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Watching flow of electricity

BY SCANNING the magnetic field generated as electric currents flow through objects, physicists have managed to picture the progress of the currents. The technology will allow manufacturers to scan microchips for faults, and in revealing microscopic defects in anything from aircraft to banknotes, according to a report in New Scientist.

Gang Xiao and Ben Schrag at the Brown University in Providence, Rhode island, visualise the current by measuring subtle changes in the magnetic field of an object and converting the information into a colour picture showing the density of current at each point.

Their sensor is adapted from an existing piece of technology that is used to measure large magnetic fields in computer hard drives.

The resulting device is capable of detecting a current as weak as 10 microamperes, even when the wire is buried deep within a chip, and it shows up features as small as 40 nanometres.

Now engineers looking for defects in a chip have to peel off the layers and examine the circuits visually. But the new magnetic microscope is sensitive enough to look inside chips and reveal faults such as short circuits and nicks in the wires

The microscope can be used to reveal the internal structure of any object capable of conducting electricity. For eg., it could look directly at microscopic cracks in an aeroplane's fuselage, faults in the metal strip of a forged banknote or bacteria in a water sample. The technique cannot yet pick up electrical activity in the human brain as the current there is too small. The microscope is currently the size of a refrigerator and takes several minutes to scan a circuit.

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