In NIST's new mini-magnetometer, light from a laser (small gray cylinder at left) passes through a small container (green cube) containing atoms in a gas. The cell and any sample being tested are placed inside a magnetic shield (large grey cylinder). When no sample is present, as in the top image, the atoms' "spins" (depicted inside red circle) align themselves with the laser beam, and the virtually all the light is transmitted through the cell to the detector (blue cube). In the presence of a sample emitting a magnetic field, such as a bomb or a mouse (middle and bottom images), the atoms become more disoriented as the field gets stronger, and less light arrives at the detector. A mouse heart produces a stronger signal than many explosive compounds found, for example, in bombs, if both are located the same distance from the sensor; at greater distances, the detected field is reduced. By monitoring the signal at the detector, scientists can determine the strength of the magnetic field.
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