A Recording Torque MagnetometerD. S. Miller Citation: Review of Scientific Instruments 21, 605 (1950); doi: 10.1063/1.1745665 View online: http://dx.doi.org/10.1063/1.1745665 View Table of Contents: http://scitation.aip.org/content/aip/journal/rsi/21/7?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Digital enhancement of torque magnetometer signals Rev. Sci. Instrum. 47, 938 (1976); 10.1063/1.1134777 Frictionless Recording Torque Magnetometer Rev. Sci. Instrum. 31, 544 (1960); 10.1063/1.1931246 Simple Recording Torque Magnetometer J. Appl. Phys. 31, S184 (1960); 10.1063/1.1984658 Improved Torque Magnetometer J. Appl. Phys. 29, 493 (1958); 10.1063/1.1723195 Some Uses of the Torque Magnetometer Rev. Sci. Instrum. 8, 56 (1937); 10.1063/1.1752236

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TilE REVIEW OF SCIENTIFIC INSTRUMKNTS VOLUME 21. NUMBER 7 JULY, \950

A Recording Torque Magnetometer

D. S. MILLER

Research Laboratory, United States Sleel Corporation of Delaware, Kearny, New Jersey (Received February 6, 1950)

This instrument produces, automatically, a curve of torque versus angle of orientation for a thin iron or steel disk in a strong magnetic field. A resistance strain gauge converts the torque acting on the disk specimen to a small d.c. voltage, which is recorded on a "strip-chart" recording potentiometer. The specimen is rotated in synchronism with the translation of the chart paper, both being driven by synchronous motors. A torque curve is completed in six minutes.

T ORQUE magnetometers of various types have been used for many years in laboratory studies

of the magnetic anisotropy of single crystals. For instance, such instruments were used by Weiss! in the study of pyrrhotine and by Webster2 in the study of iron crystals. More recently, others3-6 have used them for the study of both single crystal and polycrystalline iron alloys. N, P. GOSS7 used the same type of device in developing a treatment to produce a high degree of preferred orientation in an iron-silicon alloy.

The torque magnetometers used by these workers were manually operated, readings being made point by point. It has recently been found that the instrument is useful in both development and production where rather large numbers of specimens are to be tested. It was considered, therefore, worth while to devise a modification of the instrument which would record the data automatically and more rapidly than could be done with the manual type of instrument. Ingerson and Beck8 devised a scheme for generating the torque curve on the screen of a cathode-ray tube, which would provide the speed we wished, but was not exactly suited to our needs since we preferred a directly pro­duced permanent record such as is provided by a pen-driving type of potentiometer-recorder.

In the instrument which we have designed to fulfill these requirements the torque exerted on a disk speci­men is converted, by a resistance strain gauge, to a small direct-current voltage. This voltage is applied to the measuring circuit of a "strip-chart" voltage recorder of the proper sensitivity. The disk is rotated about an axis perpendicular to its plane by means of a synchronous motor and reduction gearing. The chart paper in the recorder is also driven by a synchronous motor, so that there is a definite relation between the angular rotation of the disk and the movement of the chart paper, As the disk rotates the torque varies and the pen in the recorder moves in accordance with this variation, By use of the proper conversion factor, the voltage record

1 Weiss, J. de Phys. 4, 469 (1905). 2 Webster, Proc. Roy. Soc. 107,496 (1925). 3 Dahl and Pfaffenberger, Zeits. f. Physik. 71, 93 (1931). 'K. J. Sixtus, Physics 6, 105 (1935). • H. J. Williams, Rev. Sci. Inst. 8, 56 (1937). • F. Bitter and L. P. Tarasov, Phys. Rev. 52, 353 (1937). 7 Trans. Am. Soc. for Metals 23, 511 (1935).

on the chart paper becomes a plot of torque versus angle of orientation of the disk in the magnetic field,

The construction of the torque transducer is shown in Fig. 1. The specimen, a disk one inch in diameter, is held between spring jaws on the holder (B), which fits on two taper pins on the cross-head (1) of the trans­ducer. The torque on the specimen is transmitted through the cross-head and the shaft (1), producing a displacement of the shaft of the strain gauge (K) by means of a bronze strip (0.OO2XO,020 in.) wrapped once around the shaft (1) and attached to the two collars (H) on the strain gauge shaft, Figure 2 shows the manner in which the strain gauge is coupled to the shaft (1). Pivots at the ends of shaft (1) ride in V-jewels mounted in the supporting rectangular frame,

The strain gauge (K), which is a product of the Statham Laboratories of Beverly Hills, California,

J

8 W. E. Ingerson and F. J. Beck, Jr., Rev. Sci. Inst. 9, 31 (1938). FIG. 1. Torque transducer, showing method of mounting specimen.

605

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606 D. S. MILLER

consists of four resistance strain elements connected as a Wheatstone bridge to which a direct current of not more than 30 ma is supplied. The gauge used here has a range of 4 oz. applied force, producing an output of approximately 20 mv maximum. The torque limit of the transducer is 50,000 dyne-centimeters, which is adequate for practically all of the steels which we have tested, in thicknesses from 0.015 to 0.080 in. To produce curves of usable amplitude from thinner speci­mens, having thicknesses from 0.010 to 0.015 in., we have made holders to take specimens of 1.5 in. diameter. The graduated scale on the base plate (D) provides a measure of the angle between the direction of the applied magnetic field and the orientation of the transducer. It is an approximate measure of the angle between the magnetization in the specimen and a chosen reference direction in the specimen. The errors involved in this reading will be discussed later. The small hand lever (Q) controls one stop which operates the limit switch (Z). With this stop in the operating position the allowed rotation is 100 degrees. A fixed stop limits rotation to 200 degrees when the lever (Q) is in the open position.

The position of the torque transducer between the poles of the electromagnet is shown in Fig. 3. It is attached, by means of three screws through the base plate (D), to a drive plate on the base of the magnet yoke. This drive plate connects, through a gear train in the base of the magnet, to a small synchronous motor mounted below and in back of the yoke. The drive rotates the gauge at a rate of 30 degrees per minute. A thumb lever (E) disengages the drive so that the gauge may be rotated by hand to set the starting position. Two switches (F and G) on the right foot of the magnet pedestal control the motors for rotating the torsion gauge and driving the chart paper in the recorder. A limit switch (Z) under the right hand magnet coil shuts off both of these motors at the completion of a run. A plug and four wire cable (U) connects the strain gauge to its supply battery and to the recorder.

The output terminals of the gauge are connected to the input of the recorder. We have used a Brown strip­chart recorder with a range, -4 to +4 mv. By making

eEARIN~ PIVOT

STRAIN GAGE SHAFT

BRONZE STRIP 0.002')( 0.020"

FIG. 2. Method of coupling strain gauge to torsion shaft.

the range of the recorder a fraction of that of the gauge we have enhanced the useful range of the instrument. A specimen whose maximum torque is SOX 103 dyne-cm is kept on scale by setting the gauge input current to 6 rna; a specimen whose maximum torque is lOX103

dyne-cm is made to record full scale by setting the gauge current at its maximum, 30 rna. Furthermore, by selection of the gauge current, a convenient factor for the torque scale on the chart can be chosen. The current required for a preselected scale factor can be determined by a simple calculation, using the volume of the specimen to be tested and the constant of the gauge.

The electromagnet is of conventional design. Each coil has approximately 4500 turns of No. 16 wire. With a current of 2.3 amp. the field at the center of the 1.75 in. gap is about 4000 oersted. The heat dissi­pation at this current is not sufficient for continuous operation but is adequate for short runs. At 2.0 amp. continuous operation is possible. With this current the field is about 3700 oersted.

The recorder and controls for the electromagnet and the recording circuit are mounted in a cabinet 22 in. wide, and 68 in. high. A field-discharge switch, a 50-ohm 200-watt rheostat, and a 3-amp. ammeter provide control of the magnet current. A milliameter (30 m,a) and two rheostats (1000 ohm for coarse, and 100 ohm for fine adjustment) for control of the input current to the strain gauge are mounted on a panel above the recorder. On the same panel a 50-ohm potentiometer wired into the recorder measuring circuit provides fine control of the zero point of the recorder. This adjust­ment is necessary because the strain gauges cannot be so precisely adjusted in manufacture that the output voltage is zero for zero strain.

Within the limits of the useful range the output of the transducer is linearly proportional, to within t percent, to the applied torque and to the input current. The transducers were calibrated by using a torsion fiber of known constant, attached, at its upper end, to a graduated circle (a converted optical goniometer), and at its lower end, to the specimen holder (B).

FIG. 3. Torque transducer mounted on electromagnet.

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RECORDING TORQUE MAGNETOMETER 607

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FIG. 4. Typical magnetic torque curve obtained on a specimen of silicon steel having a high degree of preferred orientation.

As stated previously, the plot obtained is an approxi­mation to the true torque as a function of the angle between the direction of magnetization and the refer­ence direction in the specimen. The principal error, which is in the angular measurement, arises from two sources. For any torque other than zero, the specimen must be deflected from its rest position with respect to the torsion gauge. The maximum value of this deflection is 0.7 degrees; for the torque radius of the gauge is 0.125 in. and the limit of displacement of the strain gauge shaft is 0.0015 in. The second source is the deviation of the magnetization in the specimen from the direction of the magnetizing force. Bozorth9 has calculated this effect and shown that it becomes small

9Bozorth, Phys. Rev. 42, 882 (1932).

at high fields. An error in the torque arises from the fact that in punched disks the edges are not saturated. This could be eliminated by making the specimen ellipsoidal. A more complete discussion of these errors is given in reference 6. In the applications for which we have used the instruments up to the present the errors are not large enough to be significant.

A typical curve obtained on an iron-silicon alloy specimen is shown in Fig. 4. This material was processed in a manner somewhat similar to that developed by GOSS.7 The curve approximates that of a single crystal having a (110) plane in the plane of the specimen and a (100) direction parallel to the reference direction (zero degrees). The step in the curve at about -13 degrees was caused by the operation of the automatic standardizing mechanism in the potentiometer-recorder.

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