Home Study Course in Practical Electricity: An Electrical Catechism, in Three Volumes Operation and Management of Electric, Motors, Lamps and Power Stations, Vol. 3 of 3 (Classic Reprint)

What is a direct-current motor? A direct-current motor is a machine for converting energy in the form of direct-current electricity into energy in the form of mechanical motion. 789. How does this conversion take place? Through the force of two magnetic fields mutually exerted upon each other within a small space between the stationary and movable members of the electric motor. 790. Does it make any difference how the two magnetic fields are formed? Only a difference in degree. The two magnetic fields may be formed by currents flowing in two wires (straight or coiled, with or without cores), or by two permanent magnets, or the one field may be produced by a current in a wire and the other field by a permanent magnet. 791. Explain how the magnetic fields produce mechanical motion in an electric motor. When the lines of force of two magnetic fields meet without coinciding in direction, each set exerts a pull upon the other which tends to draw them into agreement as to their direction in space; this pull is transmitted to the parts in which the magnetic lines of force originate, and will tend to draw them into such a position that the lines of force of the two fields will coincide to the greatest possible extent. The greater the strengths of the magnetic fields, the stronger will be the force thus acting, and if the fields be sufficiently strong mechanical motion will be given to that part which moves the more readily.
(Typographical errors above are due to OCR software and don't occur in the book.)

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