Magnet Myths and Misconceptions

[TinselKoala]

@Poynt99

Some interesting experiments this morning with the hall probe, more like confusing,lol.
First I would like to say when taking measurements my intent is not to prove anything one way or another but to understand what I see.


1)First my thinking was that if Mark is correct then logically I should be able to point the hall sensor face at the North pole and rotate the sensor one way or the other to "follow" the maximum field density of a given polarity. That is I pretend to follow a "loop" from the N to S pole and if there is a max value of constant polarity it should show me the way...in fact it does and Mark is right. I should note I am not following a loop I am trying to maintain a max reading of same polarity within any given space which happens to follow what appears to be a loop from pole to pole.


2)However, and this is a big however if I start with the hall face pointing towards the N pole and follow the magnet contour around the corner to the side the max reading and polarity is also maintained, the same hall face is always facing inwards towards the magnet. In fact I can follow a line from the N pole center around the edge along the side of the magnet and the reading is constant at max value and same polarity until a very small region near the side/center of the magnet at which point it reverses. The polarity remains constant on both ends/sides of the magnet as I move away from it as does the zero value at the center. This measure suggest there are no loops but straight lines acting away from the magnet center outwards in all directions not unlike rays from the Sun. The measures suggest two half spheres of different polarity with a zero boundary condition between the two. No loops but lines moving away from the magnet center in all directions.


Now measure (1 )suggests Mark is correct and it would seem I can follow imaginary lines looping from pole to pole by rotating the sensor. However measure (2) with the hall face always pointing towards a point in the center of the magnet suggests there are no loops but lines, two half spheres of differing polarity acting outwards in all directions.


The confusion would seem to lie in the fact that in between the two planes of the poles the sensor shows a constant max value and polarity if the sensor face is pointing in the same direction as the poles N-S axis. However if sensor is always facing towards the magnet center we see two half spheres of different polarity.


This was unexpected and I will leave it with everyone here to make sense of it.


AC

I think you are misinterpreting what you see in Case 2.  Look at the conventional picture of the field lines. As you slide the Hall sensor along the side of the magnet with the face parallel to the magnet's long axis, the sensor experiences changing directions of the flux and at the midpoint the flux is mostly _parallel_ to the plane of the sensor: That is, no flux or very little goes through the plane of the sensor at right angles to it, so the sensor reads minimum flux here. (In the case of my particular sensor, the voltage output will be the midpoint of the supply voltage or about 2.55 volts, meaning no flux through the sensor plane.) As you continue to slide along towards the pole, the flux begins to curl back towards the pole so more and more of it goes through the plane of the sensor. When you started, the "arrow" of the flux goes through the sensor in one direction. As you progress along, the flux becomes more and more parallel so less and less goes through the plane of the sensor. Past the midpoint the flux begins to curl back and so more and more goes through the plane of the sensor, with the "arrow" now in the opposite direction wrt the sensor. When you are on the pole itself, and have turned the corner and rotated the sensor so that the plane is now flat to the pole, you get the maximum flux reading.

What is the part number of your sensor? Is it a true linear ratiometric sensor like I am using, or is it a "switch" type? It would be nice to see the data sheet for your sensor. I've already provided the data sheet for my sensor in a previous post.

[NoBull]

I specified the field outside the magnet. Perhaps you missed that important piece of information?

If you meant to compare the direction of the flux lines on the outside to the outside, then indeed I missed it.
Of course the direction of the flux lines on the outside compared to other lines on the outside does not reverse.

[itsu]

There is one caveat here that I think Itsu has stumbled upon.

When a cylindrical bar magnet is mapped by a Hall sensor whose face is perpendicular to the cylinder's axis (which is also the magnetization axis) and scanned along the radius of the cylinder that crosses its axial midpoint, then this sensor would indicate the B magnetic field in the opposite direction outside the magnet than inside the magnet (if it could penetrate inside the magnet).

This means that at one point somewhere along this radius, the Hall sensor would indicate zero.

to stop any confusion on what i did and what i used, here a repeat of that action on video:

https://www.youtube.com/watch?v=qm718ND5WDo&feature=youtu.be


Sorry for mixing up the north south of the magnets all the time, North is on the left,   south on the right


Regards itsu

[allcanadian]

@TK
I am using a linear ratiometric sensor, it shows a signal which varies with field strength and when facing the N pole it shows a N polarity and when rotated 90 Deg it shows zero polarity and when rotated another 90 Deg it shows a polarity reversal to S pole. It does everything it is supposed to do when I tested it as it should.


The fact remains, just above and below the magnet center line along the side the sensor can be rotated 90 Deg with no change in field strength nor polarity. This only occurs in a region approximately 1/4 of the magnet diameter away from the side above and below the center line. I would suggest you do the same measurement and report your findings here.


I am measuring a 1" dia x 1" N42 neo magnet.


AC

[poynt99]

If you meant to compare the direction of the flux lines on the outside to the outside, then indeed I missed it.
Of course the direction of the flux lines on the outside compared to other lines on the outside does not reverse.