The Magnaprobe Mark II uses a gimbal mounted magnet that traces out the shape of magnetic fields in 3D around bar magnets; only $11.00.
It is a simple but elegant tool for showing the three dimensional nature of magnetic fields. It consists of a small alnico bar magnet that can rotate freely on 3 axes (i.e. behaves like a gyroscope) and will map out x, y and z coordinates as you move it around a magnetic source.
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