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A Hall effect magnetometer for small magnetic fields

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1967 J. Sci. Instrum. 44 798

(http://iopscience.iop.org/0950-7671/44/9/441)

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Notes on Experimental Technique and Apparatus

Reference 2

J. SCI. INSTRUM., 1967, VOL. 44

,, . Reference unusable. I

2. Drift in the Hall magnetometer The magnetometer was at first constructed without the

A Hall effect magnetometer for small magnetic fields

C. N. OWSTON? North-East Liverpool Technical College, Muirhead Avenue East, Liverpool MS. received 8th May 1967

Abstract. A portable survey magnetometer for field use and having an ultimate sensitivity of 5 x Oe is described. The instrument has a wide range of sensitivity and can be used in regions of large field gradient. To achieve the necessary stability, a temperature control system was required. This system is described in full.

-.

I 1 I l l I 1 I I " I

I 1.5 k I ..,.., -0v

Figure 2. Temperature control circuit. Reisstances in kn, capacitances in PF unless otherwise stated. TR 1, TR4 and TR 5, AC 128; TR 2 and TR 3, OC 71.

798

Notes on Experimental Technique and ApEaratus 100 pv degc-I, while the Hall voltage corresponding to an applied field of 5 X io-’ Oe was only 0.2 p. The heat generated by the current flowing through the probe caused a slow rise in its temperature and hence a drift in the residual voltage and the magnetometer zero. The drift was

and effectively indefinite because of the large thermal capacity of the field concentrators.

3. Temperature control system The Hall probe was covered with aluminium foil to mini-

&e temperature variations across its surface and was then themally insulated from the field concentrators. Thin

slivers of expanded polystyrene made suitable insulation. The Hall probe was heated by the alternating current used in the measuring process, together with a separate direct current. The alternating current was kept constant so that the a.c. Hall voltage depended only on applied magnetic field. The direct current could then be varied at will to control the probe temperature.

RV 1 (figure 2) set the temperature of the probe. M e n the probe was not at the correct temperature an a.c. voltage appeared at A with a magnitude and phase appropriate to the magnitude and sign of the temperature error. (The resistance of the Hall probe is temperature dependent.) This signal, after amplification and phase sensitive detection,

1 1

Hall terminals on probe

I 2 2 0

o v i

Reference 2

Figure 4. Amplitier and detector. Resistances in kn, capacitances in p~ unless otherwise stated. Components in filter, 5 % tolerance. Resistors have 10% tolerance except the components in the tuned filters which have 5%.

799

Notes on Experimental Technique and Apparatus provided a d.c. correction signal at B. This correction signal, together with the a.c. necessary to operate the magnetometer, was fed to TR 1 which operated as a constant current source for the Hall probe.

With the link L1 open, RV 2 was adjusted to give approxi- mately 20 m~ mean current through the Hall probe and the thermal insulation arranged to give a probe temperature of about 35"c. RV 1 was then adjusted to give zero signal at A and the link L1 closed. The system then stabilized at the desired temperature and required no further adjustment.

4. Remainder of the circuit The other parts of the circuit (figures 3 and 4) were con-

ventional. The oscillator was a simple multivibrator run

from a stabilized supply so as to give a constant amplitude output at about 550Hz.t The detector amplifier had a bandwidth of 50Hz at 550Hz. The probe used was a Siemens Hall generator type SV 120 I and the field concentrators were Mumetal rods, each 12in. long by +in. diameter. The instrument was powered by dry batteries.

References AITKEN, M. J., 1961, Physics and Archaeology (New York,

London: Interscience), p. 53.

t I Hz = 1 CIS.

J.-SCI. - -. INSTRUM., 1967, VOL. 44 --_.

Electron beam proiile indicator

R. KALIBJIAN and W. E. TINDALL Lawrence Radiation Laboratory, University of California, Livermore, California, U.S.A. MS. received 9th January 1967, in revised form 2nd May 1967

Abstract. An automatic plotter was developed in order to determine the current density profile of electron beams in the 10-200 v range. The system consists, basically, of a matrix collector, scanned by a stepping relay.

In certain applications of low-energy electron beams for spectroscopy, the profile of the beam both in the radial and axial directions becomes important. Beams for this use are generally of large cross section (about 0.5 cm in diameter) so that a reasonable amount of gas can be ionized. In the course of our investigations of various electron beam systems, we have developed an automatic plotter to indicate the profile (current density of the radial cross section) of an electron

beam. The prose indicator differs from the conventional type of pinhole or slit-type beam analyser (Ashkin 1957, Haskell et al. 1966). Our system consists of an 8 x 8 matrix collector which is scanned by a stepping relay; the signal is then amplified and recorded as shown in figure 1. Typical data from the chart recorder are shown for three different cross sections of the electron beam. The profile of the beam can be quickly recognized from the print-out of the

he

8 x 8 matrix colle

A-A 8-B C-C

800 Figure 1. Beam profile indicator system.