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The production of a magnetic field by a living object. The living object presently most studied is the human body, for two purposes: to find new techniques for medical diagnosis, and to gain information about normal physiology. Smaller organisms studied include birds, fishes, and objects as small as bacteria; many scientists believe that biomagnetics is involved in the ability of these creatures to navigate. The body produces magnetic fields in two main ways: by electric currents and by ferromagnetic particles. The electric currents are the ion currents generated by the muscles, nerves, and other organs. For example, the same ion current generated by heart muscle, which provides the basis for the electrocardiogram, also produces a magnetic field over the chest; and the same ion current generated by the brain, which provides the basis for the electroencephalogram, also produces a magnetic field over the head. Ferromagnetic particles are insoluble contaminants of the body; the most important of these are the ferromagnetic dust particles in the lungs, which are primarily Fe3O4 (magnetite). Magnetic fields can give information about the internal organs not otherwise available.
These magnetic fields are very weak, usually in the range of 10-14 to 10-9 tesla; for comparison, the Earth's field is about 10-4 T (1 T = 104 gauss, the older unit of field). The fields at the upper end of this range, say stronger than 10-4 T, can be measured with a simple but sensitive magnetometer called the fluxgate; the weaker fields are measured with the extremely sensitive cryogenic magnetometer called the SQUID (superconducting quantum interference device). The levels of the body's fields, whether they are fluctuating or steady, are orders of magnitude weaker than the fluctuating or steady background fields. They can, however, be measured by using either a magnetically shielded room or two detectors connected in opposition so that much of the background is canceled, or a combination of both methods. The organs producing magnetic fields which are of most interest are the brain, the lungs, and the liver. See Electroencephalography, Migratory behavior