342 THE INDUCTION MOTOR. 



will take up a position 45 from its previous one. It will 

 occupy this position whatever the strength of the current 

 may be, as long as the same current passes through both 

 coils ; consequently, if the current in the two coils is an 

 alternating one the field will always be formed along this 

 axis, but the direction and strength of the magnetic field 

 will undergo the same changes as the current. The 

 magnet would in this case tend to set itself in the direction 

 of this field, with its axis along a fixed line, but with the 

 relative position of north and south poles rapidly reversed. 

 The magnet would consequently not rotate (unless given 

 an initial rotation by hand), but would receive rapid im- 

 pulses in the nature of alternate pulls and pushes in the 

 direction of its axis at every reversal of the current. 



If the currents in the two coils do not vary simulta- 

 neously, but flow one after the other, the result will be 

 that the magnet will tend to rotate in the manner illus- 

 trated by the six diagrams in Fig. 170. 



Thus, suppose (Diagram I.) the current flows from left 

 to right, through the horizontal coil, the needle will point 

 downwards. On a current being started downwards in 

 the vertical coil (Diagram II.) the needle is deflected to 

 the left. If, now, the current in the horizontal coil ceases 

 (Diagram III.), the needle will point horizontally to the 

 left. As a current is started in a reverse direction, i.e., 

 from right to left, the needle inclines upwards (Diagram 

 IV.). And so the changes may be followed further. 



A step-by-step motion of rotation would thus be 

 maintained by such a sequence of changes in the currents. 

 The cycle of changes just suggested would be experi- 

 enced if a two-phase current were supplied to the windings, 

 one phase being connected to the vertical coil, and the 

 other phase to the horizontal coil, except that in this case 

 the changes would occur gradually, instead of step by 

 step. It will be shown that the resultant field produced 

 by a two-phase current flowing in two windings situated 

 perpendicular to each other is constant in strength, and that 

 its direction changes gradually and uniformly from the 

 axis of one coil to the axis of the other. A two-phase 

 current thus produces a rotating magnetic field of constant 

 strength, which would make one complete revolution of the 

 field formed within the two coils in the time of one period 

 of the current. 



