US1939448A - Circuit controller - Google Patents

Description

Dec. 12, 1933. w. HOPP CIRCUIT CONTROLLER Filed April 5, 1950 Inventor \X/iLheLm P16 019 I ocw/ His Attorne g.

Patented Dec. 12, 1933 CIRCUIT CONTROLLER Wilhelm Hiipp, Heiligensee, Germany, assignor to General Electric Company, a corporation of New York Application April 5, 1930, Serial No. 441,982, and in Germany July 11, 1929 I 15 Claims.

My invention relates to switches and has for an object the provision of an improved switch with thermal release which has large switching capacity and high switching speed. Though my invention is suitable for a wide variety of application it is particularly suitable as a protective switch for arc rectifiers or the like.

In the case of rectifiers with glass envelopes, protection against overload is ordinarily obtained by means of explosion-proof safety fuses, because such devices provide the variable permissible switching time which is in agreement with the heating of the anode 1ead-in caused by varying overloads. With the customary overload current switch and even a quick acting switch, the desired protection can not be obtained because every mechanically operated switch has a definite operating time of its own which is determined by the inertia of its moving parts. In order to increase the disconnecting speed, it is necessary to reduce the mass of the movable parts of the switch to a minimum.

In the case of thermally released switches, quick action is most readily obtained by direct heating of the thermal element, since indirect heating causes a delayed release. However, for heavy current service ordinarily a special cutout switch is required which is controlled mechanically or electrically by a thermal time relay. In.

this case, the particular time required for operation of the excess current switch depends on the heating of the time element as well as the retarded operation of the cutout switch, so that in the case of arc rectifiers, considerable damage may occur before the endangered circuit is interrupted. f

Compared with this, the improved switch embodying the present invention has a very short opening time and'at the same time large current controlling capacity. According to the preferred form of the invention the movable switch actuating part consists of a 'bi-metallic vthermal element formed as the main switch lever and traversed by the main current. The bi-metallicelement carries the movable contact atits free end. The thermal element itself is thus used'directly for effecting the interruption of the current. In this case the relatively high contact temperature occurring due to the heating of the bimetallic element is overcome by the use of non-oxidizing contact parts. Moreover, that the welding together of the interrupting contacts is prevented to a very considerable degree by means of an overcenter spring mechanism providing a suflicient contact pressure during the disconnecting movement, and which also insures the, immediate separation of the contacts with great switching speed. The further details of the invention may be best explained by means of the constructional example represented inthe drawing. I

In the accompanying drawing Fig. 1 is a side view partly in section of a circuit controller. em-. bodying the present invention; Fig. 2 is a chart showing the variations in the operating forces occurring during operation of the circuit con- 05 troller; Fig. 3 is an enlarged view showing a'modiw fication of the operating electromagnet for'the switch; Fig. 4 is a view partly in section showing the detailed construction of the electromagnet armature and the thermal element; Fig. 5 is a top view showing the detailed construction of. the contact mechanism and Fig. 6 shows a modi-. fication of the'switch operating electromagnet structure.

As'shown in Fig. 1 the bi-metallic strip 1' traversedby the main current is firmly secured at its lower end, and at its upper end carries the main switch contact 2, whose mass is small. The result of this is that the temperature of the bimetallic element is transferred to the main contact with hardly any diminution. Since for the safe protection of electric lines and machines the operating temperature required for the bimetallic strip fluctuates within the limits of 100-200" C., the oxidation temperature for customary copper contacts is considerably exceeded. In order to overcome this difiiculty, the main moVablecontact or the cooperating contact piece is provided .with a silver insertion 3 atthat point at which the contacts touchone another in the circuit closing position. The cooperating contact 4, moreover, is not rigid, but is arranged so as to be yielding and is limited in its switching path by means of a stop 5. In order to keep the mass of the cooperating contact small, the current is con- 5 ducted to the same through a resilient band 6. The opening movement of the main contact 2 is also limited by a fixed stop '7. The operating force of the cooperating contact spring 6 is proportioned to that of the bi-metallic strip 1. In order to effect an instantaneous disconnection a powerful overcenter spring 8 is arranged so that on the opposite sides of the dead center, or critical position, it exerts a greatly increasing additional force on the contact 2 to either to close or to openthe same.

The operation of the forces of the mechanical system described, according to Fig. 1, can be seen from the diagram of Fig. 2. The characteristic lines, that i to say, the curves of the forces exerted by the resilient contact 1 and of the resilient cooperating contact 4, are approximately straight lines. The characteristic of the force occasioned by the tension of the overcenter spring 8 is also practically'a straight line in the case of its displacement beyond the dead center position. The position and direction of these characteristic lines are so selected according to the invention that the desired quick operation is brought about in the following manner.

The closed position of the contacts indicated in Fig. 1 is selected in the diagram as the zero point, the movement of the switch elements being represented along the abscissas and the forces on the switch elements by the ordinates. The closing force acting to the left in Fig. 1 is reckoned as'positive and the opening force as negative.

The force at the contacts 2 and 4, due to the displacement of the thermal element, is represented by the line Br at normal temperature, but it will be seen that this force is zero in the closed position. If the temperatureof the thermal element is increased above the normal temperature by a predetermined amount the force at the contacts will be represented by a straight line which is parallel to the line Br and displaced from it a predetermined amount. For example, assuming that the thermal element isunrestrained, the force at the contacts will be represented by the line By and in the diagram this line corresponds to the temperature required 'toopen the contacts. Upon reaching the releasing temperature, however, the characteristic line of the bimetallic strip is displaced by the amount Bgo which represents the thermal expansion due to the heating producedby the load.

The characteristic line Z of the spring 8 (considered with respect to the force exerted on contact 2) always cuts the zero force line in the dead center of the path of movement of contact 2 which distance is represented by the distance St. along the abscissa axis. In the zero position the force of the spring has the initial value represented by the ordinate Z0. Finally there is further the characteristic line K of the counter contact 4. In the closed position the force on the contact 4 is represented by the ordinate -Ko (minus because it represents an opening force which has been plotted above the abscissa). As shown, this force diminishes somewhat before the contact 4 is arrested by the stop 5. The force at the distance Sic drops to zero. In order to mainvtain the switch in the closed position, Zo must of course be greater than theinitial force Ko of the counter contact. As shown, the contact pressure is' represented by the difference between the forces Z0 and K0. 7 With increased heating of the bimetallic element a force is developed tending to open the contacts and counteracting the residual force-due to Z and K. When the force due to the thermal strip is such as that represented by the line By the net force on the contact element is zero and a very slight further increase will produce a net motion. The contacts are opened a distance So, large enough to interrupt the current but kept as small as possible to reduce stresses in the bimetallic element as much as possible.

The overload switch fulfills the further requirement that when the bi-metallic strip cools no further automatic reclosure of the switch takes place. If the reclosure of the switch is desired, it can be accomplished without difficulty. For example, if S0 is made larger than a certain amount, the force of the bimetal tending to close the contacts (when cool) is less than that of the overcenter spring tending to keep the contacts open. This arrangement would be used when it is desired to have the switch remain open after being tripped. If automatic closing is desired, So is made smaller so that Z-Br (representing the net force on the contacts) has a positive value tending to close the contacts. The cycle for the heating and. the cooling of the thermal element is indicated byv the plotted arrows.

. The present construction requires only a small operating force in order to reclose the switch. Further, the entire system can be arranged practically without friction due to the absence of all complicated intermediate members, such as levers, pawls and rods. Thus the necessary forces for automatic operation are also small when the switch is to be reclosed or opened independently of the bimetallic strip. The new switch therefore is particularly adapted for remote operation, since its operating magnets may be exceptionally small.

It can be seen from the diagram that the conditionsare fully illustrated for-the special case mentioned. The variation of the value of the current required for automatic operation of the switch can be obtained in different ways, for example, by varying one or the other constant of the system. For convenient calibration a variation of the spring constants is desirable. The cooperating contact 4, for example, can have an additional spring 9, arranged so that the characteristic line results from the combined effect of the two springs 6 and 9. In addition to varying the pressure by tensioning or untensioningthe spring 9, its constant can be adjusted by varying the' number of turns. The spring constant of the over-center spring 8' can be conveniently regulated by meansof an adjusting nut 10. Accordingly, as its force is varied the position of its characteristic curve Z is varied, since the latter must always pass through the dead center point St.

In the case of heavy short-circuits the temperature of the bi-metallic strip increases in the same manner as in the case-of a safety fuse. Since there is no time for conducting away the heat on a sudden overload, the electric energy causes a quick rise in the temperature of the thermal element, whereby the expansion takes place ven quickly. To open the switch it is only required to accelerate the small additional mass of I 12' of the main current magnet 13 as a magnet armature. In the case of switches for small current strengths, it will be understood that the resilient current conducting band 6 may consist of a magnetizable material, for example, steel, so that a special armature 11 is unnecessary.

The blow-out magnet 13 with coil 14 is used as main current magnet according to the invention. For obtaining a large switching capacity, contrary to the customary arrangement on excess current switchboards, this is not arranged above or behind the main contacts, but in front of and somewhat below the same. A stationary auxiliary electrode 15 extends over the blow-out coil and down the front side of the switch, so. that the interruption arc can extend on the front switch surface entirely free and unhindered. At

both sides special pole horns 16 are provided,

which are bent somewhat at right angles, and

embedded in a U-shaped recess, is covered almost completely by insulating material. Only the upper part of the contact which is located in the blow-out field is exposed. In this way the switch is protected to considerable degree against the injurious effect of the arc gases in the case of disconnection through short-circuit, against re-ignition, and against interpole sparking. The blowout arrangement is not built together with the contact, but is located at the front part of the switch. In the open position of the switch, no voltage difference exists between the contact and the blow-out poles. The front pole-of the switch, with the construction described, can without difficulty be entirely covered with insulating plates 17 since the operating mechanism and the connection for the individual switch poles (cross-bar 31 in Fig. 4) are located at the lowest end at the greatest distance from the arc.

The total switching time ofa switch, as is known, is made up from its own time and the duration of the arc. In order to avoid excessive heating of the thermal element 1 while-the arc lasts, the stationary horn 15extends over the contact 2, so that after the occurrence of the are it is led over on to'the horn 15 in the shortest time. This arrangement, in this case serves the special 'purpose of preventing excessive heating of the bimetallic element and thus shortens the time interval which is necessary before the reclosing of the switch can be accomplished.

As-explained by means of the diagram Fig. 2, an automatic reclosing of the switch can be effected after the cooling of the bimetallic element by an appropriate adjustment of the spring constants. In order to be able to effect a reclosure before the co aling occurs, the dead or critical point position of the rocking lever 1 can be varied. For this purpose the lower suspension point of the rocking spring 8 is mounted in a swing 21 which, as more clearly shown in Fig. 3, is rotatable about the point 25 against the eflectof a stop 18 which is yielding in the switch closing direction and which is pressed into its end position, by the spring 19.

To operate the switch by hand only a small motion of the fixed end of the over-center spring is required. This can be shown in the diagram as a displacement of the line Z to the left or right. If Z is displaced a distance Sa to the left the force Z0 becomes equal to K0 and a position of unstable equilibrium ensues and the switch opens. If the switch is closed and Z is displaced to the right a distance Se the force of the overcenter spring is reduced to equal the force of the bimetallic element and the switch snaps closed.

If the switch is to-be remotely operated then the swing'21 isformed as a magnet armature. At either side of this swing is arranged the closing and'opening magnets 22 and 23 respectively.

A particularly effective remote release is obtained f or the reason that the torque originating from the tilting spring 8 in the closing direction is partly or entirely balanced byan auxiliary force, for example vby the springs 24. This arrangement has no. influence on the contact pressurebut to effect the opening of the switch only a small jmovement of swing 21 is required. If

thejswinging armature 21 is further mounted on leaf springs-25 like a clock pendulum, so as to be free -from'friction, the sensitiveness can be increased'to an extraordinary degree. This is of great-importance since it often is desirable that the switch be controlled by means of an auxiliary sitive' to faults other than overload. 1 rangement of an auxiliary magnet armature '11 Fig. 1 on the counter contact 4 already bove gives without difliculty the unretarded rel se in the case of excess current. This is accomplished because the contact path Sk, as

can be seen in the diagram, is greater than the dead center position. If the oscillating armature 21'has time to fall into the switch opening position before the interruption arc has been .ex-'

tinguished, then the switch opening operation is ensured. If however the arc breaks, say, as a result of the existing counter E. M. F., then the contact 2. springs back into the switch closing position since the temperature of the bi-metallic strip 1 has not yet reached the releasing temperature. This kind of release is found to be particularly effective for very high short-circuit currents. However an additional overload release independent of the heating is desirable. Therefore on the blow-out magnet 13 additional pole pieces 27 are arranged which extend as far as the oscillating armature 21 and magnetize the section of the path St which extends only to the latter independently of the releasing coil 23.

Thus with the desired excess current the switch is moved positively into the disconnected open circuit position. The releasing current strength.

can be adjusted by arranging a spring to exert a counter force on the releasing armature 21 or by varying the gap between armature and magnet pole.

The described switch construction renders possible an extremely simple arrangement of a remote release, because the forces necessary for the operation are relatively small, and particu-' larly because the movement of the operating mechanism is very small. In contra-distinction to this the known switches require complicated lever mechanisms and pawl locking devices. In accordance with the present invention the means for insuring release free of the hand lever 29 con sists in a simple movable coupling. pawl 28 formed to function simultaneously as an armature movement of handle 29.

for the magnet pole 2'7 and as a mechanical connection between the hand lever 29 and the oscillating armature 21. Thus in the case of excess current the pawl 28 is influenced by the same magnet poles 27 which attract the oscillating armature 21. e

In order to beable to maintain the switch as narrow as possible throughout its entire height, it is desirable to arrange the over center spring 8, recording to Fig. 4, between the divided bi-metallic strip. In like manner, the oscillating armature 21 may be divided at the upper part or provided with a notch 30 in order to provide room for the turns of the spring. The force of the tilting spring 8 is used, according to the invention, for securing the coupling member 31 in multi-polar switches. For this purpose the spring tension adjusting screws 10 of the individual switch poles are located below the cross-bar 31 and press the latter against the individual oscillating armature 21.

In many cases it is desirable to increase the releasing current strength, for example during the starting of motors. In the present case this can be very easily done by having the releasing armature 28 connected to the operating handle 29 so that the air gap between releasing armature 28 and pole 2'7 is enlarged by the switch closing After the completed closure of the switch the handle 29 is moved back into the starting position by means of light spring 33 without thereby effecting a release of the switch. The stop 32 is then so arranged that in the closed position of the switch, the member 28 is moved the maximum distance away from the magnetic poles 27 and thus increases the value of current required for releasing the switch.

An increase of the releasing current is also brought about for the thermo element when the switch closing lever 29 is moved entirely into the end position against the pressure of the resilient stop'18, because Z0 is increased thereby. In the case of immediate reclosure of the switch, after the release has been effected by the thermal element, however, this method is only possible once because with'the second switching-in the bi-metallic strip has become heated to a greater extent. This limitation, however, is desirable for the protection of the switch as well as for the protection of the circuit.

A shunting of current from the main contact spring 6 is effected in the following manner: To

obtain relatively great flexibility of the leaf spring '6, the same must be kept somewhat long and thin,

from which a high electrical resistance results. According to the invention now the non-oxidizing part 3 of the contact 2 is so arranged, for example, so extended, that it engages directly with the grooved-shaped contact holder 34, as shown in Fig. 5. For currents which come below the limit current a large'part of the current goes from the contact piece 3 direct through the contact holder 34 to the connecting bolt 35, and thus reduces perceptibly the currents passing through spring 6. This is due to the fact that the ohmic resistance of the contact holder 34 is small compared with the resistance of the resilient spring 6. The contact surfaces on the contact holder 34 may be provided with a coating or a layer of a non-oxidizing metal.

As I have already stated, a modern switch must be capable of general use. The applications of my inventions have already been described in connection with remote controlled switches, with beyond the point of equilibrium of the movable system, and in place of the switch closing coil, an undervoltage coil is provided. In this case the magnetism which originates fromthe undervoltage coil, acts in opposition to the force of the auxiliary spring 24. The value of the releasing voltage can be conveniently adjusted by varying the tension of the spring 24. In order to utilize the switch with undervoltage release for remote control, a series resistance is inserted in the energizing circuit of the holding magnet, which resistance is entirely or partly short-circuited by' the remote control contact maker. The magnet thereby develops the necessary-surplus pull with a large air-gapto closethe switch. The resistance prevents overheating of the coil during the time the switch is closed In the case of the hitherto known forms of overload switches with under-voltage release, such a remote control arrangement was out of the question, hecauseof the energy requirements for closing the switch. I

For certain switching purposes, remote control switches, in many cases, must be equipped with an automatic arrangement for deenergizing the operating coils after the operation has been effected. The interrupter in this case entails special difiiculties for the switch closing magnet, because the movement of the switch available for the movement of the coil deenergizing interrupter is mostly very limited. For example, .in the case of latched switches, the interruption can only be effected at the very last moment, after the holdingpawls, etc. have come into engagement. In order to make the coil deenergizing interrupter, or the interlock contacts, safe in its action, special locking devices, quick-acting circuit arrangements, and complicated rod systems are necessary, which frequently give rise to faulty operations. Another difficulty is found in the fact that the switch closing magnet consumes a large amount of energy which results in considerable arcing when the interlock contacts are opened. 7

In accordance with the present invention these difiiculties do not arise, since the forces necessary for the closing, as well as for the opening of the switch, are particularly small.- The energy consumption of the switch closing coil is conse quently very small; also by means of the present construction, the described difiiculty caused by the limited movement available only at the last moment, is avoided. In this case, the entire switching movement, from the dead center position as far as the final closing position is available for the interruption of the switch closing coil currents. Therefore, as indicated in Fig. 5, the interrupter contacts 38 and 39 for the switch closing coils can, without difficulty, be coupled with the counter contact 4. Thus in case the oscillating armature has'fallen away, then the'main contact automatically follows, and since the closing coil interrupter is moved direct by the latter, the interruption of the coil circuit is likewise guaranteed.

The interruption of the closing coil circuit by the arc. The counter contact 4 is located, according to Fig. 5, in a U-shaped recess 34 which is tightly shut off above the contact by means of a short pole horn 5 (Fig. 1). At the lower end and in front the recess is closed by means of insulating plates 36, 37 (Fig. .1).- With. the movement of the counter contact 4 a compression or a vacuum arises in the'recess, since the resilient spring 6 operates in the recess like a piston (Fig. 5). The interruption contacts 38 and 39 are arranged laterally at this recess opposite openings 40 which lead to the interior of the recess. Thus a current of air is blown against the interruption contacts for the switch closing coil, or sucked up for the interruption contacts of the switch opening coil, whereby an easier extinction of the interruption arcs is brought about.

The switch can also be used as a contactor. For this purpose it is only necessary to make the above-mentioned under-voltage arrangement and to leave out the series resistance. With the excitation of the switch closing coil 22 the contactor is closed and is automatically disconnected by the spring 24 upon the coil 22 becoming deenergized.

The described construction can also be advantageously used when a thermal quick-acting release is not desirable or is unnecessary. In this case the bi-metallic strip 1 is replaced by the usual metal (steel or bronze). On account of the easy operation of the switch it is possible without difiiculty to adjust the releasing current for low values. In this manner, that is to say, without thermal release, the third or fourth pole of a multi-polar switch for example can be constructed.

The described magnet system can be used for protection against reverse current flow. Its operation is independent of voltage fluctuations within widelimits or of the magnitude of the current flow in the normal direction. For this purpose a current coil 41 (Fig. 6) is wound about the oscillating armature 21, while the shunt coils 22, 23, are connected to the system or to a separate current source. The magnet system therefore acts as the-polarized magnet, that is to say, with normal current flow, the armature is firmly attracted while with reverse current flow the release takes place upon the reversal of the current. The character of the magnet system can be differently arranged by changing the airgaps in the left or right magnet circuit.

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

1. In combination, a thermo-responsive movable switch member having means for insuring operation thereof with a snap action after movement of the switch member to a predetermined critical position, a cooperating switch member biased for movement with said thermo-responsive switch member, and means for arresting the movement of said cooperating switch member as soon as the snap action movement of said firstmentioned switch member is initiated.

2. In combination, a movable thermo-responsive element, an over-center spring connected to said element, a movable switch contact mounted on said element, a cooperating switch contact biased for movement with said first contact, said thermo-responsive element being arranged to control the operation of said over-center spring to operate said movable switch contact with a snap action to and from a closed circuit position with said cooperating contact, and means for arresting the movement of said cooperating switch contact as soonas said over-center spring initiates the snap action movement of said movable contact from a closed circuit position to an open circuit position.

3. An overload circuit breaker comprising a current conducting bimetallic element fixed atone end, a contact mounted upon the free end of said element, an over-center spring for operating said contact with a snap action between predetermined positions, said bimetallic element being arranged to operate said over-center spring from one over-center position to the other overcen-' ter position, a cooperating contact for engaging said first-mentioned. contact, means for biasing said second contact for movement between predetermined positionsso that when said bimetallic element operates the over-center spring said contacts remain in engagement until after said over-center spring initiates the opening of said contacts with said snap action.

4. An overload circuit breaker comprising a thermo-responsive movable switch member, a co-' operating switch member biased for movement with the thermo-responsive switch member to a predetermined position, and an electromagnet energized in accordance with the current through the contacts for electromagnetically holding the contacts in engagement during movement thereof to said position.

5. In combination, a pair of cooperating movable switch members, a stop for limiting the movement of one of said members, and an electro-responsive means energized in accordance with the current through the switch members for maintaining the same in engagement during the limited movement of said one member.

6. An overload circuit breaker comprising a current conducting bi-metallic element fixed at one end, a contact mounted upon the free end of the element, a cooperating contact for engaging with the first contact when the temperature of the bi-r'netallic element is below a predetermined value and biased for movement with the first contact to a predetermined position upon the heating of the bi-metallic element above said temperature, an over-center spring for biasing the first contact into engagement with said c0- operating contact on one side of said position and out of engagement therewith on the other side of said position, and means for controlling the action of said over-center spring to return said first contact into engagement with said cooperating contact.

7. An over-load circuit breaker comprising a current conducting bi-metallic element fixed at one end, a contact mounted upon the free end of the element, a cooperating contact for engaging with the first contact, an over-center spring for biasing the first contact into engagement with said cooperating contact to conduct current to the bi-metallic element and out of engagement therewith upon a predetermined movement of the bi-metallic element due to heating, and means associated with said over-center spring for selectively biasing the first contact into and out of engagement with said cooperating contact upon the subsequent cooling of said oi-metallic element.

8. In combination, a thermal responsive element having one end fixed, an overcenter spring for biasing the free end of the element for movement on either side of a predetermined critical position depending upon the temperature of the element, and means including a movable member for controlling the overcenter biasing spring to bias the free end of the thermal element from one side of said critical position to the other independently of the temperature of the element.

9. In combination, a thermal responsive element having an overcenter biasing spring and means including a movable member for controlling the overcenter biasing spring to operate the thermal elementovercenter independently of the temperature oi tble element.

10. In combination, a movable thermal responsive element having an overcenter biasing spring and electro-responsive means for controlling the overcenter biasing spring to operate the thermal element independently of the temperature thereof.

11. In combination, a Uj-shaped thermal element having the ends thereof fixed, an overcenter spring having one end connected to the thermal element and extending between the fixed ends thereof, and a movable member connected to the other end of the overcenter spring for controlling the biasing action of the spring on the thermal element.

12. An overload circuit breaker comprising a thermo-responsive movable switch member, an over-center biasing spring operatively associated with said member, means including a voltage responsive coil for controlling the over-center biasing spring to operate said movable member independently of the temperature of said thermoresponsive member.

13. In combination, a movable thermo-respon-' sive element having an over-center biasing spring, means for controlling the over-center biasing spring to operate the thermal element independently of the temperature thereof, comprising an electromagnet provided with an armature, and energizing means associated with said armature operating said armature between predetermined positions so as to control the operation of said over-center biasing spring.

14. A circuit breaker comprising a current conducting bi-metallic element fixed at one end, a contact mounted upon the free end of said element, a cooperating contact for engaging said first contact, an over-center spring attached at one end to said bi-metallic element for biasing said first contact into and out of engagement with said cooperating contact upon a predetermined movement of said bi-metallic element, and a movable member to which the other end of said biasing spring is attached for'operating said biasing spring over-center independently of the temperature of said element.

15. A circuit breaker comprising a current conducting bi-metallic element fixed at one end, a contact mounted upon the free end of said element, a cooperating contact for engaging said first contact, an over-center spring attached at one end to said bi-metallic element for biasing said first contact into engagement with said 00- 100 operating contact to conduct current to the bimetallic element and out of engagement therewith upon a predetermined movement of said bi-metallic element, due to heating, and a movable member to which the outer end of said bias- 105 ing spring is attached and electromagnetic means for operating said movable member in one direction or the other to control said over-center biasing spring so as to operate said element independently of the temperature thereof.

WILHELM HoPP.

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