Ferrofluid bolt

Bolt on a strong magnet with ferrofluid on the bolt.

 

Ferrofluid is a really cool liquid. It has tiny magnetic particles suspended in it so that it reacts to magnets. No magnetic field; it's just a liquid. Apply a magnetic field and suddenly the liquid develops peaks and valleys. The stronger the magnetic field, and the further away the magnet is held (subject to the strength of the magnet) the bigger the peaks. Cool huh? More info here: en.wikipedia.org/wiki/Ferrofluid

Done