Monday, Nov. 20, 1972

Footprints on the Rug

It has long been known that certain materials become electrified when they are rubbed together. That is, they lose or gain enough electrons to acquire a charge of static electricity. In this state, the materials will attract dust, hair and other lightweight fluff that happen to have an opposite charge. Now a British researcher has proposed that this phenomenon be used for a practical purpose: to help track down criminals.

Distinctive Field. The idea comes from Physicist Kurt Greenwood of the British textile industry's Shirley Institute in Manchester; he has been studying ways of reducing the static electricity built up by walking across carpets and other floor coverings. Greenwood knew that static electricity may be generated wherever a shoe rubs against a rug. His research had further established that the charge can persist for hours (particularly on some synthetic rugs in dry air) and that the shape of the charged area conforms to the shape of the sole and heel that created it. Those facts were of particular interest to Greenwood, who had also done forensic research for the Home Office--which has overall control of the police service in England and Wales. Could the distinctive electrical fields be used like fingerprints as a means of identification? If so, how could they be readily made visible for investigators?

Greenwood strode purposefully across some synthetic carpeting and then rolled thousands of tiny plastic beads across it. Most of the beads could be easily blown off the rug. But some stuck in place, attracted by the local static charges that Greenwood had created by his walk. In fact, they formed clusters that looked like footprints wherever his heels and soles had come in contact with the carpet. Greenwood found that he could use the beads to detect his shoeprints up to a day after he had walked across the rug.

Greenwood's technique is obviously cruder than fingerprinting, and could provide only an indication of the size and shape of a criminal's shoes. Still, the Home Office, which encouraged Greenwood in his research, has hopes that the technique will prove useful to detectives. Electrostatic shoeprints, for instance, could give some hint of the size and sex of a culprit, reveal how many people were involved in a caper and even allow police to trace their movements.

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