China tested a handheld submarine detector. It tracked a subway train underground

The tiny device is the size of a pencil and uses just 5 watts of power but can record a train braking into a station under a city street The device, developed by Peng Xinhua and colleagues at the University of Science and Technology of China, is described in a study published on September 14 in Acta Physica Sinica. Though it is scarcely larger than a pencil and draws only 5 watts of power, it can isolate the magnetic disturbance caused by a moving train in the Earth’s vast background field, clearly recording the train braking into a station, its stop and its departure under traction.

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