Magnet Myths and Misconceptions

[MarkE]

OK,who's test method is correct? your's or TK's,as TK has the hall sensor at right angles to the plane of the magnet,and you say face flat on the magnet

It all depends on what you want to find out.  I propose placing the sensor flat up against the magnet to show what flux density is curing in or out of the magnet at any point along the dipole axis.  The perpendicular test measures flux density parallel to the dipole axis.  If the dipole is very long then the "camel humps" in that flux density will be pretty low and the flux density along most of the magnet will be relatively constant.  As the dipole gets shorter relative to the width and height of its faces the "camel humps" in the flux density become more pronounced.

[poynt99]

The field at the center of a dipole is a mixture of two different forces that are equal in strength and opposite in direction,and the net resultant force is 0.

You are referring to the field outside the magnet, correct?

If so, then that is not a correct statement. The field at the center (and outside) of the dipole is parallel to the dipole and does not change direction. The flux density outside the magnet at the center is however weaker compared to the ends. But if you were able to insert your Hall probe into the middle of the magnet material at the center, you would find the flux density just as high, if not higher than what you might measure at either end of the magnet. Agreed?

So again, when you swipe a coil near either end of a magnet, you induce a strong voltage in the coil. When you swipe the same coil across the center of the same magnet, you induce little to no voltage in the coil. Is that because there is no field or flux at the center? No. It is because you are inducing just as much positive voltage as negative voltage, and the two cancel.

[minnie]

   Tinman,
             you're goin' to have to pull a rabbit out of the hat. TheoriaApophasis  will be
able to assist you, that's a given!
     Good luck,John.

[NoBull]

OK,who's test method is correct? your's or TK's,as TK has the hall sensor at right angles to the plane of the magnet,and you say face flat on the magnet

I depends what you want to find out.

If you want to map the magnitude absolute magnetic flux density then use Method #3.
If you want to map the magnitude of component of flux density that is perpendicular to the magnetization axis of the magnet then use Method #1.
If you want to map the magnitude of the attraction force then use Method #6.

If you want to disprove that the map of the magnitude of the magnetic flux density has a shape of figure 8, then use a Method #1 or #2 or #3.

If you want to prove that the map of the magnitude of attraction force has a  shape of the figure 8, then use Method #4 or #5.

[poynt99]

That resolves that discrepancy.  However I think that if one wants to know whether flux turns back into the magnet half way along, the direct way to measure that is with the face of the sensor right up against the magnet.  If flux at a detectable density curls back into the magnet then that orientation will detect it.

Agreed, however I think tinman is saying that even with that orientation of the sensor, you will measure 0 net flux because the N-curl + the S-curl are opposite at the center and will cancel in the sensor.

tinman may correct me if I interpreted him incorrectly.