature for causing movement of the latter; a pair of electrical contacts, one of which is movable into and out of engagement with the other of said contacts in response to movement of said armature; said magnetic means comprising an electromagnetic winding disposed about a spool member; said spool having an opening extending longitudinally therethrough; a pair of substantially L-shaped magnetic members each having one leg thereof positioned in overlapping relationship within said spool member opening to form a core; the other leg of each of said L-shaped members extending in a direction away from said spool member and forming pole pieces, said housing comprises a base member interconnected with a cover member; said pole pieces being slidably received in slot portions provided by said base member; said spool member being formed of a flexible material and said cover member abutting said spool member to maintain said spool and L- shaped member in a relatively tight fitting interengagement to prevent relative movement therebetween.

6. A switch comprising first and second electrical con tacts, one of which is movable into and out of engagement with the other of said contacts; and armature movable in a first direction for eitecting contact engagement and in a second direction opposite to said first direction for eitecting contact disengagement; magnetic means for moving said armature in said first direction of predetermined electrical conditions; an electrically conductive spring member mounting said first contact and cooperating with said armature; and means mounting said spring member so that the member acts as a spring along a substantial portion of its length in positions of the armature effecting contact disengagement and as a relatively stiff spring member along a lesser portion of its length in positions of the armature effecting contact engagement.

7. A switch comprising first and second electrical contacts, one of which is movable into and out of engagement with the other of said contacts; an armature movable in a contacts-closing direction for effecting contact engagement and in a contacts-opening direction for effecting contact disengagement; magnetic means for moving said armature in said contacts-closing direction at predetermined electrical conditions; resilient means biasing said armaature for movement in said contacts-opening direction; an electrically conductive spring member mounting said first contact adjacent one end thereof and biasing said first contact for movement in a direction toward said second contact; said spring member adjacent the other end thereof being cantilever mounted on said armature; said spring mem er having a transverse bend intermediate its ends to provide first and second interconnected segments, said first segment carrying said first contact and said second segment being cantilever mounted on said armature; an abutment positioned intermediate said first segment and armature; said first and second segments pivoting as a unit about said cantilever mounted end when said armature moves a first predetermined amount in said contactsclosing direction and further movement of said armature in said contacts-closing direction beyond said first predetermined amount resulting in pivotal movement only of said first segment about said transverse bend until said first segment moves into abutting engagement with said armature through said abutment.

8. The switch as set forth in claim 7 wherein said first segment of said spring member has a bent tab member having a portion thereof engageable with said armature for preventing contact engagement except at said predetermined electrical conditions and said first segment also mounting said abutment.

9. A switch comprising first and second electrical contacts, one of which is movable into and out of engagement with the other of said contacts; an armature movable in a contacts-closing direction for effecting contact engagement and in a contacts-opening direction for effecting contact disengagement; magnetic means for moving said armature in said contacts-closing direction at predetermined electrical conditions; resilient means biasing said armature for movement in said contacts-opening direction; an electrically conductive spring member mounting said first contact adjacent one end thereof and biasing said first contact for movement in a direction toward said second contact; said spring member adjacent the other end thereof being cantilever mounted on said armature; said spring member having a transverse bend intermediate its ends to provide first and second interconnected segments, said first segment carrying said first contact and said second segment being cantilever mounted on said armature; said first and second segments pivoting as a unit about said cantilever mounted end when said armature moves in said contacts-closing direction to cause initial engagement of said contacts, and upon further movement of said armature in said contacts-closing direction said first and second segments will continue to pivot as a unit until said bend moves into abutting engagement with said armature whereby only said first segment will pivot about said bend in response to continned movement of said armature in said contacts-closing direction.

10. The switch as set forth in claim 9 and wherein movement of said armature in said contacts-closing direction after initial contact engagement results in relative movement between said contacts to provide a Wiping action therebetween.

11. A switch comprising first and second electrical contacts, one of which is movable into and out of engagement with the other of said contacts; an armature in said housing; means mounting said armature for rotary movement in a contactsclosing direction for effecting contact engagement and in a contacts-opening direction for reflecting contact disengagement; magnetic means for moving said armature in said contacts-closing direction at predetermined electrical conditions; said armature mounting means comprising a post mounted in said housing and having a part thereof disposed in an aperture in said armature; resilient means interconnected at one end with said post and at its other end abutting a pnojection on said armature for biasing said armature for movement in said contactsopening direction; an electrically conductive spring member mounting said first contact adjacent one end thereof; said spring member adjacent the other end thereof being cantilever mounted on said arma-ture; said spring member having a transverse bend intermediate its ends to provide first and second interconnected segments, said first segment carrying said first contact and said second segment being cantilever mounted on said armature; said first and second segments pivoting as a unit about said cantilever mounted end when said armature moves a first predetermined amount in said contacts-closing direction to cause initial engagement of said contacts and movement of said bend into abutting engagement with said armature; and further movement of said armature in said contacts-closing direction beyond said first predetermined amount resulting in pivotal movement only of said first segment about said transverse bend and also resulting in relative movement between said engaged contacts to provide a wiping action therebetween; said first segment having a bent tab adjacent said first contact; said bent tab having a window opening in which is received an armature projection; said tab being engageable with said projection to limit movement of said first segment relative to said armature in a direction away from said armature and toward said second contact.

References flied by the Examiner UNITED STATES PATENTS BERNARD A. GILHEANY, Primary Examiner.

ROBERT I. SCHAEFER, Examiner.

Claims

1. A SWITCH COMPRISING A HOUSING; AN ARMATURE IN SAID HOUSING; MEANS MOUNTING SAID ARMATURE FOR ROTARY MOVEMENT IN EACH OF TWO OPPOSITE DIRECTIONS; SAID MEANS CONPRISING A POST MOUNTED IN SAID HOUSING AND HAVING A PORTION THEREOF DISPOSED IN AN APERTURE IN SAID ARMATURE; SAID SWITCH INCLUDING MAGNETIC MEANS ASSOCIATED WITH SAID ARMATURE FOR CAUSING MOVEMENT OF THE LATTER IN ONE OF SAID DIRECTIONS; A PAIR OF ELECTRICAL CONTACTS MOUNTED IN SAID HOUSING, ONE OF SAID CONTACTS BEING MOVABLE INTO AND OUT OF ENGAGEMENT WITH THE OTHER OF SAID CONTACTS IN RESPONSE TO MOVEMENT OF SAID ARMATURE; RESILIENT MEANS COMPRISING A SPRING, ONE END OF WHICH IS CONNECTED TO SAID POST AND THE OTHER END OF WHICH ABUTS A PORTION OF SAID ARMATURE FOR BIASING SAID ARMATURE FOR MOVEMENT IN THE OTHER OF SAID DIRECTIONS; SAID POST BEING MOVABLE RELATIVE TO SAID OTHER END OF SAID SPRING TO AN ADJUSTED POSITION FOR ADJUSTABLY VARYING THE BIAS EXERTED AGAINST SAID ARMATURE; AND MEANS FOR MAINTAINING SAID POST IN SAID ADJUSTED POSITION.