[Dix Dogfight]

I'm gonna start off with a little theory and then decribe how real life Hall sensors work. I'm leaving most equations out for now because they are not needed to describe how a Hall sensor works and it will simplify things.

Assume we have a Current I_x from C to D in the X-axis in a bar shaped carrier (see figure).
If a magnetic field is applied perpendicular (along the z-axis) to the direction of the current there will be a force F in the y-axis.
This force will push the electrons from one side of the carrier to the other = there will be a larger concentration of electrons on the right side (B). Hence the (-) sign on this side. And off course there will be a equal amunt of electrons missing on the
other side (A).
The differences in electron concentration results in a electric field (E_y) across the carrier. The resulting voltage V_ab=E_y*W, the Hall voltage, is measurable and is used in the Hall effect sensors.

How does this help us in practice then:
There are different types of sensors that use the Hall effect.
First one is a type of switch that switches when the magnetic field changes polarity.
Second type switches when the magnetic field rises above a preset level.
A third kind deliveres a voltage that is proportionell to the magnetic field.

Depending on what kind of sensor you have you must alter the magnetic field close to the sensor accordingly.

Example: Lets say you have the first type and you are waveing a magnet close to it. The switch would not react because the field doesn't change it's polarity.
If you on the other hand rotate the magnet = change polarity between N and S then the switch will react.

Hope this will help you some in your quest for safer cooling

cheers

Edit: damn spelling errors are all over the place