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From: neufeld@aurora.physics.utoronto.ca (Christopher Neufeld)
Subject: Re: hall effect sensors
Message-ID: <1991Jun21.011115.15340@helios.physics.utoronto.ca>
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References: <3372@ria.ccs.uwo.ca> <12536@qisoff.phx.mcd.mot.com>
Date: Fri, 21 Jun 1991 01:11:15 GMT

In article <12536@qisoff.phx.mcd.mot.com> hbg6@QIS1.mcdphx.mot.com (John Schuch) writes:
>
>I understand Hall Effect Sensors produce a voltage when magnetic lines
>of flux pass through their body at a right angle. Does the output vary
>depending on the polarity of the magnetism? That is, is the output
>different depending on whether you place the North end or South end
>of a bar magnet next to the device?
>
   The Hall effect is pretty easy to understand once you hear the
details of operation. It's a four terminal measurement. Say you have a
square piece of metal, with leads at the middles of the four faces. A
steady current is fed across the face of the square through two of the
leads. If a magnetic field is threading the face of the square then the
charge carriers will experience a Lorentz force tending to push them up
against one side of the square. The other two wires are held at a
voltage so that no net current flows through them. This is the Hall
voltage, the voltage which is set up at right angles to both the
magnetic field lines and the current density vector.
   If the magnetic field lines change direction, so that they go through
the conductor in the other direction, then the Hall voltage does change
polarity. The measurement is only sensitive to the perpendicular
component of the magnetic field, though I wonder if a six terminal
device mightn't also work to measure the fields in both directions at
right angles to the current density vector.
   Note that the Hall effect is felt by the "carriers" in the metal.
This has interesting effects, among them that two different metals can
have Hall resistances of opposite signs because the carriers can be
hole-type. Dredging up old memories of undergrad courses I believe that
aluminum is an example of a metal that exhibits a Hall effect indicative
of positive charge carriers.

   I've used one of these before. We calibrated it first with a rotating
coil gaussmetre and used it to measure fields of up to a couple of
Tesla in a water-cooled magnet.


-- 
 Christopher Neufeld....Just a graduate student  | If ignorance is bliss
 neufeld@aurora.physics.utoronto.ca    Ad astra  | why aren't there more
 cneufeld@{pnet91,pro-cco}.cts.com               | happy people?
 "Don't edit reality for the sake of simplicity" |