### Re: The Functions of a Search Coil/ Hall Probe

Date: Tue Jun 20 10:16:07 2006
Posted By: Zack Gainsforth, Staff Scientist
Area of science: Physics
ID: 1146463394.Ph
Message:
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Hi Richie,

The hall probe measures magnetic field strength, what we usually call B.

There are two steps to get from magnetic field strength to the magnetic
flux density.  First you want to get the magnetic flux:

B * area is the total amount of flux travelling through the entire area.
The unit of magnetic flux is the "Weber."

Next, the magnetic flux density is how much magnetic flux travels through
one unit of area.  So let's say you have 10 Webers travelling through an
area of 1/2 square meter.  Then the magnetic flux density is how many
Webers flow through one square meter, or 20 Webers per square meter.

So, you're looking for the magnetic flux density.  What you should keep in
mind is that the field may vary a little bit between your coils.  So you
can't just take the area of your coils and multiply that times one reading
of your Hall Probe.  What you  should do is to take several readings of
your hall probe at the exact center of the coils, and then move the hall
probe gradually out towards the edge of the coils.  You'll probably see the
strength of the field drop.

My guess is that you're interested in the flux density near the center of
the coils, since that is usually where the magnetic field is the most
constant and least bent.  So for that, you can figure out an area over
which the field is approximately constant, and use that area to find out
the flux density in the middle.

So what's the search coil?  A search coil is a radio antenna in a metal
detector, so I'm a little confused what you're using a search coil for
here.  Perhaps you can clarify how the setup is designed?

Zack

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