US2425770A

US2425770A - Electromagnetic device

Info

Publication number: US2425770A
Application number: US583901A
Authority: US
Inventors: Ludwig S Walle
Original assignee: General Electric Co
Current assignee: General Electric Co
Priority date: 1945-03-21
Filing date: 1945-03-21
Publication date: 1947-08-19
Anticipated expiration: 1964-08-19

Description

Aug. 19, 1947. s. WALLE ELECTROMAGNETIC DEVICE Filed March 21, 1945 2 Sheets-Sheet 1 Inventof: Ludwig S. Wal le,

I by {W 3 4 RATED CURRENT 2 TIMES us Attorney ELECTROMAGNETIC DEVICE Filed March 21, 1945 2 Sheets-Sheet 2 TIME IN' SECONDS a 4 V i 6 TIMES RATED CURRENT Inventor: I Ludwng SWalle, Y His Attorney.

Patented Aug. 19, 1947 2,425,770 ELECTROMAGNETIC DEVICE Ludwig S. Walle, Lansdowne,

General Electric Company,

New York Pa.,- assig'nof to a corporation of Application March 21, 1945, Serial No. 583,901

My invention relates to improvement in electromagnetic devices and; more particularly to time element electromagnetic devices for operation by oscillating currents.

Electromagnetic devices which operate with a time delay dependent. on a function of an electric quantity of a circuit such. as current, voltage, etc, have been used to clear faults on circuits by operating with a. selective time. action suchas to effeet the opening of only'the circuit breakers nearest to and on each side of the fault. These. time element electromagnetic devices have been built as relays and also as direct tripdevices. To obtain the timing asctiom. many arrangements have been tried, but those with which I am familiar are deficient in some one o more respects. Some devices, for example, embody an oil dash pot for the timing action. Such devices are erratic because of the variation in viscosity oi the oil with the temperature of the ambient. Moreover, oil because of its tendency to creep soon becomes omnipresent and a nuisance. Dash pots of the air bellows type have also been used. These cause trouble in consequence of the stiflening of the material of the bellows. by drying and deterioration. Other time element electromagnetic devices have embodied rotating vanes, gear trains, flywheels andl clutching mechanisms. These are not only costly but also troublesome to maintain. With many of the foregoing devices, it has been difficult or impossible to secure at the end of'the timing period that final impulse action necessary for a direct trip device. Some time element devices have embodied a pivotally" mounted con'- tainer partially filled with mercury which flows through an orifice within the" container from the higher end to the lower end untilthe level of the mercury in the lower end bridges contacts. These containers, usually of glass toavoidtrouble with the mercury, are easily broken. More-'- over, they have to be movably mounted, and sealed tightly to prevent deterioration of'the mercury with consequent impairment of the timing action.

An. object of invention is t'o'providean improved time element electromagneticdevicewhich is in general free. of the difiicul ties and expense of, prior art devices-.- Another: object ofmy iniven tion is to' provide: an improved: time: element electromagnetic device which: has" only the: simplest of relatively inexpensive and? durable: moving parts. Still another object ofmy'invention-lto provide an improved; time. element electromagnetic device wherein amovab'le operating me'm ber has a fixed range of movement regardless of 21 Claims. (Cl. 175-372);

the timing and pick-up adjustments of the device. A further object of my invent-ion is to-pi-ovide an. improved time element electrcmagn tic device which provides at the end of the tim ng period that final strong impulse nece sary a direct" trip device. A still further objectoi my invention is to provide an improved time element electromagnetic devicewhich oi'fiers a, wide range of consistent til-hing with a minimum of difie rent parts together withandpickup adjustments which are substantiauy independent 01? each other. These and other objects of my invention will appear in more detailhereinafter.

In accordance m invention; I rov'ide for operation on oscillating currentsa, time element electromagnetic device wherein a mas'sof" relatively smaliimagnetie elementsis housed-i ascentamer having an intermediate constriction through which the: elements pass in moving from the unattracted position to the attracted position; Further; in accordance with. invention, I mount above the magnetic elements a rider which may be of amagnetic or nonmagnetic material; depending upon whether or" not animpulse actio'n is desired near the end of the-movement of the magnetic parts to the attracted position. Also accordance with my invention, I provide an operating; member which has: a time travel independentiy' of the timing and pick-up ad j us'tments: of the device This application is a continuation iri-pai tof my copending. application, Serial Ni 544 ,5754, filed July 13, 944,101: Electromagnetic device, now abandoned, the subject matter. of said prior app'licati'on being; incorporated: in: this application together with additional subject'- matter:-

My invention will be better understood: from the follow-ing'description when considered: in con nection with the accompanying two she" 5 of drawings; audits scope will be pointed out the appended claims.

Fig. 1 is a: perspective view of a time: element electromagnetic device: embodying. my invention; Fig. 2 is a sectional: elevation on the line" 2 4 of Fig; 1 showing: the movable magnetic" parts in theunattracted position; Fig; 3 is a awn elevatiorr similar to '2' showing the movable mag?- netic parts in: the attracted position; Fig; 4 iliustra-tes a family of. time -current'curvesobtainable:- with: apparatus embodying: invention under different timing adjustments; Fig. 5 is a sectional" elevationsimilar to Fig 2 illustrating amodification of my invention. embodying both timing and pick-'up adjustments; and-Fig; 6 illustrates time current curves obtainable with appet- 3 ratus embodying my invention, as shown in Fig. 5, under different timing and pick-up adjustments.

In Figs. 1, 2 and 3, I have illustrated an embodiment of my invention in a time element electromagnetic device comprising an energizing winding or coil I which has an axial central opening formed by winding the coil on a spool 2 which may be of insulating material, as shown. At the upper end of the opening, a pole piece 3 is mounted in an outer magnetic member such as a shell 4 of suitable magnetic material. As shown, the shell 4 is split longitudinally at 5 so as to reduce eddy current heating effects and also to permit clamping around the winding I by suitable means such as a bolt 6 which passes through two mounting brackets 'I. These may be integral with the shell 4, as shown, or suitably secured thereto. Slidably positioned in the pole piece 3 is an operator 8 having lugs 9 positioned in slots I in the pole piece 3. To prevent escape of the operator, a retaining ring II is provided in a groove around the upper end of the pole piece.

In accordance with my invention, the electromagnetic device has an armature comprising a mass of relatively small magnetic elements I2 and encompassing means therefor such as a container I3, which is so shaped as to restrict the movement of the elements from the unattracted position, shown in Fig. 2, to the attracted position, shown in Fig. 3, upon energization of the Winding I by current of a magnitude sufficient to move the elements. The container I3, which extends into the axial opening of the winding I from the lower end thereof, may be of nonmagnetic high resistance metallic materials, as shown, or of insulating material. As shown, the container I3 is of tubular form and has an upper portion of substan tially uniform cross section and a lower portion of greater cross-sectional area which istapered to the upper portion to provide an intermediate constriction or throat portion. Thus, in general, the container I3 is more or less funnel shaped. The magnetic elements I2 are preferably of low retentivity magnetic material, and they may be spheroidally shaped, although my invention is not limited in its application to this specific form. In general, the size of the magnetic elements I2 .is not critical except that I have found that if balls are used the diameter of the restricted portion of the container I3 should preferably be at least eight times the maximum diameter of the elements I2 for consistent timing results,

In order to adjust the time of the device, the container I3 is so adjustably mounted that the initial or unattracted position of the magnetic elements I2 can be varied relatively to the coil I. For this purpose, the container I3 has suitably secured thereto at its upper end a threaded extension pole tip I4 which engages threads in the pole piece 3, as shown in Figs. 2 and 3. In order to reinforce the action of the pole piece 3 the pole tip I4 is of suitable magnetic material. For adjustment purposes, the container is provided with a hollow cylindrical base I suitably secured to the container as by screws It. In order to maintain the time adjusting base I5 in any desired position, I provide suitable holding means which is illustrated as of the frictional type. Thus, for example, to the base of the spool 2 I secure a sub-base I1 of nonmagnetic material as by screws I8. On this sub-base I! at suitable intervals, I provide one or more holding means such as resilient angles I9, one leg of each of Which is secured to the sub-base I I as by a screw 20 and the other leg of which is biased against the inside of the adjusting base by suitable means shown as a compression spring 2I suitably pocketed in the sub-base II. On the sub-base I1, I provide an indicator scale 22 having graduations 23 with which graduations 24 on the rim of the adjusting base I5 register for desired time settings. I In order better to correlate the action of the movable magnetic elements I2 as they pass from the larger portion of the container to the smaller portion thereof through the intermediate constriction or throat, I provide a rider 25 which is seated on the elements I2 so as to move therewith from the unattracted position to the attracted position and vice versa. The size of the rider 25 is such that it will slide freely in the upper narrow portion of the container I3 without permitting the passage of any of the elements I2 between the sides of the rider and the container. If the rider 25 is made of solid magnetic material, then it should be slotted a indicated in order to reduce eddy current heating. Also, to reduce chatter, the rider may be provided with a'shading ring 26.

In order to impart the movement of the rider 25 to the operator 8, I provide a push rod 2'5 which is slidably positioned in the upper end of the container I3 and provided with suitable rotation-preventing means such as a pin 28 which rides in slots 29 in the pole tip I4. The lower end of the push rod 21 extends downwardly in the container in the path of movement of the rider 25, and the upper end is threaded into the operator 8. In order to maintain a given or fixed travel for the operator 8 regardless of the timing adjustment of the device, the pitch of the threads on the push rod 21 is made the same as the pitch of the threads on the end of the container I3 50 that, whatever movement is imparted to the container I3 in changing the adjustment, a like movement is imparted to the push rod 21 to maintain the travel of the operator 8 substantially constant. It will be obvious to those skilled in the art that the upward move ment of the operator 8 can be utilized to operate, for example, a pivotally mounted member 33 which can be a movable contact arm for controlling a circuit such as the trip circuit, of a circuit breaker or the member 30 may be a trip latch on the circuit breaker operating mechanism. I have found by the use of high speed motion pictures that with sufficient current applied to move the magnetic elements I2 there is a chaining process with the elements moving faster at the top of the mass than below. This provides a sort of stretching action which tends momentarily to relieve pressure fro-m the lower side wall of the container I 3. In general, these chains of elements maintain contact with the magnetic rider 25 and with the mass of elements below tending to draw them upward to the greater magnetic density of the coil, impeded, however, by the restriction in the container I3. As the instantaneous value of current decreases and passes through zero, the magnetic rider 25 and the ele ments I2 near it can be seen to drop downward momentarily thereby apparently tending to release the congestion at the restriction in the container I3. As the current again approaches its peak value, the elements I2 once more move upward before becoming arrested by the coalescing of the mass in the restricted portion of the container I3. This continues for th necessary number of half cycles of current to permit the magnetic rider 25 to be attracted to the pole piece 3 and the pole tip Hi.

During this stretching process of the magnetic elements 12, there comes a point in the travel where the magnetic rider 25 moves suddenly to give the push rod El a quick impulse. The point at which the rider 25 starts to give this quick impulse varies with the amount of current in the coil I. At short circuit values of current, this quick action starts without preliminary advance merit of the magnetic elements E2 to give sub-- stantially instantaneous action, as shown in the time-current curves in Fig. 4. Upon interruption of the current in the coil l, the rider 25 and the elements {2 drop back substantially instantaneously to their original position.

In order to obtain consistent action, the bottom of the magnetic rider 25 and the surfaces of the magnetic elements i2 should be kept free of corrosion. To do this, a thin coating of noncorrosive metal may be deposited thereon. A thin coating of graphite is also helpful, although probably not alone adequate under humid conditions. Both the magnetic rider 215 and elements 52 are preferably of a material of relatively high permeability and low magnetic retentivity. As a mass, however, the magnetic elements i2 will have a much lower permeability due to the multitude of air gaps between the elements.

The timing action is dependent on many fac-- tors, among which are the magnitude of the our rent, the frequency of the current impulses, the size of the magnetic elements, their surface condition, and their location relatively to the coil. This last factor can also be changed by adding to or subtracting from the elements l2. I have also found that the device can be tilted up to about 45 degrees from the vertical without materially affecting the time of operation.

In Fig. 4, I have illustrated a family oi timecurrent curves taken on an embodiment of my invention using 60-cycle alternating current and five different time settings of the container it. In this case, the armature consisted of about 36 grams of hydrogen-annealed steel balls of 0.68 to 0.04" diameter and a magnetic plunger rider thereon. The horizontal axis is plotted on the basis of the number or times minimum operating current and the vertical on the basis of the time of operation of the device. Raising the container l3 into the coil i lengthens the time delay because the flux through the elements is increased thereby binding them tighter together so that it is more difficult for the elements to pass through the throat portion of th container.

Further in accordance with my invention, I provide means for efiecting both timing and pickup adjustments substantially independently of each other and also of the fixed travel of the operator 8. For varying the pick-up indepe ently of the timing, I provide, shown in Fig. 5, biasing means such. as a spring 3i which has one end secured to the rider 7.5 by suitable 1. such as a preferably slender rod or wire wl passes through the mass of magnetic el l2. The other end of the spring 3i is secur suitable means such as a rod or wire 33 to a member 34 which is movably positioned relatively to the timing base 15. The rod 33 passes through an opening in the bottom of the member t l which is provided with a hole large enough to permit turning of the member 35 relatively to the rod without torsionally stressing the sprin 31. To prevent the rod 33 from pulling through the bottom of the member 34, it is provided with a suitable enlargement or head 35.

For varying the stress on the spring SI and thereby the current necessary to pick up the rider 25 and the magnetic elements l2 for any given time adjustment or position of the container [3, the member 34 may be of tubular form and arranged for threaded engagement with another tubular member 36 which is rigidly secured to the timing base It or to the bottom plate 31 of the container I23. Thus, as shown, the tubular mem ber 3 is threaded internally for engagement with the externally threaded tubular member 36. In order to prevent any tendenc for jamming the magnetic elements I? in consequence of the pull of the spring on the rider 25, the container I3 is preferably provided with suitable stop members such as pins 33 which project through the sides of the container l3 just sufficiently to act as a stop for the rider 25. In general, two or at the most three of these pins, which may be of relatively small diameter, are sufficient and do not oifer enough obstruction to the movement of the magnetic elements 12 materially to change the timing action of the device. In order to provide a suitable pick-up or current setting scale, the internal tubular member 35 may be faced fiat over a portion 39 of its external surface for the reception of suitable numbers which indicate current or voltage settings dependent upon the application of the relay.

From the foregoing, it will be apparent that by varying the position of the tubular member 34 relatively to the tubular member 3'6, the tension on the spring 35 and thereby the pick-up of the device may be changed Without in any way altering the position of the timing base [5. It will also be obvious that the position of the timing base may be varied without materially changing the pick-up setting. Moreover, since the timing base and the pick-up control move as a unit when the timing base is moved and the pick-up adjustment may be made without moving the timing base, any change in either has no effect on the fixed travel of the operator 8.

In Fig. 6, I hav illustrated a family of timecurrent curves based on different timing and pick-up adjustments. Thus, for example, curves a, b and 0 represent time-current characteristics of an embodiment of my invention for minimum, medium and maximum time settings on the basis of a 5-ampere pick-up setting; curves cl, eand for minimum, medium and maximum time settings on the basis of a 'l-ampere pick-up setting; and g, h and z for minimum, medium and maximum time settings on the basis of a Q-ampere pick-up setting. From the foregoing, it will be apparent that apparatus embodying my invention has great flexibility in the control of its time and pick-up responses, both of which can be effected without interfering with the fixed travel of the operator 8 and each of which can be changed independently of the other.

While I have shown and described my invention in considerable detail, I do not desire to be limited to the exact arrangements shown, but seek to cover in the appended claims all those modifications that fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electromagnetic device having an armature comprising a mass of relatively small magnetic elements, and means encompassing said elements so shaped as to restrict the movement aeaavvo thereof from the unattracted position to the attracted position upon energization of the device by current of a magnitude sufficient to initiate the movement of the elements, said encompassing means comprising a container having two portions of relatively large and small cross-sectional areas interconnected by a portion of varying cross-sectional area.

2. An electromagnetic device having an armature comprising a mass of relatively small magnetic elements and a container for said elements having two portions of relatively large and small cross-sectional areas and an intermediate throat portion of varying cross-sectional area, said throat portion constituting a constriction through which said elements pass in moving from the unattracted position to the attracted position.

3. An electromagnetic device having an armature comprising a mass of relatively small magnetic elements and a container therefor having an upper portion of small cross-sectional area, a lower portion of materially larger cross-sectional area, and an intermediate throat portion of varying cross-sectional area, the number of said elements being sufficient to fill the lower and throat portions of said container when the elements are in the unattracted position.

4. A11 electromagnetic device having an armature comprising a mass of relatively small magnetic elements, means encompassing said el ments so shaped as to restrict the move thereof from the unattracted position to the tracted position upon energization of the device by current exceeding a magnitude suilicient to initiate the movement of the elements, encompassing means comprising a container having two portions of relatively large and small cros sectional areas interconnected by a portion of varying cross-sectional area, and a member within said encompassing means seated on said elements and movable therewith.

5. An electromagnetic device having an armature comprising a mass of relatively small magnetic elements, a container for said elements having two portions of relatively large and small cross-sectional areas and an. intermediate throat portion of varying cross-sectional area, said throat portion constituting a constriction through which said elements pass in moving from t e unattracted position to the attracted position,

and a magnetic member slidably positioned in said container above said elements and movable therewith, said member being sufficiently large relatively to said constriction to prevent the entry of said elements between the member and the constriction.

6. An electromagnetic device having an armature comprising a mass of relatively small rounded magnetic elements, a tubular container for said elements having an intermediate constriction through which said elements pass in moving drom the unattracted position to the at tracted position, the volume of said elements while in the unattracted position being sufiicient to fill the lower portion of the container at least to the level of said constriction, and a rider slidably positioned in said container above said mag netic elements and seated thereon, said rider being proportioned relatively to said container to prevent the entry of said elements between the rider and the container.

7. An electromagnetic device comprising an energizing winding, a pole piece, a mass of relatively small rounded magnetic elements and a magnetic member relatively large in comparison with one of said elements, and means encompassing said member and said elements with said member seated on said elements, said encompassing means being funnel-shaped to restrict the movement of said elements from the unattracted position to the attracted position and delay the movement of said member to the attracted position upon energization of said winding by current exceeding a magnitude sufiicient to initiate the movement of said member and said elements.

8. A time element electromagnetic device having an armature comprising a mass of relatively small magnetic elements, a container for said elements having an intermediate constriction through which said elements pass in moving from the unattracted position to the attracted position, the volume of said elements while in the unattracted position being sufficient to fill the lower portion of the container at least to the level of said constriction, a rider slidably positioned in said container above said magnetic elements and seated thereon, an energizin winding around said container, means for varyin the position of said container relatively to said winding whereby to change the time of movement of said armature to the attracted position, an operator, and means for effecting a fixed linear movement of said operator independently of the timing adjustment of the armature.

9. A time element electromagnetic device comprising a winding having an axial central opena pole piece at one end of said opening, a container extending into said opening from the other end thereof and having an upper hollow portion of substantially uniform cross-sectional area and a lower portion of greater cross-sectional a mass of relatively small spheroidall'y shaped magnetic elements sufficient to fill the lower portion of said container and to project a relatively small distance into the upper portion thereof, and a magnetic member relatively large in comparison with one of said elements slidably positioned in the upper portion of said container and seated on said elements.

10. A time element alternating current electromagnetic device havin an armature comprising a :mass of relatively small magnetic elements, a container for said elements having an intermediate constriction through which said elements pass in moving from the unattracted position to the attracted position, the volume of said elements while in the unattracted position being sufiicient to fill the lower portion of the container at least to the level of said constriction, a magnetic member relatively large in comparison with one of said elements slidably positioned in said container above said elements and seated thereon, an energizing winding around said container, means for varying the position of said container relatively to said windin whereby to change the time of movement Of said armature and magnetic member to the attracted position, an operator, and means for effecting a fixed linear movement of said operator independently of the timing adjustment of the armature and magnetic member.

11. A time element electromagnetic device comprising a winding having an axial central opening, a pole piece at one end of said opening, a container extending into said opening from the other end thereof and having an upper hollow portion of substantially uniform cross-sectional area and a lower portion of greater cross-sec tional area, said container having its upper end in threaded engagement with said pole piece, an

armature comprising amass of relatively small spheroidally shaped magnetic elements sufficient to fill the lower portion of said container and to project a relatively small distance into the upper portion thereof, a magnetic member relatively large in comparison with one of said elements slidably positioned in the upper container and seated on said elements, an operator slidably positioned above said pole piece for a predetermined linear movement thereto, and means for imparting the movement of said armature and magnetic member to said operator comprising a push rod slidably positioned in the upper end of said container and in threaded engagementwith said operator.

12. An electromagnetic device having an armature comprising a mass of relatively small magnetic elements, a container therefor having an upper portion of small cross-sectional area, a lower portion of larger cross-sectional area, and an intermediate throat portion of varying crosssectionalarea, the number of said elements being sufficient to fill the lower and throat portions of said container when the elements are in the unattracted position, and a magnetic member slidably positioned in said container above said elements and movable therewith, said member having a cross-sectional area sufiicient to prevent the entry of said elements between the member and the adjacent walls of the container.

13. A time element electromagnetic device having an armature comprising a mass of relatively small magnetic elements, a container for said elements having an intermediate constriction through which said elements pass in moving from the unattracted position to the attracted position, an energizing winding around said container, means for varying the position of said container relatively to said winding, and means for varying the pick-up of said armature independently of the position of said container.

14. A time element electromagnetic device having an armature comprising a mass of relatively small magnetic elements, a container for said elements having an intermediate constriction through which said elements pass in moving from the unattracted position to the attracted position, the volume of said elements while in the unattracted position being suificient to fill the lower portion of the container at least to the level of said constriction, a rider slidably positioned in said container above said magnetic elements, an energizing winding around said container, means for varying the position of said container relatively to said Winding, means for varying the pickup of said armature independently of the position of said container, an operator, and means for effecting a fixed linear movement of said operator independently of the position of said container and the pick-up of said armature.

15. A time element electromagnetic device having an armature comprising a mass of relatively small magnetic elements, a container for said elements having an intermediate constriction through which said elements pass in moving from the unattracted position to the attracted position, the volume of said elements while in the unattracted position being suflicient to fill the lower portion of the container at least to the level of said constriction, a rider slidably positioned in said container above said magnetic elements, an energizing winding around said container, means for varying the position of said container relatively to said winding whereby to change the time of movement of said armature to the attracted relatively portion of said 10 position, means for changing the pick-up of said armature comprising a member positioned on said container for'movement relatively thereto, and biasing means interconnecting said rider and said member. I

16. A time element electromagnetic device having an armature comprising a mass of relatively small magnetic elements, a container for said elements having an intermediate constriction through which said elements pass in moving from the unattracted position to the attracted position, the volume of said elements while in the unattracted position being suificient to fill the lower portion of the container at least to the level of said' constriction, a rider slidably positioned in said container above said magnetic elements, an energizing winding around said container, means for varying the position of said container relatively to said winding whereby to change the time of movement of said armature to the attracted position, and means movably positioned'on said container for varying the pick-up or said armature independently of the position of the container.

17. A time element alternating current electromagnetic device having an armature comprising a mass of relatively small magnetic elements, a container for said elements having an intermediate constriction through which said elements pass in moving from the unattracted position to the attracted position, the volume of said elements while in the unattracted position being sufficient to fill the lower portion of the container at least to the level of said constriction, a magnetic member relatively large in comparison with one of said elements slidably positioned in said container above said elements, an energizing winding around said container, means for varying the position of said container relatively to said winding whereby to change the time of movement of said armature and magnetic member to the attracted position, and means for changing the pick-up of said armature comprising an element adjustably positioned on said container for movement relatively thereto in the direction of movement of said armature and a spring interconnecting said magnetic member and said element.

18. A time element electromagnetic device having an armature comprising a mass of relatively small magnetic elements, a container for said elements having an intermediate constriction through which said elements pass in moving from the unattracted position to the attracted position, a relatively large magnetic member slidably positioned in said container above said elements and forming therewith the armature of the device, means for preventing said member from resting on said elements when the elements and the member are in the unattracted position, an energizing winding around said container, means for varying the pick-up of said armature, and means for independently varying the time required for the armature to move from the unattracted position to the attracted position.

19. A time element electromagnetic device having an armature comprising a mass of relatively small magnetic elements, a container for said elements having an intermediate constriction through which said elements pass in moving from the unattracted position to the attracted position, a rider slidably positioned in said container above said elements, means for preventing said rider .from resting on said elements when the elements are in the unattracted position,' an energizing 11 winding around said container, and means for independently varying the pick-up of said armature and the time required for the movement thereof from the unattracted position to the attracted position.

20. A time element electromagnetic device having-an armature comprising a mass of relatively small magnetic elements, a container for said elements having an intermediate constriction through which said elements pass in moving from the unattracted position to the attracted position, a rider slidably positioned in said container above said elements, said rider having a crosssectional area suflicient to prevent the entry of said elements between the rider and the adjacent walls of the container, means for preventing said rider from resting on said elements when the elements are in the unattracted position, an energizing winding around said container, and means for independently varying the pick-up of said armature and the time required for movement thereof from the unattracted position to the attracted position.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 781,993 Bell Feb. 7, 1903 2,015,156 Richmond Sept. 24, 1935 680,704 Dion Aug. 20, 1901