Search for treasures and treasures: Magnetometers - basic information. Fluxgate magnetometers Magnetometer with ferromagnetic core and three windings

Let's construct a vector diagram of an ideal transformer at no-load (Fig. 15.34).

Magnetizing force at idle

Let us now draw up an equivalent circuit for an ideal transformer under its load (Fig. 15.35).

If a load with resistance is connected to the terminals of the secondary winding Z n, then a current will pass through it , which, in turn, will tend to reduce the magnetic flux , and this will lead to a decrease in emf. , as a result of which the current will increase to such a value at which the magnetic flux will acquire its original value and equation (15.35) will be fulfilled.

Thus, the appearance of current in the secondary circuit leads to an increase in the current in the primary circuit. In a loaded transformer, the magnetic flux in the core is equal to the magnetic flux at no-load, i.e. Always F= const. Under load magnetic flux is created under the influence of magnetizing forces of the primary and secondary windings:

Let's construct a vector diagram of an ideal transformer under load (Fig. 15.36).

Let's transform the equivalent circuit of an ideal transformer, for which we will get rid of inductive coupling. If you connect the same terminals of the transformer windings to each other, the operating mode of the transformer will not change.

Let us first consider inductively coupled elements that now have a common point. The coupling coefficient of two elements in this case equal to one, since the entire magnetic flux is completely interlocked with the turns of the primary and secondary windings, i.e.

therefore, given that w 1 = w 2, we find:

Let us now replace part of the circuit with inductively coupled elements with a common point (Fig. 15.37 A) to an equivalent circuit without inductive coupling (Fig. 15.37 b).

;

;

Taking into account what was found, the circuit takes the form shown in Fig. 15.37 V, and the equivalent circuit of an ideal transformer is the type shown in Fig. 15.38.

If we now take into account the active and inductive leakage resistance of both windings, then for a transformer in which w 1 = w 2, we get the equivalent circuit shown in Fig. 15.39.

Let us write down the equations of the primary and secondary circuits of the circuit:

Let's construct a vector diagram of the circuit (Fig. 15.40).

To measure small constant and alternating magnetic fields, fluxgates are used, which in their simplest form are rods made of soft magnetic material and having two windings, one of which creates a temporary magnetic flux, and the other is measuring.

When an alternating current of a sinusoidal shape passes through the excitation winding, the magnetic state of the core will change along a dynamic hysteresis loop, and an e-wave appears in the measuring winding. d.s., which, in addition to the fundamental frequency, will contain higher odd harmonics.

Rice. 21. Schematic diagram ballistic installation: electromagnet, measuring coil, ballistic galvanometer, primary and secondary windings of the reference coil, switches, key, rheostat system, A - ammeter

Rice. 22. Schematic design of the measuring probe

If such a probe is placed in a constant magnetic field, directed in the same way as the alternating field along the axis of the core, then the magnetic state of the core will already change in an asymmetrical private cycle. This is explained by the fact that in the direction of action of a constant field, magnetization reversal of the core will occur at lower values ​​of the alternating field than in the absence of a constant field, and in the opposite direction, a constant field will prevent magnetization reversal. In this case, in the curve e. d.s. Along with odd harmonics, even ones, mainly second harmonics, will appear. It turns out that the value of e. d.s. this

harmonics are proportional to the magnetic field strength. By size e. d.s., proportional to this harmonic, and measure the field strength.

In Fig. 22 shows a schematic design of one of the measuring probes, the core of which is made of soft magnetic material permalloy. The core is made up of 20-50 plates thick. If the same number of turns of one wire are wound on both sides of the core in opposite directions, then the magnetic fluxes created by each winding will be equal.

Rice. 23. Schematic design of a bridge-type magnetic probe

Rice. 24. To the bridge type probe device

The windings are connected to each other in series. Measuring coil 3 is placed on core 1. If alternating current is passed through the turns of winding 2, then it will not occur in the measuring coil, since changes in magnetic flux over time from each winding 2 will be equal and opposite in direction. When the core is placed in a constant uniform field, which is directed perpendicular to the cross-sectional plane of the windings and coil, a redistribution of magnetic fluxes will occur in the space between windings 2, since the constant field will add up to alternating fields, as a result of which an electromotive force will arise in the measuring coil 3. This e. d.s. will be proportional to the magnetic field strength. Using such a probe, at an alternating current frequency of 103 Hz, it is possible to measure magnetic fields of the order of

There are currently bridge-type magnetic probes. One of these bridges is shown in Fig. 23. The bridge is cut out of sheet soft magnetic material (Fig. 24). It is assembled from several sheets, one half of which is cut lengthwise and the other half across the rolling. This ensures optimal magnetic uniformity of the bridge arms and improves the magnetic contact of the arms. The segments are folded and connected to each other so that a second diagonal of the bridge is formed. Coils 1 and 2 are located on the diagonals of the bridge, and any of them can be either measuring or exciting. The excitation coil winding is powered by industrial or

increased frequency. The magnetic bridge is in equilibrium, and without an external constant magnetic field, no emission occurs in the measuring coil. d.s. If the bridge is placed in an external constant magnetic field, then the equilibrium of the bridge is disturbed, an alternating magnetic flux will appear in the diagonal of the bridge and an emission will appear in the measuring coil. d.s. induction, the magnitude of which determines the value of the external field strength. Maximum value e. d.s. occurs in the measuring coil if the external field is directed parallel to two opposite branches of the bridge. To increase sensitivity, a magnetic bridge is sometimes made with poles (Fig. 25).

Rice. 25. Schematic design of a magnetic probe with poles

Let's consider a highly sensitive compensation magnetometer for measuring magnetic field strength up to , where a magnetically saturated probe is used. The schematic diagram of the magnetometer and the section of the magnetically saturated probe are shown in Fig. 26 and 27.

The magnetometer circuit consists of an excitation and signal circuit, a compensation circuit, and a circuit for testing the sensitivity of the device

The excitation and signal circuit includes a generator 4, a frequency doubler 5, a phase discriminator 6, a resonant amplifier 7 and an indicator device 8. To increase sensitivity, the device uses a compensation measurement method, in which the measured field of solenoid 2 is compensated by another field of known magnitude and the opposite direction. This field is created by a current-carrying coil within which probe 1 is located. Compensating coil 3 is used either in the form of a conventional solenoid or in the form of a closed coil. The second type of coil is used when ferromagnetic materials are located near the magnetometer.

Compensation can also be carried out using a current that is passed through the measuring winding of the sample. In this case, the dimensions of the measuring head are significantly reduced, but the uniformity of the compensating field deteriorates. To power the compensation circuit, use rechargeable batteries large capacity. The magnetically saturated probe consists of two cores 6 made of molybdenum permalloy. The cores are assembled from plates of size that are cut along the rolled stock and subjected to heat treatment. On the cores there is an excitation winding 4 with 1400 turns of wire in diameter and a measuring winding 3 with 400 turns of wire

A voltage of 25 V Hz is supplied to the excitation winding. The excitation current is 0.3 A. Under these conditions, the installation has the greatest sensitivity. Before starting measurements, the probe is adjusted by moving the core in the Helmholtz coils. The signal received on the measuring winding is amplified by a tuned resonant amplifier and then fed to a phase discriminator. The deviation of the pointer of the zero device by 2-3 divisions corresponds to the magnetic field strength. The described magnetometer is stable in operation and its mode is practically independent of changes in external conditions (temperature, mechanical vibrations, etc.).

Rice. 26. Schematic diagram of a magnetometer with a magnetic probe: 1 - probe, 2 - solenoid, 3 - compensating coil, 4 - generator, 5 - frequency doubler, 6 - phase discriminator, 7 - resistive amplifier, 8 - indicator device, compensation circuit, circuit to check the sensitivity of the device

The work provides a calculation of the optimal operating conditions of a probe consisting of two permalloy cores with dimensions 0.18X1.75X100. The excitation winding is wound from wire with a length of 350 turns. The measuring winding consists of 1500 turns of wire. At the output of the installation, a voltmeter is turned on, which records only the value day off e. d.s. second harmonic. To calculate the effective value of the amplitude of this harmonic, use the following formula:

where is the external measured magnetic field, the sensitivity of the probe to the external field in the second harmonic. The last value is determined by the formula

where the number of turns of the measuring winding, the cross-sectional area of ​​the cores, is the frequency of the alternating current supplying the excitation windings, a coefficient that takes into account flux dissipation is a certain constant, depending on the magnetic properties of the material and the demagnetizing factor.

Sensitivity is determined at the optimal value of the bias current, the strength of which is calculated using the formula

where is the number of turns in the field winding.

The described probe has high sensitivity if a long core is used.

Grabovsky and Skorobogatov used a permalloy fluxgate to measure the coercive force. Their installation consisted of two completely identical magnetizing coils, between which a fluxgate of length, width and thickness was located. A current was passed through the coils in such a direction that in the space occupied by the fluxgate, the magnetic fields of the coils were mutually compensated. To measure the coercive force, a magnetized sample was placed in one of the coils, and the magnetic field of the sample caused a deflection of the needle of the device, which was included in the indicator winding located on the fluxgate. By passing a direct current through the magnetizing coils, the sample was gradually demagnetized. At the moment when the needle of the indicator device returned to the zero position, the current strength in the coils was measured and the value of the coercive force was calculated using the formula where is the coil constant.

Rice. 27. (see scan) Section of a magnetic probe: 1 - current-carrying petals, 2 - body, 3 - measuring winding, 4 - excitation winding, 5 - frame, 6 - core, 7 - insulating gasket

Using the described coercimeter, you can quickly measure with an accuracy of 2-3%.

In the Janus coercimeter, the fluxgate has the shape of a frame, on the sides of which there are two windings: excitation and measuring. The test sample is placed in the solenoid so that its ends protrude from the solenoid. They are adjacent to an iron yoke, the middle part of which is closed by the fluxgate core.

Drozhzhina and Friedman proposed a fluxgate

magnetometer for studying the magnetic properties of soft magnetic materials. In their magnetometer, the movable astatic system was replaced by fluxgates, which made it possible to eliminate zero fluctuations. The fluxgate consists of two cores made of permalloy. The field windings are connected in series so that the magnetic fluxes of the cores are mutually closed. The measuring windings of the fluxgate are connected differentially, and without an external constant field the sum of the induced e.g. d.s. in these windings is zero. In the presence of a constant magnetic field in e. d.s. even harmonics appear, the magnitude of which determines this field.

A fluxgate magnetometer consists of two identical solenoids located horizontally one below the other, in one of which the sample under study is placed. The differential fluxgate is located between these solenoids. The magnetic fields of solenoids without a sample are mutually compensated in the volume where the fluxgate is located.

For high-quality measurements, it is better to use an astatic fluxgate magnetometer. In this embodiment, one fluxgate is located between the solenoids, and the other is at a distance from the first one in a parallel horizontal plane. The windings of these fluxgates are connected in series towards each other.

Using a fluxgate magnetometer, you can determine the magnetization curve, hysteresis loop and coercive force of soft magnetic materials. The magnetization curve and hysteresis loop are measured using the compensation method. For this purpose, a current is passed through the compensating winding, the magnetic field of which compensates for the field of the magnetized sample in the area where the probe is located. To measure the coercive force, you need to magnetize the sample, and then, by increasing the demagnetizing field, reduce the readings of the indicator device to zero. Simple scheme and the fast measurement process are one of the advantages of a fluxgate magnetometer over other magnetometers, which will be described in Chapter V. Recently, some types of magnetic probes have begun to be used to study the magnetic field in accelerators and spectrometers. A description of the probes is also available in the works.

In magnetometers of this type, the magnetically sensitive element is a fluxgate, which consists of two thin and long rods made of permalloy (an iron-nickel alloy - a soft magnetic ferromagnet), on which the primary (exciting) winding is wound in the opposite direction. In addition, both cores, together with the primary winding, are covered by a secondary (measuring) winding (Fig. 3.15 a). Soft magnetic ferromagnets are characterized by the fact that the hysteresis loop for them is so narrow that it can be considered as one curve (Fig. 3.15 b).

Rice. 3.15. Operating principle of a fluxgate magnetometer

type of second harmonic.

The operating principle of the fluxgate is as follows. By using external source A current of frequency w (most often 400 Hz) is passed through the primary (exciting) winding. If there is no external magnetic field, then the initial magnetization of the cores is zero. When a current of frequency w is passed in each half-cycle, the induction pulses in the cores are directed in the opposite direction and compensate each other (Fig. 3.15 b). Therefore, the total induction in the space closest to the cores at each moment of time is zero and the signal is not induced in the measuring winding, i.e. is also zero.

When an external field T appears (which must be measured) in each half-cycle, this field coincides with the induction of one of the cores, and the induction of the other core is directed in the opposite direction, which is equivalent to a shift in the induction of the cores. The total (total B S) induction in space near the cores, adding up, forms an alternating magnetic flux, changing with a frequency of 2w (Fig. 3.15. b). This flux induces an electrical signal in the measuring winding with a frequency of 2w and an amplitude proportional to the “shift” of induction in the windings - the external magnetizing field T.

To measure this field, you only need to select a signal with a frequency of 2w (800 Hz) using a filter (F), amplify it with an amplifier (U), determine the sign of the field (phase) with a phase-sensitive detector (PSD) and measure its amplitude with a meter (I). In this case, the device measuring the signal amplitude can be calibrated in units of magnetic field strength or induction. Such a fluxgate is called a “second harmonic type fluxgate”.

A useful feature of such a fluxgate for magnetic surveys is that it can measure the component of the magnetic field strength directed along the axis of the probe. That is, if the field T is directed perpendicular to the cores, then there will be no “shift” of induction in the windings and there will be no signal in the secondary winding.

This feature makes it possible to carry out so-called component measurements (i.e., measurements of three components along the axes) of the magnetic field induction, which is one of the advantages of the method. The disadvantage of the method is the presence of a zero offset of the device, which, even with a high sensitivity threshold of the device of 1 nT, does not allow measurements with high accuracy.

The fluxgate also has other names: magnetic saturation probe, magnetic modulation sensor (MMD). In foreign literature it is called flux - date (flux gate) - flow-passing.

Magnetometer designed to measure magnetic field induction. The magnetometer uses a reference magnetic field, which allows, through certain physical effects, convert the measured magnetic field into an electrical signal.
The applied use of magnetometers for detecting massive objects made of ferromagnetic (most often steel) materials is based on the local distortion of the Earth's magnetic field by these objects. The advantage of using magnetometers compared to traditional metal detectors is that longer detection range.

Fluxgate (vector) magnetometers

One type of magnetometer is . The fluxgate was invented by Friedrich Förster ( )

In 1937 and serves to determine magnetic field induction vector.

Fluxgate design

single rod fluxgate

The simplest fluxgate consists of a permalloy rod on which an excitation coil is placed (( drive coil), powered by alternating current, and a measuring coil ( detector coil).

Permalloy- an alloy with soft magnetic properties, consisting of iron and 45-82% nickel. Permalloy has high magnetic permeability (maximum relative magnetic permeability ~100,000) and low coercivity. A popular brand of permalloy for the manufacture of fluxgates is 80НХС - 80% nickel + chromium and silicon with a saturation induction of 0.65-0.75 T, used for cores of small-sized transformers, chokes and relays operating in weak fields of magnetic screens, for cores of pulse transformers, magnetic amplifiers and contactless relays, for magnetic head cores.
The dependence of relative magnetic permeability on field strength for some varieties of permalloy has the form -

If a constant magnetic field is applied to the core, then a voltage appears in the measuring coil even harmonics, the magnitude of which serves as a measure of the strength of a constant magnetic field. This voltage is filtered and measured.

double rod fluxgate

An example is the device described in the book Karalisa V.N. "Electronic circuits in industry" -



The device is designed to measure constant magnetic fields in the range of 0.001 ... 0.5 oersted.
Sensor field windings L1 And L3 included counter. Measuring winding L2 wound over the field windings. The excitation windings are powered by a current frequency of 2 kHz from a push-pull generator with an inductive feedback. The generator mode is stabilized by DC resistor divider R8 And R9.

fluxgate with toroidal core
One of the popular design options for a fluxgate magnetometer is a fluxgate with a toroidal core ( ring core fluxgate) -

Compared to rod fluxgates, this design has less noise and requires creation much lower magnetomotive force.

This sensor is excitation winding, wound on a toroidal core, through which an alternating current flows with an amplitude sufficient to bring the core into saturation, and measuring winding, from which the alternating voltage is removed, which is analyzed to measure the external magnetic field.
The measuring winding is wound over the toroidal core, covering it entirely (for example, on a special frame) -


This design is similar to the original fluxgate design (a capacitor is added to achieve resonance at the second harmonic) -

Applications of proton magnetometers
Proton magnetometers are widely used in archaeological research.
The proton magnetometer is mentioned in the science fiction novel "Trapped in Time" by Michael Crichton. Timeline") -
He pointed down past his feet. Three heavy yellow housings were clamped to the front struts of the helicopter. “Right now we’re carrying stereo terrain mappers, infrared, UV, and side-scan radar.” Kramer pointed out the rear window, toward a six-foot-long silver tube that dangled beneath the helicopter at the rear. “And what’s that?” “Proton magnetometer.” "Uh-huh. And it does what?" “Looks for magnetic anomalies in the ground below us that could indicate buried walls, or ceramics, or metal.”

Cesium magnetometers

A type of quantum magnetometers are alkali metal atomic magnetometers with optical pumping.

cesium magnetometer G-858

Overhauser magnetometers

Solid State Magnetometers

The most accessible are magnetometers built into smartphones. For Android good app using a magnetometer is . The page for this application is http://physics-toolbox-magnetometer.android.informer.com/.

Setting up magnetometers

To test the fluxgate you can use. Helmholtz coils are used to produce a nearly uniform magnetic field. Ideally, they represent two identical annular turns connected to each other in series and located at a distance of a turn radius from each other. Typically, Helmholtz coils consist of two coils on which a certain number of turns are wound, and the thickness of the coil should be much less than their radius. In real systems, the thickness of the coils can be comparable to their radius. Thus, we can consider a system of Helmholtz rings to be two coaxially located identical coils, the distance between the centers of which is approximately equal to their average radius. This coil system is also called a split solenoid ( split solenoid).

In the center of the system there is a zone of uniform magnetic field (magnetic field in the center of the system in a volume of 1/3 of the radius of the rings homogeneous within 1%), which can be used for measurement purposes, for calibrating magnetic induction sensors, etc.

Magnetic induction at the center of the system is defined as $B = \mu _0\,(\left((4\over 5)\right) )^(3/2) \, (IN\over R)$,
where $N$ is the number of turns in each coil, $I$ is the current through the coils, $R$ is the average radius of the coil.

Helmholtz coils can also be used to shield the Earth's magnetic field. To do this, it is best to use three mutually perpendicular pairs of rings, then their orientation does not matter.

Recently, there have been no significant changes in the principles of magnetic field measurement. In the field of magnetic surveys, methods based on the phenomenon of magnetic resonance, optical orientation of atoms, etc. have been established. Flux-gate installations are used to determine the magnetic properties of rocks and observations in wells, and astatic magnetometers and rock generators are used to measure remanent magnetization. Let us dwell in more detail on such a device as a magnetometer.

Magnetometer- a device for measuring the characteristics of a magnetic field and the magnetic properties of substances (magnetic materials). Depending on the value being determined, instruments are distinguished for measuring: field strength (oerstedmeters), field direction (inclinators and declinators), field gradient (gradientometers), magnetic induction (teslameters), magnetic flux (Webermeters, or fluxmeters), coercive force (coercimeters) , magnetic permeability (mu-meters), magnetic susceptibility (kappa-meters), magnetic moment.

In a narrower sense, magnetometers are instruments for measuring the strength, direction and gradient of a magnetic field.

The most important parameter of a magnetometer is its sensitivity. At the same time, it is almost impossible to formalize this parameter and make it uniform for all magnetometers, and not only because magnetometers differ in the principle of operation, but also in the design of the converters and the function of signal processing. For magnetometers, sensitivity is usually denoted by the magnitude of the magnetic induction of the field that the device is capable of registering. Typically, sensitivity is measured in nanotesla (nT) 1nT = (1E-9) T.

The Earth's field is approximately 35000nT (35µT). This is an average value - in different parts of the globe it varies in the range of 35000nT (35µT) - 60000nT (60µT). Thus, the task of searching for ferromagnetic objects is to detect, against the background of the Earth’s natural field, an increase in the field caused by distortions from ferromagnetic objects.

There are several physical principles and types of magnetometric instruments based on them that make it possible to record minimal changes in the Earth's magnetic field or distortions introduced by ferromagnetic objects. Modern magnetometers have a sensitivity from 0.01nT to 1nT, depending on the principle of operation and the class of problems being solved.

There are magnetometers for measuring absolute values ​​of field characteristics and relative changes in the field in space or time. The latter are called magnetic variometers. Magnetometers are also classified according to operating conditions and, finally, in accordance with the physical phenomena underlying their operation.

There are several types of magnetometers based on different principles of operation, such as: fluxgate, magnetoinductive, Hall effect, magnetoresistive, quantum (Proton).

Let us dwell in detail on fluxgate magnetic field converters, consider their operating principle, design and measurement technology.

The discovery of the properties of high magnetic permeability in iron-nickel alloys - permalloys led to the creation of fluxgate or flux-sensing magnetometers, the operation of which sensors is based on the effect of the reaction of the magnetic permeability of permalloy cores to the action of the Earth's constant magnetic field when powered by alternating current.

The fluxgate magnetic field transducer, or fluxgate, is designed to measure and indicate constant and slowly changing magnetic fields and their gradients. The action of a fluxgate is based on a change in the magnetic state of a ferromagnet under the influence of two magnetic fields of different frequencies. Depending on the magnitude of the applied voltage, the fluxgate can operate on the peak-type and second harmonic principles. Devices operating on the second harmonic principle have become more widely used(3).

Ferromagnetic probes are characterized by:

High sensitivity - the minimum change in the measured field element that the device is capable of registering when the power component changes; the sensitivity of the best devices is 1 nT, for an angular value - 01 sec;

Possibility of accurate (0.1%) calibration;

Low temperature coefficient, less than 0.01 nT/deg. Celsius in the temperature range from -20 to +50 degrees. Celsius;

Low level of own noise;

Small in size (10-20 cm) and weight (1-2 kg with a measuring unit);

Low power consumption(2).

In Fig. Figure 1 schematically shows some design options for fluxgates.

Rice. 1

In its simplest version, a fluxgate consists of a ferromagnetic core and two coils located on it: an excitation coil powered by alternating current and a measuring (signal) coil. The fluxgate core is made of materials with high magnetic permeability. An alternating voltage with a frequency of 1 to 300 kHz is supplied to the excitation coil from a special generator (depending on the level of parameters and purpose of the device). In the absence of a measured magnetic field, the core, under the influence of an alternating magnetic field H created by the current in the excitation coil, is remagnetized in a symmetrical cycle. A change in the magnetic field caused by the magnetization reversal of the core along a symmetrical curve induces an emf in the signal coil that varies according to a harmonic law. If at the same time a measured constant or slowly changing magnetic field Ho acts on the core, then the magnetization reversal curve changes its size and shape and becomes asymmetrical. In this case, the magnitude and harmonic composition of the EMF in the signal coil changes. In particular, even harmonic components of the EMF appear, the magnitude of which is proportional to the strength of the measured field and which are absent during a symmetric magnetization reversal cycle.

Fluxgates are divided into:

single-element rod (a)

differential with open core (b)

differential with a closed (ring) core (c).

A differential fluxgate (Fig. b, c), as a rule, consists of two cores with windings that are connected in such a way that the odd harmonic components are practically compensated. This simplifies the measuring equipment and increases the sensitivity of the fluxgate. Fluxgate probes are characterized by very high sensitivity to magnetic fields. They are capable of recording magnetic fields with strengths up to 10-4-10-5 A/m (~10-10-10-11 T).

Modern fluxgate designs are compact. The volume of the fluxgate with which domestic G73 magnetometers are equipped is less than 1 cm 3, and the three-component fluxgate for the G74 magnetometer fits into a cube with a side of 15 mm

As an example in Fig. Figure 2 shows the design and dimensions of a miniature fluxgate rod.

Rice. 2

The design of the fluxgate is quite simple and does not require special explanations. Its core is made of permalloy. It has a cross-section that varies along its length, decreasing by approximately 10 times in the central part of the core, on which the measuring winding and the excitation winding are wound. This design provides, with a relatively short length (30 mm), high magnetic permeability (1.5x105) and a low value of the saturation field strength in the central part of the core, which leads to an increase in the phase and time sensitivity of the fluxgate. Due to this, the shape of the output pulses in the measuring winding of the fluxgate is also improved, which makes it possible to reduce the errors in the time-pulse signal generation circuit. The measurement range of fluxgate converters of a standard design is ±50… ±100 A/m (±0.06… ±0.126 mT). The magnetic noise density in the frequency band up to 0.1 Hz for fluxgates with rod cores is 30 - 40 μA/ m (m x Hz1/2) depending on the excitation field, decreasing as the latter increases. In the frequency band up to 0.5 Hz, the noise density is 3 - 3.5 times higher. An experimental study of ring fluxgates revealed that their noise level is an order of magnitude lower than that of fluxgates with rod cores(3).