This Flux Detector makes magnetic fields visible.
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It's accomplished by a foil filled with nickel particles in a gelatinous suspension.
The Flux Detector turns dark in colour when the magnetic field is perpendicular to the foil (for example, near the poles) and turns lighter when the magnetic field runs parallel to the foil.
Other sizes upon request.
This customer video illustrates beautifully which patterns can be created by different magnets and arrangements.
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A common fridge magnet made of ferrite.
These magnets are often magnetised in stripes in order to increase the holding force in direct contact with metal. The magnetisation can be nicely seen here with the flux detector.
The detector maintains the last pattern until a new magnetic field is shown. Therefore, the flux detector won't be all green anymore once you started using it, but you'll see dark shadows.