Brian said:
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