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ONE of the world’s most sensitive magnetic field detectors, called a SQUID,
can be used to spot the corrosion that fatally weakens aircraft.

A plane’s fuselage is made from overlapping sheets of aluminium and the
joints between them slowly corrode from the inside out. You can’t spot the
damage eating away at the joints without taking them apart every few years. But
physicists have devised a way to detect corrosion before it becomes a problem:
by sensing its magnetic field.

Such fields are up to 100 000 times weaker than the Earth’s magnetic field.
But physicists John Wikswo and Grant Skennerton of Vanderbilt University in
Nashville, Tennessee, were convinced they’d find a way to detect them. They
decided to try an ultra-sensitive magnetometer called a “superconducting quantum
interference device”—better know as a SQUID. By measuring quantum effects
in a superconducting probe ring at cryogenic temperatures, SQUIDs can detect
fields so weak they’re measured in nanoteslas.

By passing the tiny probe over an airframe joint, the researchers mapped out
an image of the hidden corrosion, Wikswo told a conference of the American
Physical Society in Seattle.

The new technique is already changing ideas about what makes planes corrode.
Bizarrely, spraying joints taken from US Air Force planes with distilled water
caused as much damage as exposing them to highly corrosive salt water. Most
likely, distilled water releases corrosive chemicals already trapped within the
joint, Wikswo says.

Meanwhile, Helene Grossman of the University of California at Berkeley told
the APS that SQUIDs can now be used to detect microbes tagged with magnetic
particles. Her team are sticking tiny magnetic beads onto antigens, which in
turn latch on to target cells, allowing detection.