US3903777A

US3903777A - Electromagnetic valve for wind-pipe organs

Info

Publication number: US3903777A
Application number: US381084A
Authority: US
Inventors: George Douglas Corbett
Original assignee: Individual
Current assignee: Individual
Priority date: 1971-04-07
Filing date: 1973-07-20
Publication date: 1975-09-09
Anticipated expiration: 1992-09-09

Abstract

A pallet operator for a valve of a wind-pipe organ comprises a solenoid having an armature but no dead stop, and an air damper for damping the movement of the armature to reduce pallet acceleration on valve opening and reduce pallet bounce on valve closure. In one example, the armature is a piston which is a sliding fit in a cylinder closed by a non-magnetic part to provide a piston and cylinder damper.

Description

United States Patent Corbett 1 Sept. 9, 1975 1 ELECTROMAGNETIC VALVE FOR 1.741196 1/1930 Dugzin .4 335/257 l,767,738 6/1930 Brown..... 84/338 2,167,407 7/1939 Marr 84/339 [76] Inventor: George Douglas Corbett, 29 Chantry 2 190 50 2 1940 Carew ct a1, 84/339 Hurst, Epson, Surrey, England 2,484,930 10/1949 Cornelius 84/337 3,071,664 1/1963 Priesemuth .r 335/61 [22] F11ed: July 20, 1973 [21] Appl 381384 Primary Examiner-Joseph W. Hartary Related US. Application Data Assistant Examiner-U. Weldon [63] Continuation of Ser. No. 239,650, March 30, 1972, Afmmeyv 8 F Dav. Miller abandoned.

Mosher [30] Foreign Application Priority Data Apr, 7, 1971 United Kingdom r. 8999/71 [57] ABSTRACT Sept. 29, 1971 United Kingdom 45452/71 A pauet Operator for a valve of a p p Organ comprises a solenoid having an armature but no dead 2? 84/341 l0i 3i3 stop, and an air damper for damping the movement of q i 339 335 the armature to reduce pallet acceleration on valve opening and reduce pallet bounce on valve closure. In 84/341 335/257 one example, the armature is a piston which is a sliding fit in a cylinder closed by a non-magnetic part to References Cited provide a piston and cylinder damper,

UNITED STATES PATENTS 20 Claims, 7 Drawing Figures 899,598 9/1908 Howard 335/257 I 1 l 79 L) g 3 V =1 PATENTEDSEP 91975 39031777 PATENTEDSEP 9191s SHEET 3 BF FIGS ELECTROMAGNETIC VALVE FOR VVIND-PIPE ()RGANS This is a continuation of application Ser. No. 239,650, filed Mar. 30, 1972, now abandoned.

BLACKGROUND OF THE INVENTIGN This invention relates to electromagnetic pallet operators for wind-pipe organs.

Such operators comprise a solenoid having an armature coupled to a pallet for causing the pallet to open and close an aperture which provides communication between an organ wind chest or sound box and an organ pipe. Energisation of the solenoid withdraws the pallet and a spring is provided to return the pallet to its seating.

Such solenoids do not have a so-called dead stop to avoid noise in operation. A dead stop is often provided in a solenoid for other applications and is a normally magnetic sometimes adjustable member in the path of the armature to give an optimum pull on the armature. For obvious reasons, such a solenoid without a dead stop will be referred to herein as a magnetically-openended solenoid.

However, there is some reservation in the use of such operators because it is difficult to obtain the required slow acceleration of the pallet on opening of the aperture for proper speech from the pipe, whilst the pallet tends to bounce on its return to its seating.

SUMMARY OF INVENTION The invention is based on my discovery that a better pallet acceleration and minimum pallet bounce can be achieved by the addition of an air damper to the solenoid. Thus, according to the invention, there is provided an electromagnetic pallet operator suitable for a pallet valve ofa wind-pipe organ, the operator comprising: a solenoid without a dead stop; an armature of said solenoid for displacing a pallet; and air damping means able to damp said armature to an extent reducing pallet acceleration on opening of a pallet valve and substantially preventing bounce on return of the pallet to its seating.

When the operator is to be used on an orientation such that gravity will act on the armature and pallet to oppose the closing of the valve, the operator will have a return spring. By virtue of the damping means, this spring need to be strong enough merely to counteract gravity and to hold the pallet onto its seating. This is to be contrasted with conventional organ pallet operators which require a relatively powerful spring in an attempt to overcome bounce.

The air damping means comprises a piston and cylinder arrangement. In one embodiment, the cylinder is a tube encircled by the solenoid winding. In another case, the cylinder is part of a magnetic circuit having an energising winding which does not need to encircle the cylinder. The piston may be provided by the armature itself. In any case, the pallet may be secured to the armature so as to be coaxial with it to provide the pallet with a rectilinear action.

The damping means will have a degree of leakage depending upon the extent of damping required. As will be readily apparent, the leakage may be between the said piston and cylinder, preferably made for this purpose as a precision sliding fit with each other. An adjustable leakage or bleed could be provided to adjust the degree of damping. In that case, leakage between said piston and cylinder is not essential so that a sealing ring could then be positioned between the piston and cylinder. However, such leakage is preferred as it pro vides, in effect, an air bearing between the piston and cylinder to reduce friction and noise.

In a development, a second solenoid is provided normally fully, or at least partially, to close a bleed aperture of the damping means to achieve the degree of damping aforementioned. However, that second solenoid may be so operated as to open the aperture to reduce the damping appreciably, preferably substantially to zero, to achieve a modified tone from the organ pipe, eg for staccato playing. The second solenoid may thus be operated by a key contact of an organ manual only when the key is displaced with a certain degree of acceleration. For example, the second solenoid may be operated by a secondary contact of the key, which contact is operated after the normal contact so that the second solenoid is substantially inefi'ective in normal,

non-staccator, use.

In a further embodiment, the damping means may be constructed a) as to be able to damp the armature during an opening stroke whilst providing a lesser degree of damping during the initial stage of a closing stroke and an increased degree of damping during the final stage of the closing stroke when the pallet would normally accelerate owing to suction on the pallet. For example, the operator may comprise a member for connection to a pallet, this member being loosely coupled to the armature so that it can be driven by the armature yet carry out a movement relative to the armature in the operating direction, and the damper comprising a valve connected to be operated by the relative movement of the member and armature so as to be substantially closed on movement of the armature in a direction to open a pallet valve and to be opened, to reduce damping, when the armature reverses to the opposite direction of movement, the valve operating towards its closure position to increase damping if the member accelerates in said opposite direction relative to said armature.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

FIG. 1 is a cross-section of an electromagnetic pallet operator for a wind-pipe organ;

FIG. 2 is a cross-section of a portion of a wind-pipe organ;

FIG. 3 is a cross-section of a modified operator;

FIG. 4 is a cross-section of another embodiment of operator;

FIG. 5 is a view of a modification which can be applied to any of the operators described herein;

FIG. 6 is a cross-section of yet another embodiment of pallet operator in combination with a pallet valve shown in the closed position; and

FIG. 7 is a cross-section taken at right angles to that of FIG. 6 and showing the pallet in a withdrawn position.

FIG. 1 is a cross-section of an electromagnetic operator l for a pallet valve of a wind-pipe organ a portion of which is shown in FIG. 2, FIG. 2 including a plurality of such operators 1.

Each operator carries a pallet 2 arranged to engage a valve seating 3 of a sound board 30 (FIG. 2) surrounding a vlave outlet 5 which, when opened, admits air under pressure from a wind chest 4 to an associated organ pipe connected to its valve outlet 5. Each pallet 2 comprises a supporting washer 6, an optional felt pad 7 and a sheepskin washer 8.

Each operator comprises a solenoid having a winding 9 and an armature 10, the armature carrying the associated pallet 2. The armature is substantially a precision sliding fit in a brass cylinder 11 on which the winding 9 is supported with the aid of a metal frame 12 by which the operator is attached to the chest 4. The cylinder 11 is fitted with a non magnetic plug 13 having a bleed opening 14 operable to the outside of the cylinder I 1 via an aperture 15. A screw 16 enables the effective aperture size of the bleed opening 14 to be varied to achieve, in conjunction with the leakage about armature 10, the desired degree of damping which gives proper speech from a pipe by reducing the pallet acceleration on its withdrawal from the seating and which prevents bounce on return to the seating. In fact damping will exist throughout the movement of the pallet to prevent flutter during the intermediate movement of the pallet.

The pallet is acted upon by a spring 17 which is of just sufficient strength to hold the pallet 2 against its seating against the force of gravity.

The operator is connected to the wind chest 4 by the part 18 of the frame 12, the part 18 containing a slot 19 through which a securing screw will pass and which enables the position of the operator to be adjusted before the screw is tightened. The corners of the part 18 are bent over to dig into the wood of the chest to hold the operator in its finally selected position.

An antifriction sealing ring may be provided in a groove in the armature 10. But this is not necessary when the armature is a sliding fit in the cylinder 11 because the leakage about armature can then provide part or the whole of the required leakage and can also provide a thin film of flowing air to act as an air bearing to reduce friction and noise.

A plurality of such operators will be assembled in a chest, as illustrated for three operators in FIG. 2. A base member 20 is removable to allow the screws 16 to be adjusted.

A modification is shown in FIG. 3 in which like parts are given the same references.

The embodiment differs from the earlier in the provision of a second solenoid, having a winding 21 and armature 22, and an internal spring 17 acting between the armatures. For operation according to the embodiment of FIG. 1, the winding 21 is not energised and the cylinder 11 can bleed slowly through the gap between the armature l0 and the cylinder 11. However, the damping can be substantially reduced or eliminated, for staccato effects, by energising winding 21 to raise armature 22 to open the cylinder 11 to an aperture 24 via a groove in the armature 22. Normally, the armature 22 rests on a sealing member 25 to seal the aperture 24.

The winding 9 may, of course, be operated by a conventional contact of a key of an organ manual. On the other hand the winding 2] will be operated by a secondary contact acted upon after the conventional contact. The secondary contact will be operated in time to affect damping whenever the key is operated in a sharp, staccato, fashion, i.e. whenever a predetermined level of acceleration is imparted to that key.

If required, an adjustable bleed could be provided between the armatures to bleed air to and from the cylinder.

FIG. 4 shows substantially to scale a non-adjustable form of the pallet operator of FIG. 1 suitable for most wind-pipe organ applications and operable with an energising voltage of 15 or 24 volts, as are all the em' bodiments illustrated herein. In this case the leakage about the armature 10 is relied on and the armature is accordingly a precision sliding fit in its cylinder 1 l. The cylinder 11 is closed by a non-magnetic, plastics, plug 13 which is shielded by a non-magnetic cup 26.

FIG. 5 shows a further modification in which the frame is formed in two

separable parts

29 and 30 allowing the solenoid to be adjusted relative to a sound board. An adjusting screw 31 engages in a hole in frame part 29 and an adjusting slot in frame part 30. Pellets 32 held in part 29 extend into slots in part 30 to locate the frame parts laterally. In this case, as in the other embodiments, adjustment can also be made by screwthreaded adjustment of the position of the pallet relative to the armature, but in the case of FIG. 5, and in other cases, the pallet could be non-adjustably secured.

FIG. 6 is a cross-section of a further form of electromagnetic operator for a pallet 2 of a windpipe organ.

The operator comprises a solenoid having a winding 9 formed on a brass cylinder 11, and an armature 10 in the form of a tube. The armature is a precision sliding fit in the cylinder 11, on which the winding 9 is supported with the aid of a metal support or frame 12 by which the operator is attached relative to a sound board. The pallet 2 is coupled to the solenoid by way of a threaded rod or wire 33 on which are threaded two

plastic nuts

34 and 35 at opposite ends of the armature 10. The nuts 34 and 35 are not screwed tightly against the armature 10, thereby to allow a small movement of the rod 33 relative to the armature 10 in the axial direction.

A spring 36 acts on the lower end of the armature 10 to urge it upwardly into engagement with the nut 34, thereby to urge the pallet 2 against its seat. However, the nut 34 does not seal the end of the tubular armature I0 because of slots 37 provided at the upper end of the armature 10. Such slots do not exist at the lower end of the armature 10 so that, when the nut 35 contacts the surface 38 at the lower end of the armature 10, the interior of the armature 10 is effectively sealed from the zone 39 below the nut 35.

As already indicated, FIG. 6 illustrates the condition in which the valve is closed. In this condition, it will be seen that the zone 39 communicates with the atmosphere through the interior of the armature l0 and via the slots 37. When the coil 9 is energised so as to draw the armature l0 downwardly, the armature 10 is driven against the nut 35 and at the same time compresses the spring 36 and draws the pallet 2 away from its seating. This condition is illustrated in FIG. 7. As the surface 38 contacts the nut 35 under these circumstances, the zone 39 is sealed from the environment, apart from leakage past the armature, so that the armature 10 is damped in its movement away from the valve seating by the compression of the air in the zone 39.

When the solenoid is tie-energised, the armature 10 is pushed away from the nut 35 by the spring 36 to allow the air trapped in the zone 39 to escape through the armature 10 to atmosphere. Accordingly, during this phase of the return stroke, the damping effect is substantially at a minimum. However, in practice, it is found that the pallet 2 still reaches the valve seat substantially without bounce. It is believed that this is caused by the sudden acceleration of the pallet 2 as it approaches its seat, this acceleration being produced by the suction on the pallet 2 caused by the air flow about the pallet. This sudden acceleration causes the rod 33 and the nuts 34 and 35 to move upwardly relative to the upwardly moving armature 10, thereby causing the nut 35 to seat on the surface 38 and again substantially seal the zone 39 from the environment so as to re-create the damping efiect during the final stages of movement of the pallet 2 by the reaction of the depression created in zone 39.

Therefore, it will be seen that this arrangement gives maximum damping on opening of the valve, to give a desired quality to the starting speech of the associated organ pipe, minimum damping during the initial stage of the closing stroke, to avoid undue delay in the termination of a note, and increased damping during the final stage of closing the valve, when the pallet would normally accelerate very rapidly, substantially to prevent pallet bounce and noise. It is also noted that the free movement of the armature l enables kinetic energy to be stored in the armature before it acts on the rod 33. Therefore, the operator can open a larger valve, or a valve at higher pressure, than would be the case with no free movement.

It is also to be noted that, in addition or in the alternative to slots 36, there may be slots or passages in the nut 34.

As will be apparent, the various embodiments described with reference to FIGS. 1 to 5 may all be modified to include features of FIGS. 6 and 7. For example, a second solenoid could be added to the embodiment of FIGS. 6 and 7 in the manner indicated by way of example with reference to FIG. 3. In one such modification, a second solenoid is added to the embodiment of FIG. 7 and its armature is clamped to wire 33 in cylinder ll, suitably extended, between armature l0 and the upper, plastics, end cap. When the second solenoid is operated, it will drive nut 35 away from its seating to reduce the damping. On the return stroke, the operation will be substantially as with the form shown in FIG. 7. Thus, the second contact could be operated by a secondary manual key contact which operates after the main contact to break the connection to winding 9 and make the connection to the second solenoid. In normal use, the secondary contact may be operated, but too late to be of substantial effect. On staccato operation, the secondary contact will bring in the second solenoid quickly enough to overcome the damping effect initiated by the major contact controlling the winding 9.

I claim:

1. An electromagnetic pallet operator suitable for the substantially noise-less operation of a pallet valve of a wind pipe organ, the operator comprising a magnetically-open-ended first solenoid without dead stop, comprised of a first winding and a first armature of magnetic material, said first armature being coupled to said pallet and mounted within a cylinder for displacement on a path relative to the winding so that energisation of said winding will displace said armature along said path towards a condition of minimum reluctance and an air damping means to damp said first armature to an extent reducing pallet acceleration on opening of a pallet valve and preventing bounce on return of the pallet to its seating, said air damping means including a second solenoid comprising a second winding and second armature and a bleed opening, said second armature mounted for displacement into the said second winding 5 and at least partly closing said bleed opening when unenergised and opening said bleed opening when energised to produce a lower damping effect than when said second solenoid is without said energisation.

2. The operator as claimed in claim 1, including a 10 spring positioned between said armatures.

towards a condition of minimum reluctance, a metal frame having portions at the axial ends of the winding joined by a further portion of the metal frame, an at least partial closure for the cylinder which together with said armature and said cylinder acts to damp said armature, and valve means for controlling the degree of damping of the armature in dependence upon armature movement to enable said armature to be damped during an opening stroke while providing a lesser de gree of damping during the initial stage of a closing stroke and an increased degree of damping during the final stage of the closing stroke when said pallet would normally accelerate owing to suction on said pallet.

4. The operator as claimed in claim 3, and comprising a member for connection to a pallet, this member being loosely coupled to said armature so that it can be driven by said armature yet carry out a movement relative to said armature in the operating direction, and said damping arrangement of cylinder and armature further comprising a valve connected to be operated by the relative movement of said member and said armature so as to be substantially closed on movement of said armature in a direction to open a pallet and to be opened, to reduce damping, when said armature reverses to the opposite direction of movement, said valve operating towards its closure position to increase damping if said member accelerates in said opposite direction relative to said armature.

5. The operator as claimed in claim 4, wherein said armature is hollow and said member extends through the interior of said armature, and said armature is disposed so as to act between two spaced abutments carried by said member, one of said abutments constituting with a portion of said armature said valve releasably to close an opening to said interior of said armature.

6. A wind chest for a Wii d-pipe organ and having a plurality of valve seatings for communicating with organ pipes, each of at least some of said seatings having closure pallets each with an operator comprising a magnetically-open-ended solenoid without dead stop comprised of a winding, a metal frame extending from one axial end to the other of the winding and an armature of magnetic material, said armature being coupled to said pallet and mounted within a non-magnetic cylinder for displacement on a path relative to the winding so that energisation of said winding will displace said armature along said path towards a condition of minimum reluctance and an at least partial closure for said cylinder which together with said armature and said cylinder acts to damp said armature to an extent reducing pallet acceleration on opening of a pallet valve and preventing bounce on return of the pallet to its seating.

7. The wind chest as claimed in claim 6, wherein said damping arrangement of cylinder and armature has an adjustable air bleed.

8. An organ having a wind chest with a plurality of valve seatings for communicating with organ pipes, each of at least some of said seatings having closure pallets each with an operator comprising a magnetically-open-ended solenoid without dead stop comprised of a winding, a metal frame extending from one axial end to the other of the winding and an armature of magnetic material, said armature being coupled to said pallet and mounted within a non-magnetic cylinder for displacement on a path relative to the winding so that energisation of said winding will displace said armature along said path towards a condition of minimum reluctance and an at least partial closure for said cylinder together with said armature and said cylinder acting to damp said armature to an extent reducing pallet acceleration on opening of a pallet valve and preventing bounce on return of the pallet to its seating 9. The organ as claimed in claim 8, wherein said damping arrangement of cylinder and armature has an adjustable air bleed.

10. An electromagnetic pallet operator suitable for the substantially noiseless operation of a pallet valve of a wind pipe organ. the operator comprising a magnetically-open-ended solenoid without dead stop comprised of a non-magnetic cylinder, a winding on said cylinder, an armature of magnetic material and a metal frame extending from one to the other of the axial ends of the winding. said armature being mounted within said cylinder for displacement on a path relative to the winding so that energisation of said winding will displace said armature along said path towards a condition of minimum reluctance, and a member providing an at least partial closure for said cylinder so that said armature, said cylinder and said member provide air damping for said armature to an extent reducing pallet acceleration on opening of a pallet valve and substantially preventing bounce on return of the pallet to its seating.

11. The operator as claimed in claim 10, including a return spring for said armature.

12. The operator according to claim 1 1, wherein said pallet is connected directly to said armature.

13. The operator as claimed in claim 10, wherein said pallet is secured coaxially with respect to said armature and cylinder.

14. The operator as claimed in claim 10, including mounting means connected to said metal frame for mounting the operator upon a sound board, said mounting means being designed for mounting said operator in a predetermined position in relation to an organ sound board.

15. The operator as claimed in claim 14, wherein said mounting means includes adjustment means whereby said predetermined position is adjustable.

16. The operator as claimed in claim 14, wherein said mounting means defines a slot enabling the operators position to be adjusted relative to a seating.

17. The operator as claimed in claim 16, wherein said mounting means comprises a plate containing the slot, said plating being curved to dig into material on which said plate is mounted.

18. The operator as claimed in claim 10, wherein said damping arrangement of cylinder and armature has an adjustable air bleed. I

19. The operator as claimed in claim 10, wherein said armature is slidably fitted in said cylinder to provide leakage from said damping between said cylinder and armature.

20. The operator as claimed in claim 10, wherein said cylinder extends beyond said winding and has a nonmagnetic closure beyond said armature when in said condition of minimum reluctance.

Claims

1. An electromagnetic pallet operator suitable for the substantially noise-less operation of a pallet valve of a wind pipe organ, the operator comprising a magnetically-open-ended first solenoid without dead stop, comprised of a first winding and a first armature of magnetic material, said first armature being coupled to said pallet and mounted within a cylinder for displacement on a path relative to the winding so that energisation of said winding will displace said armature along said path towards a condition of minimum reluctance and an air damping means to damp said first armature to an extent reducing pallet acceleration on opening of a pallet valve and preventing bounce on return of the pallet to its seating, said air damping means including a second solenoid comprising a second winding and second armature and a bleed opening, said second armature mounted for displacement into the said second winding and at least partly closing said bleed opening when unenergised and opening said bleed opening when energised to produce a lower damping effect than when said second solenoid is without said energisation.

2. The operator as claimed in claim 1, including a spring positioned between said armatures.

3. An electromagnetic pallet operator suitable for the substantially noise-less operation of a pallet valve of a wind pipe organ, the operator comprising a magnetically-open-ended solenoid without dead stop, comprised of a winding and an armature of magnetic material, said armature being coupled to said pallet and mounted within a non-magnetic cylinder for displacement on a path relative to the winding so that energisation of said winding will displace said armature along said path towards a condition of minimum reluctance, a metal frame having portions at the axial ends of the winding joined by a further portion of the metal frame, an at least partial closure for the cylinder which together with said armature and said cylinder acts to damp said armature, and valve means for controlling the degree of damping of the armature in dependence upon armature movement to enable said armature to be damped during an opening stroke while providing a lesser degree of damping during the initial stage of a closing stroke and an increased degree of damping during the final stage of the closing stroke when said pallet would normally accelerate owing to suction on said pallet.

6. A wind chest for a wind-pipe organ and having a plurality of valve seatings for communicating with organ pipes, each of at least some of said seatings having closure pallets each with an operator comprising a magnetically-open-ended solenoid without dead stop comprised of a winding, a metal frame extending from one axial end to the other of the winding and an armature of magnetic material, said armature being coupled to said pallet and mounted within a non-magnetic cylinder for displacement on a path relative to the winding so that energisation of said winding will displace said armature along said path towards a condition of minimum reluctance and an at least partial closure for said cylinder which together with said armature and said cylinder acts to damp said armature to an extent reducing pallet acceleration on opening of a pallet valve and preventing bounce on return of the pallet to its seating.

8. An organ having a wind chest with a plurality of valve seatings for communicating with organ pipes, each of at least some of said seatings having closure pallets each with an operator comprising a magnetically-open-ended solenoid without dead stop comprised of a winding, a metal frame extending from one axial end to the other of the winding and an armature of magnetic material, said armature being coupled to said pallet and mounted within a non-magnetic cylinder for displacement on a path relative to the winding so that energisation of said winding will displace said armature along said path towards a condition of minimum reluctance and an at least partial closure for said cylinder together with said armature and said cylinder acting to damp said armature to an extent reducing pallet acceleration on opening of a pallet valve and preventing bounce on return of the pallet to its seating.

9. The organ as claimed in claim 8, wherein said damping arrangement of cylinder and armature has an adjustable air bleed.

10. An electromagnetic pallet operator suitable for the substantially noiseless operation of a pallet valve of a wind pipe organ, the operator comprising a magnetically-open-ended solenoid without dead stop comprised of a non-magnetic cylinder, a winding on said cylinder, an armature of magnetic material and a metal frame extending from one to the other of the axial ends of the winding, said armature being mounted within said cylinder for displacement on a path relative to the winding so that energisation of said winding will displace said armature along said path towards a condition of minimum reluctance, and a member providing an at least partial closure for said cylinder so that said armature, said cylinder and said member provide air damping for said armature to an extent reducing pallet acceleration on opening of a pallet valve and substantially preventing bounce on return of the pallet to its seating.

12. The operator according to claim 11, wherein said pallet is connEcted directly to said armature.

16. The operator as claimed in claim 14, wherein said mounting means defines a slot enabling the operator''s position to be adjusted relative to a seating.

18. The operator as claimed in claim 10, wherein said damping arrangement of cylinder and armature has an adjustable air bleed.

20. The operator as claimed in claim 10, wherein said cylinder extends beyond said winding and has a non-magnetic closure beyond said armature when in said condition of minimum reluctance.