FerroFluid is a black, oily liquid that can be used to visualize magnetic fields.
When a magnetic field is applied to ferrofluid, the magnetic moments of the particles orient along the field lines almost instantly. The magnetization of the ferrofluid responds immediately to the changes in the applied magnetic field and when the applied field is removed, the moments randomize quickly.
In other words, when you bring the ferrofluid near a magnetic field, little spikes form along the field lines. If the magnetic field is dense then the spikes will be numerous and close together. If the field is weak, then the spikes will be larger and further apart. This is easy to demonstrate by moving a magnet close to and away from the ferrofluid.
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