Most common materials are diamagnetic. The tendency of a
diamagnetic substance to move out of a magnetic fi eld can
be detected by hanging a long, thin sample from the arm of
a balance and letting it lie between the poles of an electromagnet.
This arrangement, which was once the primary
technique used to measure the magnetic properties of samples,
is called a Gouy balance. When the electromagnet is
turned on, a diamagnetic sample tends to move upward, out
of the fi eld, so it appears to weigh less than in the absence of
the fi eld. The diamagnetism arises from the effect of the
magnetic fi eld on the electrons in the molecule: the fi eld
forces the electrons to circulate through the nuclear framework.
Because electrons are charged particles, this circulation
corresponds to an electric current circulating within the
molecule. That current gives rise to its own magnetic fi eld,
which opposes the applied fi eld. The sample tends to move
out of the fi eld so as to minimize this opposing fi eld.
Compounds with unpaired electrons are paramagnetic.
They tend to move into a magnetic fi eld and can be identifi
ed because they seem to weigh more in a Gouy balance
when a magnetic fi eld is applied than when it is absent.
Paramagnetism arises from the electron spins, which behave
like tiny bar magnets that tend to line up with the applied
fi eld. The more that can line up in this way, the greater is
the lowering of energy and the more the sample is drawn