Field Magnet

Field Magnet, in Magnetism, is a special magnet placed suitably in a given mechanism so that its field of force may be utilised to control the motions of other magnets, to generate motion in armatures through which electric currents are passed, or to generate electric currents through armatures that are set in motion in the given field of force. The first function is performed in several electrical measuring instruments, the second in electric motors, and the third in dynamos. The design of a field-magnet depends on the qualities required of its field of force. For this field to be very intense - i.e. for the action on a small magnetic pole to be very great at any part of the field - the magnet should be built of soft iron, and should be excited by being covered with coils of wire round which a powerful electric current is flowing. The two ends or poles should be brought as near as possible together, the field of force being then confined practically to the space between the pole-pieces. Special shaping of these pieces will render the field locally intense, or will produce general uniformity in its intensity. [Galvanometer, Magnetism.]