GB1595824A - Magnetic transducer device - Google Patents

Description

(54) MAGNETIC TRANSDUCER DEVICE (71) We, MOSKOVSKY ENERGETICHESKY INSTITUT., a USSR corporate body of Krasnokazarmennaya ulitsa, 14, Moscow, USSR., do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to devices for converting a magnetic field intensity into an electric signal.

According to the present invention there is provided a transducer device arranged to develop an electric signal representative of the intensity of a magnetic field, comprising one ferromagnetic member having one end rigidly fastened to a support and having its free end overlapping the free end of another ferromagnetic member one of said one ferromagnetic members and said support being a resilient member, so that the presence of a magnetic field gives rise to bending stresses in said resilient member, at least one resistance strain gauge being mounted on said one of said one ferromagnetic member and its support, in the immediate vicinity of the point at which said one ferromagnetic member is fastened to the support, said strain gauge or gauges being connected to a resistance-responsive circuit arrangement.

Conveniently the source of the magnetic field may be a current conductor and the device may be provided with an assembly allowing rotation of the axis of the resistance strain gauge with respect to the longitudinal axis of the current conductor, the assembly being rigidly coupled with the supports of the ferromagnetic members.

Conveniently the transducer device may be provided with a permanent magnet mounted so that it can be shifted with respect to the ferromagnetic members in order to vary the resistance rating of the strain gauge the value of which is used as an additional indication of the permanent magnet position.

The transducer device may also comprise a control magnetizing coil connected via an amplifier-converter to an a.c. voltage source so as to produce periodic variations in the strain gauge resistance.

The invention by way of example with reference to the accompanying drawings in which: Figure 1 represents an embodiment of the invention comprising a transducer device for converting the intensity of a magnetic field into an electric signal, the device being provided with one resistance strain gauge; Figure 2 shows the device as shown in Figure 1, viewed from above; Figure 3 represents an embodiment of transducer device with two resistance strain gauges; Figure 4 represents an embodiment of transducer device with four resistance strain gauges; Figure 5 represents in longitudinal cross section an embodiment of transducer device with four resistance strain gauges installed on membranes which are secured to a supporting body; Figure 6 represents in partial longitudinal cross section an embodiment of transducer device including a field source formed by a current conductor; Figure 7 represents an embodiment of transducer device including a permanent magnet field source Figure 8 is a diagram showing preferred connections between the resistance strain gauges of the transducer and the measuring circuit; and Figure 9 shows an embodiment of transducer device wherein the resistance strain gauges are connected, via an amplifierconverter, to a magnetizing control coil.

The transducer shown in Figure 1 for converting the intensity of a magnetic field into an electric signal comprises a resiliently deformable element made as a ferromagnetic member 1 of which one end is rigidly fixed in a support 3, a further ferromagnetic member 2 has one end rigidly fixed in a support 4 and the members 1 and 2 are arranged so that their free ends overlap each other. The device also comprises an element for sensing the deformation due to displacement of the free end of the fer romagnetic plate 1. tlis element is formed by one resistance strain gauge mounted in the immediate vicinity to the point where the ferromagnetic plate is fixed in the support, where maximum strain occurs. The embodiment of the invention described herein uses one resistance strain gauge 5 mounted on the ferromagnetic plate 1.

In use the terminals 6, 7 of the resistance strain gauge 5 are connected to a measuring circuit, as shown in Figure 2.

Figure 3 presents another embodiment of transducer device for converting the intensity of a magnetic field into an electric signal which comprises an additional resistance strain gauge 9, having terminals 10 and 11, which is mounted in a way similar to that of the resistance strain gauge 5. in the immediate vicinity to the point where the ferromagnetic plate 1 is fixed in the support 3 but on the opposite surface of the plate 1 so that it is oppositely affected by the deformation.

The presence of the additional resistance train gauge 9 subjected to strain opposite to that of the resistance strain gauge 5 and located in its immediate vicinity, makes it possible to increase the magnitude of the useful signal (in the form of resistance increment) and to reduce the temperaturedependent conversion error caused by variations with temperature of the strain gauge.

Figure 4 represents another embodiment of the transducer device for converting the intensity of a magnetic field into an electric signal which comprises in addition to those in the embodiment of Figure 3, two additional resistance strain gauges 12 and 13 having terminals 14, 15, 16, 17 which are mounted on the ferromagnetic member 2, which in this case must also be resiliently deformable.

The use of the two additional resistance strain gauges 12 and 13 makes it possible to increase the magnitude of the useful signal still further though the deformation of the ferromagnetic plates 1 and 2 is the same.

The transducer device for converting the intensity of a magnetic field into an electric signal shown in Figure 5 differs from the embodiment shown in Figure 4 in that it comprises a tubular casing 18 enclosing the ferromagnetic plates 1 and 2. The casing 18 is made of non-ferromagnetic material. The opposite ends of the casing 18 are provided with expanded sections having membranes 19 and 20 secured to them. The diameter of these membranes 19, 20 exceeds to a certain extent that of central section of the casing 18. The membranes 19 and 20 have their edges rigidly fixed (e.g. welded) to the casing 18. Connected rigidly to the centers of said membranes 19 and 20 are the ferromagnetic plates 1 and 2 respectively. The connections may be either sealed or unsealed. In the embodiment described herein the membranes 19 and 20 serve as resilient elements and perform the function of supports for the ferromagnetic plates 1 and 2.

Resistance strain gauges 5, 9 and 12, 13 respectively are placed on the outer surfaces of the membranes 19 and 20, and are secured at points at which deformation occurs in the immediate vicinity of the points where the ferromagnetic plates 1 and 2 are fixed.

The use of the casing 18 and the membranes 19 and 20 which are rigidly connected to it makes it possible. firstly, to osi- tion the resistance strain gauges 5. 9. 12, 13 after the major components, including the ferromagnetic plates 1 and 2 have been assembled; secondly, to arrange the resistance strain gauges 5, 9, 12, 13 on the outer surfaces of the membranes 19, 20, which facilitates the connection of other components to the terminals of the resistance strain gauges 5, 9, 12, 13, and thirdly. to increase the number of resistance strain gauges in the system by securing them upon sections of the membranes 19, 20 subjected to deformation. Still another advantage of the last-described embodiment of the invention described herein consists in that its design features allow automation of the process for its production.

Figure 6 represents another embodiment of the transducer device wherein the field source is current conductor 21. In addition the device is provided with an assembly 22 for rotating the axis of the resistance strain gauge 5 in a plane parallel to the longitudinal axis of the current conductor 21. The rotating assembly 22 is rigidly coupled with the supports 3 and 4 through a tubular casing 18. The supports 3 and 4 are conveniently made as washers of insulating material. The casing 18 is made of nonferromagnetic material. In order to increase the accuracy with which the intensity of a magnetic field is converted into an electric signal the casing 18 is preferably made of copper so as to form a short-circuited winding.

The use of the rotatable assembly 22 allows the range and the sensitivity of conversion of the intensity of a magnetic field generated by the current conduit 21 into an electric signal to be varied by altering the angular position to which the assembly is rotated.

Figure 7 shows an embodiment of the transducer device wherein the field source is a permanent magnet 23 which serves as a component of the device. The permanent magnet 23 also performs the function of a means for transferring data as to its position, to which end it is mounted so that it can be shifted with respect to the ferromagentic plates 1 and 2 and thereby vary the resistance of the strain gauge 5. The permanent magnet 5 is shifted with the help of a holder 24 wherein the permanent magnet 23 is fixed and by means of a rod 25 housed in guides 26 and rigidly coupled with the holder.

The directions indicated by the doublearrows "X" in Figure 7 in which the permanent magnet 23 shifts with respect to the longitudinal axes of the ferromagnetic plates 1. 2 are not the only possible directions. The embodiment allows the position of the permanent magnet 23 to be changed by rotation of rod 25 as well as by its movement along the length of the ferromagnetic plates 1 and 2.

The reference "X" indicates the effect caused by a sensor responsive to the parameter to be measured (not shown) on the rod 25 which shifts the permanent magnet 23. The functions of the sensor for the parameter to be measured may be performed by a pressure pick-up, a temperature sensor, etc., which converts the parameter to be measured into a displacement. The assembly described above is especially suitable in transferring the results of measurements to a remote user.

One of the possible modifications of the embodiments described above is a device for converting angular position of the rod 25 carrying the permanent magnet 23 into variations of the resistance of the strain gauge 25 and hence into an electric signal. Such a transducer is used most preferably in cases when the rate of rotation of the rod 25 is close to zero. The above circumstance is due to the fact that the transducer device for converting the intensity of a magnetic field into an electric signal, in which the functions of a field source are performed by the permanent magnet 23, combines in a most advantageous way both a means allowing the amount of displacement of the permanent magnet to be determined and a means presenting information on the position (coordinate) of the permanent magnet 23.

In particular the proposed embodiment makes it possible reliably and accurately to solve the problem of determining the position of slowly rotating rotor in electric machines such as step-by-step motors.

Figure 8 shows an embodiment of the invention wherein four resistance strain gauges 5, 9, 12 and 13 are interconnected with one another and connected to the measuring circuit. Specifically, there is shown in Figure 8 the connection of the resistance strain gauges 5, 9, 12, 13 into a bridge circuit of which one disposal is connected to a source of supply voltage U and the other to the measuring circuit 8. The above method of interconnecting the resistance strain gauges 5, 9, 12, 13 and the measuring circuit 8 makes it possible to obtain a large electric signal in the form of a change in output current. This is due to the fact that a variation of the magnetic field intensity will cause a change of the resistance of all the four strain gauges 5, 9, 12, 13. In addition, the error introduced by ambient temperature variations may be reduced drastically by selecting resistance strain gauges having similar temperature responses. The transducer device employing the circuit arrangement of Figure 4 may be used to advantage as a remotely variable resistor, the range of which may be varied either by changing the current that flows through a magnetizing coil or by means of spitting the position ot a permanent magnet.

An additional advantage of the described method of interconnecting the resistance strain gauges is that it allows an output to be obtained in the form of a signal current proportional to the product obtained by multiplying the supply voltage "U" applied to a level by the load current "I" which is used to determine the intensity of the magnetic field which is converted by the transducer device into an electric signal.

When the supply voltage "U" is set equal to the voltage drop across the ohmicinductance load having the current "I" flowing through it the output of the circuit will be proportional to the power consumed by the load.

Figure 9 illustrates an embodiment of transducer device for converting the intensity of a magnetic field into an electric signal which comprises a control magnetizing coil 27 connected, via an amplifier-converter 28, to the resistance strain gauges 5, 9, 12, 13.

This arrangement makes it possible to obtain a feedback loop using the position of the ferromagnetic plates 1 and 2 as the source of a feedback signal which can be either positive or negative, which allows modification of the conversion characteristic of the transducer for converting the intensity of a magnetic field into an electric signal. In particular the device can operate as a null-inductor. However, in this case it is required that the electromagnetic field generated by the control magnetizing coil 27 should compensate for the effect of the external magnetic field to be converted into an electric signal. If there are several control magnetizing coils (not shown in Figure 9) the displacement of the ferromagnetic plates 1 and 2 and their resultant position will be determined by the sum (or difference) effect of the coils.

In another version of the embodiment of Figure 9, the control magnetizing coil 27 is connected, via the amplifier-converter 28, to an a.c. voltage source, thereby providing periodic variations in the resistance of the strain gauges 5, 9, 12, 13 and enabling a.c.

amplifiers to be used for handling the signals from the strain gauges.

The transducer device, for converting the intensity of a magnetic or an electromagnetic field into an electric signal which is shown in Figures 1 and 2 operates as follows.

As soon as a control signal in the form of a longitudinal or transverse magnetic or electromagnetic field is applied to the ferromagnetic plates 1 and 2 the latter will start approaching each other and with the further increase of the control signal they will diverge, provided that the initial gap and the amount of overlapping between the ferromagnetic plates 1 and 2 exceed certain limiting values i.e. the values at which the ferromagnetic plates 1 and 2 still contact each other. In case the above requirements are not met the proposed device will operate within the time interval from the moment the ferromagnetic plates 1 and 2 start approaching each other till the moment when they start moving abruptly to make contact with each other. In any, case, however the above mutual approach or the terromagnetic plates 1 and 2 is accompanied by their resilient bending strain since they are rigidly fixed in the supports 3 and 4. The outer surfaces of the ferromagnetic plates 1 and 2 expand while the inner ones contract.

In the particular case of a cantilever arrangement of the ferromagnetic plates 1 and 2 the section subjected to the maximum strain will be located in the immediate vicinity of the point where the ferromagnetic plate is fixed in the support 3. It is at this section that the resistance strain gauge or gauges should be located. A properly selected and mounted resistance strain gauge will vary in resistance in proportion to the displacement of the free end of the ferromagnetic plate 1. The variations of the resistance of the strain gauge 5 are sensed by the measuring circuit 8 so that its output indicates the intensity of the magnetic field.

measuring circuit 8 so that its output indlcates the intensity of the magnetic field.

The operation of the embodiment of the transducer device shown in Figure 3 is generally similar to that of the arrangement described with reference to Figures 1 and 2.

However the use of the complementary resistance strain gauge 9 makes it possible to compensate for temperature errors and to raise the level of the useful signal in any of the known ways.

The operation of the transducer device for converting the intensity of a magnetic field into an electric signal arranged as shown in Figure 4 is similar to that of the preceding embodiments of the device. In order to raise the level of the output still higher and to reduce errors to the minimum it is necessary to select the resistance strain gauges 5, 9, 12 and 13 so that they have similar responses and to install the ferromagnetic plates 1 and 2 symmetrically so as their free ends bend to one and the same extent while the resistances of the strain gauges 5. 13 and 9. 12 are equal.

The operation of the transducer device as shown in Figure 5 is basically the same as that of the preceding embodiment. The fact that the interior of the device is protected by a casing allows its response to be stabilized to a certain extent against variations of the amount of dust, gas and other components of the environment. The major difference.

however, consists in the use of the deformation suffered by the membranes 19 and 20, here performing the functions of resilient elements. The ferromagnetic plates 1 and 2 and the membranes 19, 20 in the device are designed so that the increase of the control signal represented by a longitudinal or transverse magnetic or electromagnetic field causes the ferromagnetic plates 1 and 2 to approach each other and create an undulating deformation in the membranes rather than bending. In this case the sections of maximum strain in every membrane 19 and 20 will be located at the points where they are fixed to the ends of the casing 18 and in the immediate vicinity of the place where the ferromagnetic plates 1 and 2 are rigidly fixed in the membranes 19 and 20. It is at this location that the resistance strain gauges 5, 9 and 12, 13 are to be located. Properly selected and mounted resistance strain gauges 5, 9, 12, 13 will vary in resistances in proportion to the displacement of the free ends of the ferromagnetic plates 1 and 2.

The operation of the device shown in Figure 6 is basically similar to that of the embodiment described in relation to Figures 1 and 2. The only difference consists in that the control signal represented by a longitudinal magnetic field affecting the system of ferromagnetic plates 1 and 2 is generated by a direct current flowing through the current conductor 21. The use of copper to form the casing 18 allows errors caused by expected variations of the current I to be reduced.

The device will exhibit maximum sensitivity when the longitudinal axis of the ferromagnetic plates 1, 2 is orthogonal to that of the current conductor 21, and minimum sensitivity when the above axes are arranged parallel to each other. If it is found that the electromagnetic fields to be converted saturate the ferromagnetic plates 1 and 2 of the device the casing 18 will be turned into a position where saturation of the plates 1 and 2 does not occur. The above-described transducer device permits strong direct currents (tens and hundreds of kiloampers to be reliably and accurately converted into electric signals as increments of the resistance ratings of the strain gauge 5. The operation of the transducer device in Figure 7 is basically similar to that of the embodi ment presented in Figures 1 and 2. The difference, however. consists in that the control signal represented by a longitudinal (or transverse) magnetic field that effects the system of ferromagnetic plates 1 and 2 is generated by the permanent magnet 23. The displacement of the permanent magnet 23 will vary the intensity of the magnetic field to be converted by the ferromagnetic plates 1 and 2 and the resistance strain gauge 5 into an electric signal. In case it is required to determine discrete displacements of the permanent magnet 23 when, for instance, the amount of the production is counted piece by piece, the resistance strain gauge 5 will be connected, via an amplifierconverter, to a counter responsive to the discretely varying resistance of the strain gauge 5 (the amplifier-converter and the counter are not shown in the drawing).

A circuit arrangement wherein the resistance strain gauges and the measuring circuit are interconnected as shown in Figure 8 will operate in case the ferromagnetic plates 1 and 2 are being displaced and the resistance ratings of the strain gauges 5, 9 and 12. 13 are changing.

Since the resistance strain gauge 5 and 9 as well as 12 and 13 are subjected to deformation effects caused by the displacement of the ferromagnetic plates 1 and 2, the deformations being of opposite signs, their arrangement in adjacent arms of the bridge will ensure the maximum sensitivity of the circuit with respect to the intensity of the magnetic field to be converted (the sensitivity of the circuit is the ratio between the relative increment of the current at the output of the bridge to the relative variation of the intensity of the magnetic field). The initial adjustment of the bridge requires that the resistance strain gauges 5, 9, 12, 13 should be accurately selected. However this adjustment could be facilitated by adding a complementary balancing resistor (not shown) to the circuit of the device.

The operation of the device shown in Figure 9 depends on the mode of converting the intensity of a magnetic or electromagnetic field into an electric signal. This mode can be either continuous or intermittent. In the continuous conversion mode, using negative feedback, the output of the amplifier-converter 28 is applied to the control magnetizing coil 27 so as to cause the ferromagnetic plates 1 and 2 to return to their initial positions since it is arranged that the electro-magnetic field generated by the control coil 27 will compensate for the effect of the magnetic field to be converted. This mode of operation of the device ensures high accuracy and sensitivity of the conversion process. In the intermittent conversion mode with negative feedback the output of the amplifier-converter 28 is applied to the control magnetizing coil 27 in a manner similar to that described above. However the shape and the duration of the signal should be different.

The operation of the device discussed herein is intended to obtain a commutation mode that will be free from vibration and to ensure the optimum rate of commutation whereat the time required for the ferromagnetic plates 1 and 2 to shift from one stable position into another will be minimal.

The proposed mode of operation of the device is based upon the theory and methods of optimal control and a complement part of them which is known as dynamic programming. In order to obtain the above effect the control action represented by the number of ampere-turns should be made as high as possible. The ferromagnetic plates 1 and 2 to be closed are accelerated to the maximum speed and then the control action is dropped abruptly to the zero level. Thus, the ferromagnetic plates 1.

2 are "self-braked" and close at a zero speed. After that the control action is generated again to ensure that the ferromagnetic plates 1 and 2 remain in the closed position. The value of the speed at which the ferromagnetic plates 1, 2 are displaced and their positions are determined with the help of data provided by the resistance strain gauges 5,9, 12 and 13.

The embodiment in which the control magnetizing coil 27 is connected to an a.c.

voltage source via the amplifier-converter 28 operates as follows. The amplitude and frequency of the supply a.c. voltage are set in such a manner as to provide forced oscillations of the ferromagnetic plates 1 and 2, accompanied by periodic variations in the resistance of the strain gauges 5, 9, 12, 13.

In the presence of an invariable or slowly variable control action in the form of the magnetic field intensity, amplitude modulation of the periodicity variation in the resistances of the strain gauges takes place.

WHAT WE CLAIM IS: 1. 1. A transducer device arranged to develop an electric signal representative of the intensity of a magnetic field, comprising one ferromagnetic member having one end rigidly fastened to a support and having its free end overlapping the free end of another ferromagnetic member, one of said one ferromagnetic member and said support being a resilient member, so that the presence of a magnetic field gives rise to bending stresses in said resilient member, at least one resistance strain gauge being mounted on said one of said one ferromagnetic member and its support, in the immediate vicinity of the point at which said one ferromagnetic member is fastened to the support, said strain gauge or gauges being connected to a resistance-responsive circuit arrangement.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (8)

1. **WARNING** start of CLMS field may overlap end of DESC **.

   ment presented in Figures 1 and 2. The difference, however. consists in that the control signal represented by a longitudinal (or transverse) magnetic field that effects the system of ferromagnetic plates 1 and 2 is generated by the permanent magnet 23. The displacement of the permanent magnet 23 will vary the intensity of the magnetic field to be converted by the ferromagnetic plates 1 and 2 and the resistance strain gauge 5 into an electric signal. In case it is required to determine discrete displacements of the permanent magnet 23 when, for instance, the amount of the production is counted piece by piece, the resistance strain gauge 5 will be connected, via an amplifierconverter, to a counter responsive to the discretely varying resistance of the strain gauge 5 (the amplifier-converter and the counter are not shown in the drawing).

   A circuit arrangement wherein the resistance strain gauges and the measuring circuit are interconnected as shown in Figure 8 will operate in case the ferromagnetic plates 1 and 2 are being displaced and the resistance ratings of the strain gauges 5, 9 and 12. 13 are changing.

   Since the resistance strain gauge 5 and 9 as well as 12 and 13 are subjected to deformation effects caused by the displacement of the ferromagnetic plates 1 and 2, the deformations being of opposite signs, their arrangement in adjacent arms of the bridge will ensure the maximum sensitivity of the circuit with respect to the intensity of the magnetic field to be converted (the sensitivity of the circuit is the ratio between the relative increment of the current at the output of the bridge to the relative variation of the intensity of the magnetic field). The initial adjustment of the bridge requires that the resistance strain gauges 5, 9, 12, 13 should be accurately selected. However this adjustment could be facilitated by adding a complementary balancing resistor (not shown) to the circuit of the device.

   The operation of the device shown in Figure 9 depends on the mode of converting the intensity of a magnetic or electromagnetic field into an electric signal. This mode can be either continuous or intermittent. In the continuous conversion mode, using negative feedback, the output of the amplifier-converter 28 is applied to the control magnetizing coil 27 so as to cause the ferromagnetic plates 1 and 2 to return to their initial positions since it is arranged that the electro-magnetic field generated by the control coil 27 will compensate for the effect of the magnetic field to be converted. This mode of operation of the device ensures high accuracy and sensitivity of the conversion process. In the intermittent conversion mode with negative feedback the output of the amplifier-converter 28 is applied to the control magnetizing coil 27 in a manner similar to that described above. However the shape and the duration of the signal should be different.

   The operation of the device discussed herein is intended to obtain a commutation mode that will be free from vibration and to ensure the optimum rate of commutation whereat the time required for the ferromagnetic plates 1 and 2 to shift from one stable position into another will be minimal.

   The proposed mode of operation of the device is based upon the theory and methods of optimal control and a complement part of them which is known as dynamic programming. In order to obtain the above effect the control action represented by the number of ampere-turns should be made as high as possible. The ferromagnetic plates 1 and 2 to be closed are accelerated to the maximum speed and then the control action is dropped abruptly to the zero level. Thus, the ferromagnetic plates 1.

   2 are "self-braked" and close at a zero speed. After that the control action is generated again to ensure that the ferromagnetic plates 1 and 2 remain in the closed position. The value of the speed at which the ferromagnetic plates 1, 2 are displaced and their positions are determined with the help of data provided by the resistance strain gauges 5,9, 12 and 13.

   The embodiment in which the control magnetizing coil 27 is connected to an a.c.

   voltage source via the amplifier-converter 28 operates as follows. The amplitude and frequency of the supply a.c. voltage are set in such a manner as to provide forced oscillations of the ferromagnetic plates 1 and 2, accompanied by periodic variations in the resistance of the strain gauges 5, 9, 12, 13.

   In the presence of an invariable or slowly variable control action in the form of the magnetic field intensity, amplitude modulation of the periodicity variation in the resistances of the strain gauges takes place.

   WHAT WE CLAIM IS: 1. 1. A transducer device arranged to develop an electric signal representative of the intensity of a magnetic field, comprising one ferromagnetic member having one end rigidly fastened to a support and having its free end overlapping the free end of another ferromagnetic member, one of said one ferromagnetic member and said support being a resilient member, so that the presence of a magnetic field gives rise to bending stresses in said resilient member, at least one resistance strain gauge being mounted on said one of said one ferromagnetic member and its support, in the immediate vicinity of the point at which said one ferromagnetic member is fastened to the support, said strain gauge or gauges being connected to a resistance-responsive circuit arrangement.
2. 2. A transducer crevice in accordance

   with claim 1 in which said other ferromagnetic member is rigidly fastened at its other end to a second said support, one of said other ferromagnetic member and its second support being a resilient member, and at least one respective resistance strain gauge is mounted on said one of said other ferromagnetic member and said second support.
3. 3. A transducer device in accordance with claim 1 or claim 2 wherein the or each said resilient member has associated therewith two strain gauges arranged to be oppositely affected by deformation of said member.
4. 4. A transducer device in accordance with claim 3 as dependent upon claim 2 wherein four resistance strain gauges are connected in a bridge circuit, similarly strained gauges being in opposite arms of the bridge, and said bridge circuit has one diagonal connected to a supply and one diagonal connected to said resistanceresponsive circuit arrangement.
5. 5. A transducer device in accordance with any one of claims 1 to 4 in combination with a field source formed by an electric current conductor and means rigidly coupled with said supports for rotating the assembly of said ferromagnetic members, support and resistance strain gauges with respect to the longitudinal direction of said current conductor.
6. 6. A transducer device in accordance with any one of claims 1 to 4 wherein a permanent magnet is mounted so that it can be displaced with respect to said ferromagnetic members whereby the resistance of said at least one resistance strain gauge varies in accordance with the position of said permanent magnet.
7. 7. A transducer device in accordance with any one of claims l to 4. wherein a control magnetizing coil, in the field of which said transducer is immersed, is connected via an amplifier-converter to an a.c.

   voltage source so as to produce periodic variations of the strain gauge resistance.
8. 8. A transducer device arranged to develop an electric signal representative of the intensity of a magnetic field substantially as hereinbefore described with reference to and as shown in the accompanying drawings.