magnetic drain plug (engine lubrication system component). A drain plug in the sump of either a reciprocating engine or a gas turbine engine. Two small permanent magnets built into the drain plug attract and hold ferrous metal chips or shavings that may be in the lubricating oil. Metal on the drain plug is an indication of internal engine failure.
Some magnetic drain plugs in gas turbine engines have the two magnets connected into an electrical warning system. When bits of metal short across them, a circuit is completed, and a warning light is turned on to show that metal particles are loose in the engine.
magnetic field. The invisible, but measurable, force surrounding a permanent magnet or current-carrying conductor. This field is produced when the orbital axes of the electrons of the atoms in the material are all in alignment.
When a piece of iron or steel is held in the magnetic field, it is pulled toward the magnet, and when a conductor passes through a magnetic field, current is induced into it.

magnetic flux. Invisible lines of force passing between the poles of a magnet. These invisible lines are considered to leave the north pole of a magnet and enter its south pole, and they always follow the path of least resistance. Lines of flux form complete loops through the magnet and across the gap between the poles, and the loops never cross one another. When an electrical conductor cuts across the lines of magnetic flux, a voltage is produced that causes current to flow in it.
magnetic flux density. The number of lines of magnetic flux per unit of area. Magnetic flux density is measured in gausses. One gauss is equal to one line of flux (one maxwell) per square centimeter.
magnetic heading (navigation). The direction an aircraft is pointed with respect to magnetic north. Magnetic north is the direction to the magnetic north pole, rather than the geographic north pole.
magnetic hysteresis. The characteristic of a magnetic material that causes it to retain some magnetism after the magnetizing force is removed. In order to remove all the magnetism from a part, it must be demagnetized with a force opposite to the one that magnetized it.