The Mark I Magnaprobe is an ultrasensitive detector of magnetic fields as low as 0.3-0.7 Gauss at 10mm only: $39.95
The original Cochranes of Oxford Mark 1 Magnaprobe; sensitivity at an incredibly low price.
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).
The Mark I Magnaprobe is especially useful for locating very weak magnetic leakage to test the effectiveness of magnetic shielding or to investigate electromagnetism in coils using different current intensities.
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.