=Magnet, Horseshoe.= A magnet of U shape with the poles or ends brought closer together than the other parts of the limbs. A soft iron bar is placed across the poles when not in use, as this serves to conserve the magnetism. =Magnet, Permanent.= A term applied to a hard steel magnet possessing high retentivity, or the power to hold its magnetism indefinitely. =Magnet, Regulator.= An electro-magnet whose armature moves in such a manner as to automatically shift the commutator-brushes, on a motor or dynamo, to a position which insures the preservation of both brushes and commutator-bars, and also produces a constant current. =Magnet, Simple.= A magnet made of one piece of metal. =Magnet Wire.= Insulated wire used for coils. Cotton or silk covered wire is the most serviceable for winding magnets. =Magnetic Adherence.= The tendency of a mass of iron to adhere to the poles of a magnet. =Magnetic Attraction and Repulsion.= The attraction of a magnet for iron, steel, nickel, and cobalt; also of unlike poles of magnets for each other. The like poles repel. =Magnetic Circuit-breakers.= An automatic switch, or breaker, whose action is excited and controlled by an electro-magnet. =Magnetic Concentration of Ores.= The separation of iron and steel from their gangue by magnetic attraction. It is applicable only when either the ore or the gangue is susceptible to the magnet. =Magnetic Control.= The control of a magnetic needle, magnet, index, armature, or other iron indicator in a galvanometer, ammeter, or voltmeter by a magnetic field. =Magnetic Dip.= The inclination from the horizontal position of a magnetic needle that is free to move in a vertical plane. =Magnetic Field, Rotary.= A magnetic field resulting from a rotary current. =Magnetic Field, Shifting.= A magnetic field which rotates. Its lines of magnetic force vary, therefore, in position. =Magnetic Field, Uniform.= A field of uniform strength in all portions, such as the magnetic field of the earth. =Magnetic Force.= The power of attraction and repulsion exercised by a magnet; the force of attraction and repulsion which a magnet exercises, and which, in its ultimate essence, is unknown to science. =Magnetic Induction.= (_See_ Induction, Magnetic.) =Magnetic Needle.= A magnet having a cup or small depression at its centre, and poised on a sharp pin of brass, so as to be free to rotate. Its N pole points to the north, and its S pole to the south. A compass needle. =Magnetic Poles.= The terrestrial points towards which the north or south poles of the magnetic needle are attracted. There are two poles: the arctic, or negative, which attracts the positive or N pole of the magnetic needle; and the antarctic, or positive, which attracts the S pole of the needle.