magnetic north pole. The location on the earth near the north geographic pole to which compass magnets point. The earth is a huge magnet spinning about its axis in space. Lines of magnetic flux leave the earth at its magnetic north pole and enter at its magnetic south pole, a location near its south geographic pole. Magnetic compasses align with this magnetic flux, with one end pointing to the magnetic north pole.
magnetic particle inspection (nondestructive inspection method). A method of inspecting ferrous metal components for cracks and other types of flaws. The part being inspected is magnetized and then flooded with iron oxide suspended in a light oil much like kerosine. Any flaw, either on the surface or slightly below the surface, forms a north and a south pole, and the iron oxide attracted to these poles helps locate the flaw.
The iron oxide is often treated with a fluorescent dye, and the inspection is conducted in a darkened booth. When an ultraviolet light (black light) is shone on the part, the treated iron oxide shows up as a brilliant line.
magnetic poles. The locations on a magnet at which the lines of flux leave and enter. All magnets have both a north pole and a south pole. The lines of flux leave the magnet at right angles to the face of the north pole and enter at right angles to the face of the south pole.
magnetic saturation. The condition in which all the magnetic domains in a piece of magnetized material are lined up in the same direction. When a piece of unmagnetized iron or other magnetizable material is held in a magnetic field, increasing the magnetizing force increases the magnetism. But when all the domains are aligned in the same direction, increasing the magnetizing force no longer increases the strength of the magnet. The magnet is said to be saturated.
magnetic shunt (electrical measuring instrument component). A piece of soft iron placed across the air gap in a magnet used in an electrical measuring instrument to vary the amount of magnetic flux across the gap. The permeability of soft iron is much higher than that of air (flux can pass through iron much easier than it can pass through air), and the flux passes through the iron, rather than crossing the air gap. The position of the magnetic shunt is changed to calibrate the instrument.
magnetic variation (navigation). A compass error caused by the difference in the locations of the earth’s geographic north pole and its magnetic north pole. All navigation charts and maps are drawn with reference to the geographic north pole, and all magnetic compasses point to the magnetic north pole. These two locations are not the same, and they are only in alignment along the agonic line.
At any particular location on the surface of the earth, the variation error is the same for all headings. Variation is called declination in land navigation. See agonic line, isogonic lines.