SU798654A1 - Apparatus for determining magnetic properies of substances - Google Patents

Description

The invention relates to the measurement of magnetic quantities and can be used to determine the magnetization and magnetic susceptibility of weakly magnetic substances, materials and rocks under the conditions of a scientific or factory laboratory.

Magnetometers are known, which serve to determine the magnetization and magnetic susceptibility of non-magnetic substances containing a primary transducer, which is made in the form of strings, suspended on a torsion thread and associated with it measuring and one or two astasic magnets spaced apart for some distance along the rod and oriented parallel to the common paths, and supplied with secondary photoelectric converters, captured by a negative feedback (soy) over a magnetic field. A characteristic element of the OOS circuit is a Holmholtz type coil, completely covering either a measuring or an astazing magnet.

A primary transducer consisting of two identical magnets spaced apart vertically and oriented in opposite directions does not react to an external or alternating magnetic field, provided it is homogeneous. The sample to be measured, located on the line passing through the centers of the magnets, is closer to one of them, called the measuring one, and deflects the latter by some angle. Photoelectric converter with two

0 differential input cells at the input converts the angular magnitude into electrical voltage, which is a measure of the measured parameter: magnetization or susceptibility, depending on what parameter the instrument is calibrated. From the output of the secondary transducer, the voltage also enters the holm0 coil of the hollow ring type OZ, after which it compensates the initial angular deviation of the magnet 1.

Have a relatively high

5 metrological characteristics of 1 mi and simplicity, known astatic (magnetometers are not without serious drawbacks, the main of which is the low accuracy of measurements under conditions of a non-uniform external field.

The aim of the invention is to improve the measurement accuracy.

This goal is achieved by the fact that in the device containing the primary transducer in the form of rigidly connected and suspended measuring and astasizing magnets, the secondary photoelectric converter, negative feedback windings and the stage for the sample being measured, the measuring and astasizing magnets are selected by different according to the ratio of their lengths and cross sections, they are aligned with the centers of gravity and the axes of magnetization and are installed in the vicinity of two windings in series-opposite direction, included in negative feedback circuit, the axes of which are oriented in the direction of possible movement of the poles of one of the magnets, as well as the fact that the astasizing magnet is made in two symmetrically positioned relative to the measuring magnet and associated with it with the possibility of mutual axial movement and rigid fixation elements, between the ends of which the plate with extensions is strengthened.

The drawing shows the functional diagram of the device for determining the magnetic properties of substances.

The primary transducer consists of a measuring magnet IT selected in the form of a thin rod, an astasing magnet 2 in the form of two short hollow cylinders, a plate 3 and a brace 4. The sleeve 5 is pressed onto the middle part of the magnet 1 and the sleeves 6 pressed into the magnet 2 are connected carving. The straps 4 are attached to the plate 3 by welding or soldering. The plate is sandwiched between the ends of the elements of an astasis magnet. It simultaneously performs the functions of a mirror required for optical communication with the secondary transducer.

Near the poles of the measuring magnet, two coils 7 are installed, the axes of which are perpendicular to the axis of the magnet. The ends of magnet 1 coincide with the axes of the coils and have the ability to move in the direction of these axes.

The secondary photoelectric converter consists of an illumination lamp 8, differentially included photodiodes 9, and an operational amplifier 10. A recording device is connected to the output of the amplifier 10. To the same output is connected an input of a negative feedback circuit consisting of a differentiating chain 11 and two windings 12 wound on coils 7 and interconnected in series with each other.

A stage 13 is fixed above one of the poles of magnet 1, which serves to set and fix the sample to be measured 14. The sample's symmetry axis passes through the pole of the measuring magnet perpendicular to it.

The device for fastening and tensioning the straps is not shown in the drawing.

The principle of operation of the device is as follows.

The magnetic moments of the magnets 1 and 2 are chosen equal and oriented in opposite directions. Consequently, the primary converter is insensitive to both homogeneous magnetic fields and non-uniform ones. The latter is ensured by the fact that the centers of symmetry of the magnets are aligned. In this case, the total torque of the primary converter is determined by the expression.

) o1

where M and M are the magnetic moments of the magnets 1 and 2, and PD is the average value of the intensity of the external magnetic field, referred to the point coinciding with the centers of the magnets. Since M M, then Mg, p O for any configuration field ND.

A reduction in the susceptibility to mechanical vibrations can be achieved by precisely matching the center of gravity of the oscillating system with its point of suspension. In the construction in question, this is achieved by moving the elements of the astasis magnet along with the plate 3 relative to the magnet. 1. The threaded connection of the elements of the primary converter is also used for the exact selection of astaticity by changing the distance between the two halves of the astasis magnet 2 (this changes the magnetic moment of magnet 2). Appropriate spacers are used to compensate for gaps (not shown). The force F acting on a certain volume of a substance placed in a non-uniform magnetic field is proportional to the intensity and the gradient of the intensity of the field, i.e.

F- n |. (1) The magnetic field on the permanent magnet axis is expressed by the following relation

(ff Kfr--. (2)

where m is the magnetic moment of the magnet X is the distance from the center of the magnet to the point where field 1 is defined is the magnet half length. From (2) it follows

.10YCH .. (| Y ... (3)

With a fixed position of the sample near the measuring pole

The magnet from expressions (2) and (3) shows that the force between the sample and the magnet depends on the length of the latter. The calculation shows that the force acting on the sample from the pole of the measuring magnet is 36 times greater than that from the side of the magnet. Thus, the effect of the latter on the sensitivity of the transducer can be arbitrarily small, if the geometrical parameters of the magnets are selected accordingly.

Sample 14 mounted on the stage causes a primary deviation of the primary converter, which is converted into electrical voltage, by registering an analog or digital instrument OM. A current proportional to the output voltage flows across the windings 12 and creates in them a magnetic feedback field. The interaction between the negative feedback windings and the poles of magnet 1 is the same as with the measured sample only in the opposite direction.

The peculiarity of the negative feedback circuit is the following. The force acting on the poles of magnet 1 from the side of coils 7 is determined in accordance with expression (1). Since the gradient field exists near the poles of the magnet in all directions (the field is strongly non-uniform), the force action on the magnet will be carried out in all directions. This provides omnidirectional feedback: any movement of a magnet caused, for example, by a mechanical effect, will be prevented by the force from the negative feedback coils.

The considered property does not possess negative feedback, being introduced by means of Helmholtz-type coils, completely covering the magnet. In this case, only one component field acts on the magnet in the direction of the coil axis. In this case, only the torsional vibrations of the magnet stabilize: for all other motions of the magnet (the most characteristic of which are transverse oscillations of the primary converter), it will not experience the effect of negative feedback. The consequence of this is the observed on

In practice, a decrease in the signal-to-noise ratio with an increase in the depth of negative feedback.

The proposed device can be calibrated either in absolute units of magnetization or magnetic susceptibility, or in relative units. In the latter case, measurements are made by the method of comparison with a sample whose magnetic properties are known.

o

Unlike the widely used method of measuring magnetic susceptibility, which uses samples with the correct geometry (cube, cylinder), the proposed device uses samples with only one flat surface, which coincides with the plane of the objective peaks.

Claims (2)

1. A device for determining magnetic properties of substances containing a primary transducer in the form of rigidly coupled and suspended measuring and astasizing magnets, a secondary photoelectric transducer, negative feedback windings and an object table for the sample being measured, differing in 0 that, in order to improve the measurement accuracy, in it the measuring and astasizing magnets are selected by different according to the ratio of their lengths and cross sections, are aligned with the centers of gravity 5 and the axes of magnetization and are installed near two windings in series-opposite, connected to the negative feedback circuit, the axes of which are oriented in the direction 0 possible movement of the poles of one of the magnets. 2. The device according to claim 1, characterized in that the astasizing magnet is made in the form of two, symmetrically located relative to 5 measuring magnet and associated with it, with the possibility of mutual axial movement and rigid fixation of elements, between the ends of which the plate with braces is fixed. 0 Sources of information taken into account in the examination 1. Journal of Physics,. E.Scientifics Instruments, 19/4, 7, P, 4, p. 266-168. .-.-.....-.:; v. V.