Re: detecting a magnet

From: Mark (makolber_at_yahoo.com)
Date: 02/03/05 

Date: 3 Feb 2005 15:03:54 -0800

Bob Eldred wrote:
> "Brian" <brian@w3gate.com> wrote in message
> news:VJadnZKl8qofnp_fRVn-tA@centurytel.net...
> > If I had a flat, 2 foot by two foot coil lying flat, how far above
it
> could
> > I reasonably expect the detect a rare earth magnet, perhaps the
size of an
> > M&M?
>
> An ordinary inductive coil requires the magnet to be in motion and
the
> voltage induced is a function of the number of turns and the rate of
change
> of the flux, dphi)/dt. Therefore the speed of a moving magnet is very
> important. Practically you might be able to sense the magnet moving a
foot
> away or so depending on the number of turns in the coil and the speed
of the
> magnet. It is a signal to noise issue and power line interference
(hum)
> would limit what you can detect.
>
> However it is possible to detect small stationary magnetic fields
(DC) by
> arranging and energizing coils in a device called a flux gate. These
things
> can be designed to measure fields as low as 1/1000 of the earths
magnetic
> field or even lower, less than a milligauss and could easily measure
your
> magnet many feet from the fluxgate. Being DC, it's easy to get rid of
the
> power line interference. There is also hall effect devices which has
been
> mentioned but they are not particularly sensitive. Beyond that, there
is a
> quantum mechanical device called a "squid" that can measure
extremely small
> fields like the field from the iron in a single blood cell. These are
> probably way beyond what you need or could pay for but it gives you
an idea
> of the technology available. What are you trying to do?
> Bob

I'm guessing, maybe wrongly that the OP is interested in those vehicle
detection loops burried in the ground at red lights. These sometimes
fail to respond to motorcycles and some people sell magnets that you
can attach to your motorcycle that alegedly allow the coil to see your
motorcycle.

I believe the principal of these coils is a tuned resonant tank that is
detuned by a large hunk of metal. I don't believe that the magnet
helps in this case but there are people out there that will swear that
they do work.

Mark

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