KOTAKY MAGNETIC FIELDS. 



147 



i sin pt, i sin (pt + ^), and t sin (pt + -^ ) (see p. 144), the direc- 



tion of the magnetic field is indicated by the arrows in the cases 

 where the current is zero in A, B, and respectively, the arrow 

 head pointing towards the S pole. 



The magnetic field, in this case, rotates through a complete 

 revolution in the periodic time of the alternating currents. 



FIG. 51. 



FIG. 52. 



FIG. 53. 



If, now, a metal cylinder was placed coaxially within the anchor- 

 ring, and free to rotate round its axis, it is easy to see that rotation 

 would occur, for the rotating magnetic field would induce currents 

 in the body of the cylinder, more or less parallel to its axis, and 

 the mutual action of the rotating field and the field due to the 

 induced currents would give rise to a couple tending to make 

 the cylinder rotate in the same direction as the magnetic field. 

 The couple would be greatest if the induced currents were con- 

 strained to flow in directions parallel to the axis of the cylinder. 

 This can be effected by building the cylinder up of thin circular 

 discs of soft iron, and reducing the magnetic reluctance of the 

 circuit by imbedding copper conductors parallel to the axis near 

 the periphery, and connecting them together at the ends. The 

 iron greatly increases the strength of the rotating magnetic field, 

 while the conductors localize the induced currents. 



INDUCTION MOTORS. 



98. An arrangement such as is here described constitutes an 

 Induction Motor ; sometimes called a Rotary Field 

 Motor, or a Polyphase Motor. If the rotating field is 

 produced by two alternating currents, we get a di-phase 

 motor, and if by three, a tri-pnase motor. 



The stationary part, which is magnetized by the alternating 



