Instrumentation
operating on the input flux signal to obtain a desired function of flux output. This ability results from the multiplying ability of the Hall effect; that is, output is proportional to current times flux. Many other applications take advantage of this capability. Since the Hall probe yields a continuous output reading as long as the field is present, it is useful for driving the H axis on an X-Y plotter in field plotting and in tests using laboratory electromagnets. Other uses include field monitoring in spectrometers, beam deflection coils, cyclotron or linear accelerator magnets, and magnetic resonance measurements. Field programming and control of electromagnets have been carried out using Hall probes. Hall instruments have advantages in the study of flux transients in solenoids, motors, relays, etc., since thin probes will fit into narrow gaps in confined spaces and can feed signals directly to an oscilloscope for display and analysis of the field waveforms. Magnetic fields within solid material cannot be directly measured with a Hall probe for obvious reasons, but a proportional field value can often be indicated if a suitable air gap is available. A Hall generator positioned to sense the tangential field alongside a fluxcarrying member can read the value of surface II and is used for this purpose in B-H plotters, permeameters, and coercimetcrs. Magnetic susceptibility and saturation magnetization (a) of small samples have been measured using Hall devices. Industrial applications include the control and adjustment of field strength in the manufacture of dc motors, relays, and similar devices, and in the nondestructive testing of parts by magnetization or by eddy current methods. Noncontacting measurement of electric current is accomplished using Hall generators to sense the field around the conductor. Electric power is measured by Hall multipliers designed for use in power circuits. A host of other applications exist beyond the scope of this article, many of which have become feasible as a result of recent advances in Hall device technology. References
(1) "The Hall Effect and Its Applications," F. W. Bell Inc., Columbus, Ohio, 1962. (2) M. Epstein, L. J. Greenstein, H. M. Sachs, Proc. Nat. Electron. Conf., 15, 241 (1959). (3) A. J. Monks, U. S. Patent 3,469,092 (September 23, 1969). (4) R. J. Higgins, Rev. Sci. Instrum., 36 (11), 1536 (1965). (5) W. Viehmann, ibid., 33 (5), 537 (1962).
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ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
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