The Magnaprobe Mark I uses jewel mounts to support an alnico bar magnet that responds to magnetic fields. It is so sensitive that it tilts in response to the earth's magnetic field & indicates magnetic north (northern hemisphere only). It is especially useful for locating hard to find magnetic leakage & the effectiveness of magnetic shielding.
Note: Avoid contacting other magnets. If you do so by accident, detach the Magnaprobe by holding its magnet. Do not simply pull back with the handle as you will ruin the device.
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I have bought several of these over the years, originally from the British company that makes them, and more recently from this company. I have trouble keeping them because everyone seems to want one. They are indispensable for showing the angle of the magnetic field in three dimensions. It is like a 3 dimensional compass, and very sensitive too. It will even show the vertical angle of the Earth's magnetic field! If your business is magnets, you need one of these.