I am a bit puzzled by the effects of getting a permanent magnet next
to a yoke.

Here is my understanding of the BH curve in the third quadrant. It
denotes the magnet performance under ``load'', i.e. how much flux
emanates from the magnet face when the magnet is in an external field
H.

More specifically, when the magnet is completely enclosed in a yoke of
very high permeability, which is not saturated, integral of H is
approximately 0, and the flux on the magnet face is maximum, Br.

If however the magnet is in air, integral of H#0, and the flux on the
magnet face decreases.

Now to reality. I took several permanent magnets (Dexter, Bonded
NdFeB), and used a gauss probe to measure the field strength at the
magnet face. It turned out that the field strength was greater when
the magnet was in air than when it was stuck to a piece of iron. This
is exactly opposite from what I expect.

Can someone correct my understanding? Thank you very much,

Mirko

I think your expectation is basically correct, but your setup is
inappropriate for the properties that you are trying to measure. It
may be that the iron yoke has caused a redistribution of the flux
concentrations in such a way that the flux density at your measurement
point has indeed decreased, while it has increased at some other
point.

Regardless, a gauss probe is a poor tool for measuring bulk induction
in materials, as it only senses at one point in space. A better tool
for this would be an integrating fluxmeter, which will measure the
bulk induction in the permanent magnet.

Wrap 10 or 20 turns of fine magnet wire around your permanent magnet,
such that it can be slipped on and off the magnet. Connect the coil
to a fluxmeter, and reset the meter with the coil removed from the
magnet. Then measure the total flux change as you slip the coil over
the magnet while it is in air, and then again while it is attached to
the iron. You should see a higher total flux reading when the magnet
is on the iron.

Eric

www.netdenizen.com/emagnet



On 20 Aug 2003 09:39:44 -0700, (Mirko Vukovic)
wrote:

I am a bit puzzled by the effects of getting a permanent magnet next
to a yoke.

Here is my understanding of the BH curve in the third quadrant. It
denotes the magnet performance under ``load'', i.e. how much flux
emanates from the magnet face when the magnet is in an external field
H.

More specifically, when the magnet is completely enclosed in a yoke of
very high permeability, which is not saturated, integral of H is
approximately 0, and the flux on the magnet face is maximum, Br.

If however the magnet is in air, integral of H#0, and the flux on the
magnet face decreases.

Now to reality. I took several permanent magnets (Dexter, Bonded
NdFeB), and used a gauss probe to measure the field strength at the
magnet face. It turned out that the field strength was greater when
the magnet was in air than when it was stuck to a piece of iron. This
is exactly opposite from what I expect.

Can someone correct my understanding? Thank you very much,

Mirko