At room temperature, the element mercury is not very magnetic at all.
It has a very small, negative magnetic susceptibility, meaning that
when you put mercury in a magnetic field, it magnetizes just a little
tiny bit in the opposite direction. We say that mercury is a weakly
diamagnetic substance at room temperature.
Many compounds containing mercury are also weak diamagnets, but
some are a little stronger than mercury itself. It probably is possible
to make an alloy of iron and mercury which is magnetizable, but that
wouldn't be the mercury's doing.
Mercury is much more interesting magnetically at very low
temperatures. At temperatures below about 4 degrees Kelvin, elemental
mercury becomes a superconductor. In fact, superconductivity was
discovered by Kamerlingh Onnes in 1911 by studying mercury at low
Superconductors generally expel magnetic fields, so you could say
that below 4 K, mercury is a perfect diamagnet. To expel a magnetic
field from a material, a canceling field must be created by that
material with currents flowing on the surface. These currents flow with
no resistance in superconductors.
You can also make a permanent magnet out of a loop of
superconducting mercury. Simply cool down a loop of mercury in an
external magnetic field (the temperature at which the mercury will
superconduct will get lower as the applied field gets stronger). After
the mercury becomes superconducting, it locks in the total magnetic
flux through the loop. Switch off the external magnetic field, and a
persistent current will flow around the mercury loop, making a
permanent magnetic field.
(republished on 07/13/06)