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AFTER decades of trying, researchers have finally recreated the effect that
is thought to produce the Earth鈥檚 magnetic field.

The centre of the Earth is believed to consist of a solid inner core
surrounded by a fluid outer core of molten iron and nickel. The core is too hot
to create a permanent magnetic field like that in a bar magnet. So early this
century, researchers proposed the dynamo theory, which suggests that when liquid
metal such as the material that forms the outer core flows under certain
conditions, it creates a magnetic field. Now scientists based in Germany and
Latvia have confirmed this prediction, by producing magnetic fields in liquid
metal in a laboratory in Riga, Latvia.

According to the dynamo theory, a moving conductive fluid can create and
sustain its own magnetic fields. But for such a fluid dynamo to get going, it
has to be jump-started by a disturbance such as a tiny electrical field. This
鈥渟eed field鈥� is stretched and contorted by the moving liquid, creating new
electric currents and magnetic fields. This sets up a feedback loop that
eventually leads to a large, stable magnetic field. 鈥淭he flow creates a new
magnetic field with its own specific properties,鈥� says Gunter Gerbeth, a team
member from the Rossendorf Research Centre in Dresden, Germany.

The lab dynamo contains liquid sodium flowing at 15 metres per second down a
25-centimetre-wide pipe, while a propeller gives the flow a twist. The liquid
metal is then forced back to the top in small channels along the side of the
pipe. When a small field was applied to the fast-flowing metal, it produced a
magnetic field that remained after the seed field was removed, confirming the
dynamo theory, the team reported last week.

The experiments were stopped earlier than planned because of problems with
the equipment鈥檚 seals. But another group of German researchers claims to have
created a fluid dynamo at about the same time with similar, unpublished
results.

The findings may help researchers understand how other astronomical bodies
create their own magnetic fields. 鈥淭his is good work,鈥� says fluid dynamo
researcher Cary Forest at the University of Wisconsin in Madison. 鈥淚t鈥檚 neat
that they鈥檝e got this going and I think it points to great things in the
future.鈥� Forest says he wants to see experiments where fluid flow is less
constrained, as in the Earth鈥檚 core, to show how the field affects the flow.

The scientists at Riga had to overcome a number of hurdles to build their
dynamo. Molten iron and nickel cover massive distances around the Earth鈥檚 outer
core. To recreate the effect, the fluid in the lab dynamo had to flow at very
high speeds across short distances. But liquid sodium explodes when exposed to
water or air, and spinning the molten metal at high speeds increases the risks.
In 1987, the risk of explosion meant an earlier version of the experiment had to
be shut down before a fluid dynamo was created.