US3295023A

US3295023A - Circuit-breaker devices, especially for semi-conductor circuits

Info

Publication number: US3295023A
Application number: US245301A
Authority: US
Inventors: Peras Lucien
Original assignee: Renault SAS
Current assignee: Renault SAS; Regie Nationale des Usines Renault
Priority date: 1961-12-19
Filing date: 1962-12-17
Publication date: 1966-12-27
Anticipated expiration: 1983-12-27
Also published as: FR1322838A; DE1438113B2; FR82590E; DE1438113A1; GB1026301A

Description

Dec. 27, 1966 CIRCUIT-BREAKER DEVICES,

Filed D80. 17, 1962 L. PERAS ESPECIALLY FOR SEMI-CONDUCTOR CIRCUITS 5 Sheets-Sheet 1 Fig. I

N 2 7 T V S Fig. 2

vvvjog K 1 v Lugiefn DCIdL '1 I Hf; tm'neys) Invenkor Dec. 27, 1966 L. PERAS 3,295,023

CIRCUIT-BREAKER DEVICES, ESPECIALLY FOR SEMI-CONDUCTOR CIRCUITS Filed Dec. 17, 1962 5 Sheets-Sheet 3 l ig- 3 s l 204 A N v 1 1 205 s 218 r j 343 344 315 Inmnkor LLLLLIIQY) p87 85 At iorneys United States Patent Office Patented Dec. 27, 1966 3,295,023 CIRCUIT-BREAKER DEVICES, ESPECIALLY FOR SEMI-CONDUCTOR CIRCUITS Lucien Per-as, Billancourt, France, assignor to Regie Nationale des Usines Renault, Biiiancourt, France Filed Dec. 17, 1962, Ser. No. 245,301 Claims priority, application France, Dec. 19, 1961, 882,497, Patent No. 1,322,838; Nov. 15, 1962, 915,553, Patent No. 82,590

Claims. (Cl. 317-54) The present invention has for its object a simplified form of magnetic circuit-breaker with rapid action and resetting, mainly intended for circuits utilizing semi-conductors. The device is intended to be as economical as possible by the simplification of its form and ofthe design of the magnetic circuit, which comprises essentially a soft iron blade excited by a solenoid and arranged to pivot between permanent magnets.

Polarized relays which could carry out the same functions have been known for a long time, but they employ a different magnetic circuit. The precautions taken in these relays to separate the control functions by winding and magnetic return are such that on the one hand these relays are very expensive and on the other their contact pressures are very small.

In order to avoid these drawbacks, a circuit-breaker switch according to the invention is characterized by a magnetic frame constituted by a strip having at least two opposite faces, provided internally with stabilized magnets having a large attractive field, between which pivots a blade of soft iron, the opposite faces of the magnets being opposite poles and the distance between these faces being such that the blade has only two stable pos tions of equilibrium, against the magnets. A coil, constituted by at least one separate winding, surrounds the blade and for the purpose of the protection to be ensured, provides ampere-turns of such direction that they tend to detach the blade from the magnets against which it is applied. One or a number of electric contacts are actuated by the blade in at least one of its positions of equilibrium. The device is set by mechanical action on a portion of the blade which passes outside the apparatus.

The set of moving electric contacts of the circuitbreaker may be carried by the blade. In this case, the pressure applied between these contacts is a function of the current passing through the solenoid, and it becomes smaller as the value of this current becomes closer to the point of break or separation. This possible drawback can be remedied by placing the electric contact on the side of the magnet which holds the device in the open-circuit position, so that the pressure in the normal position (that is to say with the contacts closed) is effected solely by a spring.

In accordance with a first form of embodiment of the invention, the strip forming the magnetic frame has the shape of a U, and there are two magnets, fixed to the inner extremities of the arms of the U, the pivotal axis of the blade being close to the base of the U.

As an alternative to the preceding form, the blade can pivot about its center of gravity on an axis located substantially in the center of the U.

According to a further form of embodiment of the invention, the magnetic frame is a closed strip having a rec: tangular profile and the magnets are four in number, symmetrical in form and in position with respect to the central planes of the sides of the rectangular profile, the blade pivoting about the intersection of the two central planes referred to above.

The excitation coil of the blade may be carried by the latter or alternatively may be fixed to the frame. It may be constituted by a single winding when it is only necessary to protect one semi-conductor circuit against shortcircuits which may take place in the utilization apparatus. It may however comprise a number of windings or parts of windings when it is desired to extend the protection, that is to say the circuit-breaker action, to the cases of over-voltage and connection of the source of direct-current the wrong way around. In addition, the release current may be made dependent on the supply voltage.

To this end, auxiliary windings are provided on the coil and are connected in series with devices sensitive to the voltage and to the direction of the current, these wind ings being wound in such a direction that they act in the sense of or in opposition to the interruption of the circuit, depending on the effect desired. When so required, one winding may be coupled directly to the current source through the break contact in order to make the point of interruption dependent on the voltage of the source.

However, in order that the invention may be more clearly understood, its various forms and alternatives referred to above will now be described, by way of example and without any limitative sense, reference being made to the accompanying drawings, in which are shown, in diagrammatic section:

In FIGURE 1, a circuit-breaking switch is illustrated according to the invention with a U-shaped magnetic frame carrying two magnets and with moving electric contacts carried by the blade;

FIGURE 2 shows an alternative form of the previous embodiment;

FIGURE 3 shows a circuit-breaker, also with a U- shaped frame, but in which the moving contacts are carried by the U;

FIGURE 4 shows an alternative form of the preceding embodiment, with a balanced blade;

FIGURE 5 shows a circuit-breaker with a closed, symmetrical, magnetic frame, carrying four magnets and with a balanced blade, while FIGURE 6 is an end-view of this form;

FIGURE 7 shows an alternative form of the preceding, with a coil made in several parts for the purpose of multiple protection.

In the circuit-breaker shown in FIGURE 1, a magnetic frame 1 in the shape of a U carries magnets 2 and 3 fixed on its extremities and facing towards the interior, the magnets having a large attractive field and being generally of barium ferrite. These magnets are arranged in such manner that their magnetizing fields are perpendicular to the large sides of the U, of which they constitute a kind of closure, the oppositely-facing poles being of opposite sign.

A moving blade 4 of soft iron, pivoted at 5 on the magnetic circuit at the base of the U, gives the assembly the shape of a capital E. This blade is subjected to the action of a magnetic field created in the air-gap. It carries a contact 6 which faces another contact 7 carried by a flexible blade 8 which is electrically insulated from the U by a deformable insulating plate 9.

The flexibility of the blade 8 tends to accompany the blade 9 when the latter passes from the magnet 2 to the magnet 3, this association being however limited to about half the travel.

The side of the U which carries the magnet 2 and the contact 7 is cut-out in such manner that a tongue 10, obtained by deformation of the frame by a tool, serves as a support for the contact 7 through the intermediary of the insulating plate 9 when the blade 4 approaches the magnet 2.

A flexible electric connection 11 connects the blade 4 to one of the terminals of the source of direct or pulsating current 13, the other terminal of which is connected through a contact key 14 and the utilizing apparatus 15 to one of the extremities of a coil 12 wound round the blade 4, the other extremity of the winding being connected to the flexible blade 8. The direction of the winding of the coil 12 is chosen in such manner as to produce, on the side facing the magnets, a pole of the same sign as the magnet 2.

The operation is as follows:

If it is assumed that the voltage delivered by the source 13 is small or zero, a thrust applied to the blade 4 at the point A and in the direction of the arrow, causes the attraction and the holding of the blade by the magnet 2. The device being thus set and the electrical connection established, the apparatus 15 can be energized. If the current increases beyond a certain critical value, the blade 4 will be abruptly repelled by the magnet 2 and attracted by the magnet 3, against which it comes into abutment.

The apparatus 15 is no longer energized. The elasticity of the contact 7, carried by the blade 8, prevents a premature interruption of the current before the blade 4 has crossed the central zone between the magnets 2 and 3. Without this arrangement, there would be a risk of oscillation at currents in the vicinity of the tripping current. After opening the key 14, the apparatus can be again re-set by a mechanical action on the blade, for example at the point A.

FIGURE 2 relates to the use of the apparatus on alternating current.

In this figure, 113 is a source of alternating current, 116 is a resistance of low value and 117 is a rectifying device with a voltage threshold such as a silicon diode junction. The current return passing through the apparatus 115 is effected on the one hand through the resistance 116 and on the other through the rectifier element 117 and the coil 112.

The operation is as follows:

After setting the device as in the case of FIGURE 1, an excessive value of the current in the apparatus 15 causes a voltage drop in the resistance 116 sufiicient to bring the rectifier element 117 out of its direct con-duction bend. There is then a current circulating in the winding 112 and, if this is sufficient, the blade 104 is displaced and the circuit is interrupted.

This arrangement has the advantage of creating a threshold in the appearance of current in the coil, thus avoiding reduction of the pressure of contact between 106 and 107, provided that the current is not too close to the nominal release or tripping value.

The action of the apparatus is sufiiciently rapid for the half-wave rectification system to be found satisfactory in most cases, as shown in FIGURE 2.

In the form of embodiment given in FIGURE 3, there is again found the magnetic frame 201 in the form of a U, its

magnets

202 and 203 and the blade 204 pivoted at 205 on the circuit 201. In this case, however, the blade 204 carries at its free extremity a non-magnetic and insulating piece 218 which rests in its normal position against a stop 210, obtained by deformation of the frame 201 by a tool and located on the side of the magnet 202. The other arm of the U carries a flexible conducting 'blade 208, fixed by an insulator 209 towards the base of the U. This blade carries a contact 207 by which (when the blade 204 is close to the magnet 202) it rests elastically against a contact 206 rigidly fixed to the U.

The solenoid 212 is traversed by the current of the utilization apparatus 215 supplied by the source of directcurrent 213 through the

contacts

206 and 207 of the circuit-breaker switch, the connection 211 and the key 214. The magnetization induced in the blade 204 by the solenoid 212 produces, at the movable extremity of the said blade, a pole of the same sign as that of the magnet 202.

The operation of this apparatus is as follows:

In setting the circuit-breaker switch by actuating the blade 204 in the direction of the arrow A, the insulating piece 218 comes against the stop 210 and the blade 204 comes close to the magnet 202. The contact is made between 206 and 207 and current flows in the apparatus 215 if the key 214 is closed. If the current in the solenoid reaches a suflicient value, the blade 204 is repelled by the magnet 202 and attracted by the magnet 203, against which it comes into abutment, and the piece 218 comes into contact with the extremity of the blade 208 which bends and separates the contact 207 from the contaet 206. The current ceases to flow in theappa'ratus 215.

The contact pressure between 206 and 207 in the normal position only depends on the elasticity of the blade 208. The strength of the blade 208 will be chosen so that it can be pushed back under all conditions of temperature by the force of attraction applied by the magnet 203 on the blade 204. Since there is no fine adjustment to be pfovided, the magnet 203 can work without an air-gap when the blade 204 comes in contact with it. By way of example, a circuit breaker switch having a magnetic frame of 40 mm. in length and 20 mm. in width can readily produce, with cheap magnets, contact pressures of the order of grams, it being possible to increase this value very considerably in this same dimension with magnets of higher quality.

This circuit-breaker switch is in general slightly less rapid than that described with reference to FIGURE 1 or FIGURE 2, but it will be preferred for large currents.

In this form of embodiment, as in that preceding, the working air-gap, regulated by the stop 210, may be small, which results in a truly irreversible operation, the displacement force which is applied on the blade 204 by the effect of the solenoid 212 increasing very rapidly as the air-gap increases, contrary to other devices in which one of the air-gaps diminishes while the other increases.

In addition, there is no longer any current return through the blade 204, which makes it possible, if required, by isolating the contact 206, to avoid having any point of the U or of the blade connected to the elec tric circuit.

In a further alternative form of embodiment, more particularly intended to mobile applications and shown in FIGURE 4, the moving blade 304 can rotate about an axis 310 perpendicular to the plane of the drawing, so that the weights located on each side of the axis are substantially balanced. I

This pivot 319 rotates in a housing forming part of a non-magnetic part 320 held on the U by fixing means 321 and supporting the coil, these fixing means 321 being of small section compared with the U. From the base side of the U, the blade 304 is bent at 322 in order to increase the surface facing the base of the U and to reduce the air reluctance 323 of the magnetic circuit with out producing substantial friction.

The flexible blade 308 is bent at 324 in order to eliminate the nipple previously provided on the part 318 and thus to reduce the inertia of the moving member.

The operation remains identical with that of the device described with reference to FIGURE 3, except for the presence of the small permanent air-gap 323 of constant reluctance. If necessary, the device may be made symmetrical by providing a blade 308 on each side of the U.

The mechanical setting may be replaced by an electrical setting obtained by means of a small auxiliary solenoid placed at the same point as the break solenoid and disconnected after setting.

The adjustment of sensitivity may also be made by acting on the air-gap 323.

A further form of embodiment shown in FIGURE 5 is somewhat less economical than those preceding, but makes it possible to obtain readily a balancing of the moving parts while taking advantage of a magnetic circuit which is symmetrical and very rigid in its external portion. In addition, the accessibility of the setting lever and fixing on a wall are facilitated.

To this end, the magnetic frame 401 is a closed strip having a substantially rectangular profile, thus giving the apparatus the external shape of a parallelepiped rectangle. The two small sides of this frame are provided respectively with two

openings

422 and 423, though which pass the extremities 418 and 418 of a magnetic blade 40 pivoting on a pin 419 close to the center of symmetry of the strip. The extremities 411.8 and 418 are preferably of non-magnetic and insulating material.

Four magnets 402 4e3 4M 4%3 are arranged inside the strip 401 in order to be symmetrical in position, in shape and in magnetic polarity with respect to the central plane xy of the large sides of the rectangle formed by the magnetic frame in FIGURE 5.

With respect of the center line W of the small sides, this symmetry is limited to the position and to the shape of the magnets, the magnetic polarities of the magnets being opposite.

The magnetization of the magnets is substantially perpendicular to the planes of the large sides of the frame. When the blade 404 is in the end position shown in FiG- URE 5, the magnetomotive force of the magnet 402 is added to that of the magnet 462 located on the opposite large side, and their common flux is closed through the blade 404 and the frame 401.

In the same way, in the other end position of the blade 404, the magnetomotive forces of the magnets 463 and 403 are additive. These two positions are therefore stable. The normal position of the blade is that shown in FIGURE 5, the extremity 413 resting against a stop 410 produced by deforming the strip dill with a tool.

In this position, an elastic blade 468 fixed on an insulating support 409 brings its contact 467 against a fixed contact til-*6 carried by the strip 461, these contacts 4497 and 4% closing the electric circuit, which is composed of a direct current source 413, a contact key 4-14, the apparatus 435, the consumption of which is to be verified, and a coil 412 connected to the current source by the return connection 411. The support 429 of the coil 412 is held on the strip 491 by fixing means 42-1. For convenience of operation of the lever 413 this strip may be fixed by fastenings 425 to a wall 426 provided with a window corresponding to the opening 423 and permitting the passage of the setting lever 418 After setting the lever 418 (in the direction of the arrow A) and closing the key 414, when the consumption of the apparatus 415 approaches the value of the regulation, the magnetization induced in the blade 4&4 produces poles at the extremities of this latter which are respectively identical to those of the magnets 443 2 and 402 but opposite to those of the magnets 4-83 and M3 and there comes a moment when the blade pivots and comes up against the magnets 4-33 and 4% by pushing back the extension 424 of the elastic blade 408, separating the contacts 4% and 407 and interrupting the current in the apparatus 415 until the device has been reset.

The magnets will preferably have a shape such that their surfaces are parallel to the blade 404 in its extreme positions. The same result can obviously be obtained with magnets having parallel faces and special profile of the blade 404.

According to another form of the invention illustrated in FIGURE 7, the field winding of the blade comprises three elements 312 312 312 connected in series and making it possible to obtain multiple protections, taking account of the external circuit employed.

In FIGURE 7, there is again seen the frame 501 having a rectangular profile identical with that previously described and also carrying four magnets between which pivots the blade 504 of soft iron. After setting the blade in the direction of the arrow, the

contacts

506 and 507 are applied against each other by the action of the spring blade 508 connected electrically through the key 514 to the positive pole of the current source 513, the negative pole of which is connected through the utilization ap- 6 paratus 515 to the current coil 512 of the circuit-breaker switch, the other extremity of the said coil being connected to the contact 506.

In series with the coil 512 is connected a coil 512 the free extremity of which is connected to the negative pole of the battery through a resistance 551 and a Zener diode 550 which is either double (symmetrical) or single but connected in series with an ordinary diode which eliminates direct conduction.

Between the contact 506 and the negative pole of the battery there is also connected in series the winding 512 and the diode 552 connected in the non-conducting sense. The winding 512 will only receive current if the source 513 is connected the reverse way round, or again, with the normal connection, upon the break of the contacts 506- 5417 or 514 if the utilization apparatus 515 has a large inductance (the diode 552 then serving in recuperation). The direction of this winding 512 is such that it tends to cause the circuit to be broken when the current passing through it is in the direction of the arrow shown along the conductor which connects it to the diode 552.

The interruption of the circuit is thus obtained in the following cases:

1) The current in the utilization apparatus, passing through the winding 512 exceeds the pre-determined value;

(2) The voltage of the Zener diode 550 is exceeded, the action being possibly made progressive by the effect of the resistance 551;

(3) The polarity of connection of the source 513 has not been observed.

The elimination of one or the other of the

diodes

550 or 552 would render the circuit-breaker switch sensitive to the supply voltage either above or below.

The protection of the contacts by recuperation by the diode 552 would also be ensured by the Zener diode 550 in the event of elimination of the said diode 552 and the winding 512 The key 514 may be an electronic apparatus supplying the apparatus 515.

I claim:

1. A circuit-breaker device having rapid action and resetting comprising a fixed magnetic frame constituted of a strip in a general U shape having two arms, at least one permanent magnet fixedly mounted on the inside of each of said arms, a blade movably supported between said magnets, the opposite faces of said magnets being opposite poles and the distance between such magnets being such that the blade has only two stable positions of equilibrium against either of said magnets, a coil constituted by at least one separate winding surrounding said blade, an elec trical switch in series with said coil, said switch having at least two contacts at least one of which is flexibly supported on said fixed magnetic frame and at least one of which is actuated 'by said blade, the switch being opened when said blade is in one of said positions and shut when said blade is in the other of said positions, a circuit, to be broken comprising at least a source of current and a load apparatus in series with said coil and said switch, the device being set by mechanical actuation on a portion of said blade which extends out of said device.

2. A circuit-breaker device in accordance with claim 1 in which said switch has its movable contact carried by the blade itself, the pressure between the contacts being elastic and adjustable in such manner that the current in the coil is only interrupted about the middle of the travel of the blade.

3. A circuit-breaker device in accordance with claim 1 in which one of said contacts of said switch is arranged on the arm of the U-shaped frame opposite to that against which the blade is at rest when said contacts are closed, the other contact being supported by means of an auxiliary flexible blade so that, when the current in the coil exceeds the predetermined regulated value, the displacement of the blade acts in opposition to said auxiliary blade and opens the contacts.

4. A circuit-breaker device in accordance with claim 3 in which the blade is pivotally supported about an axis in the vicinity of its center of gravity by a non-magnetic piece, said non-magnetic piece also supporting the coil and insuring the rigidity of the U-shaped frame.

5.- A circuit-breaker device with rapid action and resetting comprising a fixed magnetic frame of a C shape, at least two permanent magnets having a large attractive field therebetween mounted on opposing arms of said frame, a movable blade of soft iron pivotally mounted between said magnets, the opposite faces of said magnets being opposite poles and the distance between the said magnets being such that the blade has only two stable positions of equilibrium, a coil constituted by at least one winding surrounding the blade and producing ampere turns of such direction that they tend to move the blade away from the magnet against which it is applied, at least one electric switch actuated by said blade in at least one of its positions of equilibrium, the switch being set by mechanical action of a portion of said blade, at least one contact of said switch being actuated by said blade, at least one contact being resiliently mounted on said frame, the pressure between the contacts being adjusted in such manner that the current in the coil is only interrupted about .the middle of the travel of said blade.

6. A circuit-breaker device in accordance with claim 5, in which said switch is arranged on the side of the U opposite to that against which the moving blade is at rest, with the contacts closed, the pressure necessary for the contacts being obtained by means of a spring, so that when the current in the coil exceeds the pre-determined regulated value, the displacement of the blade acts in opposition to the spring and opens the contacts.

7. A circuit-breaker device in accordance with claim 5, in which the magnetic frame is of U-shape and carries two magnets, the coil winding being in one single section, and the blade rotates about an axis in the vicinity of its center of gravity, a small reluctance in the magnetic return at the base of the U being effected by a widening of the facing surfaces between the blade and the U, especially by bending, without producing substantial friction between the said surfaces.

8. A circuit-breaker device in accordance with claim 7, in which a non-magnetic piece in which pivots the pin of the blade, also supports the coil and ensures the rigidity of the U.

9. A circuit-breaker device in accordance With claim 5, in which the outer magnetic circuit is a closed strip having a substantially rectangular profile and carrying on the internal faces of the large sides, four magnets which are geometrically symmetrical with respect to the two central lines of the rectangle, but the polarities of which are symmetrical only with respect to the central dividing line of the large size, the magnetic blade pivoting about an axis passing through its center of gravity and fixed on the axis of symmetry of the external magnetic circuit, the said blade having two stable positions of equilibrium in the absence of ampere-turns generated by the coil, on each side of the central dividing line of the small sides of the rectangle, while in each of its positions the blade is in proximity to two magnet faces of opposite polarity, the flux return being completed by the external circuit.

10. A circuit-breaker device in accordance with claim 9, in which the magnets have a shape such that their opposite faces are parallel to the blade in its end positions.

11. A circuit-breaker device in accordance with claim 9, in which the two extremities of the blade are fixed to insulating members provided for re-setting and which extend beyond the external magnetic circuit respectively through two windows formed in the small sides of the said circuit and the purpose of which is at the same time to permit the setting of the device and to push back, at the end of its travel, a flexible rod carrying the set of moving contacts of the said circuit-breaker switch.

12. A circuit-breaker device in accordance with claim 5, in which the winding is in a single section and in which the magnets are made of barium ferrite or the like having a temperature coeificient such that the tripping current varies with the temperature and follows substantially the variations of consumption with temperature of the utilization apparatus.

13. A circuit-breaker device in accordance with claim 5, in which the coil winding is in a plurality of sections and comprises a main winding traversed by the current of the utilization apparatus to be protected, and at least one auxiliary winding.

14. A circuit-breaker device in accordance with claim 13 in which the main winding in the auxiliary portion of the winding form portions of the same coil.

15. A circuit-breaker device with rapid action and resetting for use in circuits utilizing semiconductors, said semiconductor circuits comprising a source, a circuit switch, and a load in series with said circuit breaker, said circuit breaker comprising a fixed magnetic frame having at least two opposing arms, at least one permanent magnet mounted on each opposing arm of said frame, with a large attractive field therebetween, a movable blade of soft iron pivotally mounted between said magnets, the opposite faces of said magnets being opposite poles and the distance between said magnets being such that the blade has only two stable positions of equilibrium, a coil constituted by at least one winding surrounding the blade and producing ampere turns of such direction that they tend to move the blade away from the magnet against which it is at rest, an electric switch means mechanically actuatable by said blade in at least one of its positions of equilibrium, at least one contact of said switch being actuated by said blade, and the other contact being mounted on said frame, the pressure between said contacts being adjusted in such manner that the current in the coil is only interrupted about the middle of travel of said blade.

References Cited by the Examiner UNITED STATES PATENTS 928,582 7/1909 Burlingame. 1,546,432 7/1925 Branson. 1,773,336 8/1930 Atherton. 1,837,188 12/1931 Keller 317 58 X 2,036,277 4/1936 Johnson. 3,156,849 11/1964 Epstein 317-27 3,161,744 12/1964 Thorne et al. 3,187,140 6/1965 Lindsay.

MILTON O. HIRSHFIELD, Primary Examiner.

SAMUEL BERNSTEIN, Examiner.

L. T. HIX, Assistant Examiner.

Claims

1. A CIRCUIT-BREAKER DEVICE HAVING RAPID ACTION AND RESETTING COMPRISING A FIXED MAGNETIC FRAME CONSTITUTED OF A STRIP IN A GENERAL U SHAPE HAVING TWO ARMS, AT LEAST ONE PERMANENT MAGNET FIXEDLY MOUNTED ON THE INSIDE OF EACH OF SAID ARMS, A BLADE MOVABLY SUPPORTED BETWEEN SAID MAGNETS, THE OPPOSITE FACES OF SAID MAGNETS BEING OPPOSITE POLES AND THE DISTANCE BETWEEN SUCH MAGNETS BEING SUCH THAT THE BLADE HAS ONLY TWO STABLE POSITIONS OF EQUILIBRIUM AGAINST EITHER OF SAID MAGNETS, A COIL CONSTITUTED BY AT LEAST ONE SEPARATE WINDING SURROUNDING SAID BLADE, AN ELECTRICAL SWITCH IN SERIES WITH SAID COIL, SAID SWITCH HAVING AT