Scientists in Switzerland have explained, for the first time, the microscopic
cause of hum in those massive yokes of iron which help transform AC
electricity from one voltage to another.
When current reverses 60 times a second the iron core of the transformer
undergoes magnetetostriction twice during each cycle. In other words,
120 times per second induced fields cause the core to stretch slightly;
a meter-sized transformer might stretch or shrink by only a micron but
this would be enough to set up an audible 120-Hz hum.
(Earth itself is a magnet. Because of its own magnetic field, Earth's
diameter is shrunk by about 10 cm. Turn off the terrestrial field and
the Earth would spring back; its surface area would immediately increase
by about 10 square kilometers.)
The new experimental work probes theories, going all the way back to
Werner Heisenberg in the 1920s, about how the shrinkage arises from
the magnetic interactions (spin exchange) among pairs of atoms (dimers),
which share a common electron. The two magnetic ions want to be closer
For studying this effect iron itself is not the best test material
and the Swiss scientists (ETH Lab in Zurich and the University of Bern)
use another magnetic atom, manganese. Mn is a common ingredient in the
magneto-resistance data storage systems found in most disk drives. Normally
in a pure crystal, Mn atoms would be arrayed in endless straight lines.
But in this experiment the Mn atoms are isolated, two by two, with plenty
of intervening magnesium atoms. This allows the researchers to variably
"dilute" the magnet interactions between Mn atoms.
The strength of these interactions (or to be more precise the energy
levels of the excited Mn atoms) is measured by scattering a beam of
neutrons from the sample, a process called neutron spectroscopy. The
observed microscopic magnetostriction mimics the striction at the macroscopic
level, but it does depart considerably from the predictions of the traditional
Heisenberg model. (Straessle
et al., Physical Review Letters, June 25, 2004; contact Thierry
Straessle, Universite P&M Curie, 44-27-38-31, firstname.lastname@example.org,
33 44 27 38 81)