[Illustration: FIG. 444.--Diagram of a gramme ring. It is shown connected to a single-phase current so as to produce a rotating magnetic field, similar to that obtained with a three-phase current. (_Ahrens, Harley and Burns._)] [Illustration: FIG. 445.--The "stator" of an induction motor.] =444. The Induction Motor.=--Another common type of a.-c. motor is the _induction motor_. Its advantage lies in its simplicity. It has neither commutator nor brushes, the armature having no connection with an external circuit. If the wires of a three-phase line be connected to a coil wound in the form of a _gramme ring_, the connections being 120 degrees apart as in Fig. 444, the magnetic field within this coil will change in the same manner as if a magnet were spinning upon a pivot at the center of the coil. Suppose the _N_ pole at one instant is at _A_, in one-third of a cycle it moves to _B_, in another third to _C_, and in one cycle it makes a complete revolution. Thus we have a _rotating magnetic field_. If a cup of some non-magnetic metal such as aluminium or copper be placed on a pivot in the center of this coil, the cup is cut by the moving lines of force and currents are induced in it. Because of these currents, the cup has a magnetic field of its own, and the action of the two magnetic fields is such as to pull the cup around and cause it to rotate in the same direction as that in which the field of the coil rotates. The coil represents the stationary part, the _stator_ (Fig. 445) and the cup the rotating part, the _rotor_, of an induction motor. While the cup rotates in the same direction, it does not rotate so rapidly as the magnetic field. If it should it is plain that it would not cut the lines of force. The difference between the rate of rotation of the rotor and that of the magnetic field is called the "slip." The rotating part in small induction motors is frequently made in a single casting. In large motors, it is built up of heavy copper bars. Thus, from its appearance the common form of rotor is known as the "squirrel cage" rotor. (See Fig. 446.) [Illustration: FIG. 446.--The "rotor" of an induction motor.] [Illustration: FIG. 447.--Diagram illustrating the principle of the synchronous motor. The armature coil passes the position shown in the figure at the instant the current in the line reverses. Thus the armature keeps with the line current, making one revolution with each "cycle."]