The zinging detector

 作者:蓬巾     |      日期:2019-02-26 08:09:00
By Justin Mullins A TINY “xylophone” that resonates in the presence of a magnetic field has been developed by scientists at Johns Hopkins University. It could soon be helping guide microsurgical instruments within the body or to detect ships and vehicles. The device was conceived by Ben Givens, an engineer at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland. It exploits a fundamental law of electromagnetism which says that a current-carrying wire in a magnetic field will experience a force. Passing an alternating current through a wire in a magnetic field makes it vibrate, and the amplitude of the vibrations depend on the strength of the field. Givens realised that this principle could be used to make a magnetic sensor. His first attempt passed an alternating current through a thin aluminium bar a few centimetres long. The amplitude of the vibration was measured by bouncing a laser beam off the bar. Using a range of currents the bar can sense fields over nine orders of magnitude. Existing devices have a range of only three orders of magnitude. At the xylophone magnetometer’s lower limit, it can sense fields of only 0.1 nanotesla. The Earth’s magnetic field has a strength of about 40 000 nanoteslas. The device is so sensitive that Givens has to work in an isolated cabin in the mountains—just to get away from the stray magnetic fields caused by things like post trolleys moving around his lab. Givens and his colleagues are now working on a silicon version of the device only 1 millimetre square and carved onto a chip. This will sense vibrations by measuring changes in the capacitance in the space between the xylophone and the chip. “We’re still working on this problem,” says Tom Kistenmacher, one of Givens’s co-workers. Because the silicon magnetometer is so small,