Fig. 295.—The magnetic field about a wire carrying a current.
Fig. 296.—The magnetic field between two unlike poles.
Fig. 297.—The crowding of the lines of force above the wire, pushes it downward.
In a practical motor, the wires upon the armature are so connected that those upon one side (see Fig. 298), carry currents that pass in, while on the other side they pass out. To represent the direction of the current in the wires, the following device is employed; a circle with a cross (to represent the feather in the tail of an arrow) indicates a current going away from the observer, while a circle with a dot at its center (to represent the tip of an arrow) indicates a current coming toward the observer.
Fig. 298.—The crowding of the lines of force causes the armature to revolve in a clockwise direction.
In Fig. 298 the north pole is at the left and the south pole at the right. The field of the magnets therefore passes from left to right as indicated in the figure. Now in the armature the currents in the wires on the left half of the armature are coming toward the observer while those on the right move away. Applying the right-hand rule, the magnetic lines will crowd under the wires on the left side of the armature while they will crowd over the wires on the right side. This will cause a rotation up on the left side and down on the right, or in a clockwise direction.
Fig. 299.—View of a one-half horse-power motor.