Field Magnets

—In a dynamo or motor, strong magnets terminating in pole pieces between which the armature is rotated, the axis of the armature being at right angles to the general direction of the lines of induction of the field. The object of field magnets is to produce an intense magnetic field within which the armature revolves. They may be either permanent or electromagnets. Electromagnets, however, po e such a number of important advantages over permanent magnets that they are now invariably used in all machines intended for practical work. The field magnets of a dynamo may be excited, either by the current furnished by an independent dynamo or battery, in which case the machine is said to be ‘“‘separately excited,” or by the current generated in the armature of the machine of which the field magnet forms part, when the machine is said to be self-excited. The latter type of machine depends for its action upon the presence of residual magnetism in its field magnet. Owing a to this residual magnetism, a weak magnetic field on is always present between the pole pieces of a field magnet; hence, when the armature is rotated in the armature chamber, its conductors cut the lines of force contained in this magnetic field, and a small E. M. F. is set up in the armature in consequence.