=Magnetic Poles, False.= It has been established that there are other poles on the earth that attract the magnetic needle when the latter is brought into their vicinity. These are called false poles, and are probably caused by large deposits of iron lying close to the surface of the earth. =Magnetic Separator.= An apparatus for separating magnetic substances from mixtures. It is used chiefly in separating iron ore from earth and rock. The mineral falls on an iron cylinder, or drum, magnetized by coils, and adheres there, while the earth or crushed rock drops below. The particles of iron are afterwards removed by a scraper. The machine is also used in separating iron filings and chips from brass, copper, or other metals, the iron adhering to the magnet, while the brass and other chips drop underneath. =Magnetism.= The phenomena of attraction exerted by one body for another. It has been commonly understood that magnetism and electricity are very closely related, for without electricity magnetism could not exist, although it has not been shown clearly that magnetism plays any part in the generation of electricity. Magnetism is the phenomenal force exerted by one body having two poles (negative and positive) for like bodies. The horseshoe magnet or a bar of magnetized steel are the simplest examples of this. If both ends of the horseshoe were positive they would not attract, but would repel. If both ends of a bar were positive they would repel; but as one is negative, or north-seeking, and the other positive, they exert lines of force which attract like bodies, such as bits of iron, nails, and needles. No energy is required to maintain magnetism in a tempered steel object, such as the wiring about a soft iron core when it has been magnetized, but electric current must flow about the soft iron core in order to render it a magnet. So soon as the current ceases to flow the magnetism ceases and the soft iron fails to attract. =Magnetism, Uniform.= Magnetism that is uniform throughout a mass of magnetic steel, or a core that is electro-magnetic. =Magnetize.= To impart magnetic property to a substance capable of receiving it. =Magnetizing-coil.= (_See_ Coil, Magnetizing.) =Magneto Call-bell.= A call-bell used principally in telephone systems, and operated by a current from a magneto-electric generator. The current is excited by turning the handle at the side of the telephone-box before removing the receiver from the hook. =Magneto-generator.= A current-generator composed of a permanent magnet and a revolving armature which is rotated between the poles of the permanent magnet. =Main Circuit.= (_See_ Circuit, Main.) =Main Feeder.= The main wire in a district to which all the feeder wires are attached. =Main Switch.= The switch connected to the main wire of a line, or the main-switch controlling a number of auxiliary switches. =Mains, Electric.= The large conductors in a system of electric light or power distribution.