An instrument used in magnetic surveying for detecting changes in magnetic field intensity; it takes intermittent measurements of absolute field strength. The detector consists of a bottle of alcohol or water around which is wound an electrical coil of 1,000 turns. The protons, which form the nuclei of hydrogen atoms in the liquid, spin and gyrate in their attempt to align themselves in the direction of the earth's magnetic field intensity. A current of one amp is passed through the coil for three seconds, which aligns the majority of the protons in the direction of the magnetic field thus produced. When this current is cut off, the protons attempt to realign in the direction of the earth's magnetic field; the speed of gyration, or frequency of precession, is amplified and measured in the instrument. This measurement reflects any alteration in the magnetic intensity caused by the presence of fired structures, soil disturbances (e.g. pits, ditches, etc.), or iron objects. It is a highly sensitive magnetometer, used in subsurface detection to record variations in the earth's magnetic field caused by buried iron, kilns, hearths, pits, or ditches.