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remains there after it has once been excited by an electric current.
These horseshoe-magnets are called the "Field Magnets" of a generator, because, between their ends where the armature is located is what is known as the field of force, taking its name from the lines of force, which is the technical term used for designating the power eminating from the poles of a magnet; and finally, in the practical generator the magnet ceases to assume the form of a horseshoe.
In Fig. 7 is shown a generator of the type now commonly used for supplying the current for the incandescent electric light. The ring having the alternate black and white spaces, is the commutator, which you will remember is secured to the same shaft as the armature and is practically the same. The large circle about the commutator is intended to represent the outer surface of the armature, which runs very close to the field pieces, N S, as these enlarged ends of the magnet are called. The heavy
