Electromagnetic induction

Most people know that current traveling through a coil of wire can create a magnetic field -- that's how electromagnets work. A less familiar phenomenon, called electromagnetic induction, occurs when a wire is in a changing magnetic field. In this case, a current is generated in the wire. The magnetic field can change because the magnet is moving, the wire is moving, or the strength or direction of the field is changing. In any case, the field must be changing for a current to be generated.

The current generated is proportional to the change in the number of field lines going through the coil (flux), so scientists are able to use this phenomenon to measure the strength and direction of a magnet. A simple magnetometer consists of a coil of wire hooked up to an ammeter to measure current. By moving a magnet through the coil three different times, in a different orientation each time (one for each axis, x, y, and z), scientists are able to determine the overall direction and strength of the magnetic field.




In order to make extremely precise measurements, cryogenic magnetometers use mechanisms called superconducting quantum interference devices (SQUIDs) to detect minute currents. This kind of magnetometer consists of a superconducting pickup coil (through which the sample moves), and a Josephson junction.
The coil is kept extremely cold so that it superconducts, removing all resistance to currents in the wire. The current created by the moving magnet thus moves freely through the circuit. The Josephson junction, however is too narrow for many electrons to flow through at once, so as soon as a current begins to flow, it is overloaded and stops the circuit from superconducting. However, since the circuit is no longer superconducting, the junction is no longer overloaded, so the circuit goes back to superconducting. The circuit keeps switching in this manner many times a second as long as the magnet's motion generates a current. Scientists count how many times it switches to determine the change in magnetic flux and therefore the strength of the magnet. As with simpler magnetometers, measuring at least three times with the magnet in different positions lets scientists calculate the magnetic field's properties in all three dimensions.
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Copyright © 2003 Genevieve Tauxe