226 ALTERNATING CURRENTS 



as shown in the figure. As these currents find themselves in a 

 magnetic field, they tend to move across this field, just as the 

 currents in the conductors of a direct-current motor tend to move 

 across its magnetic field. By Lenz's law, the direction of the 

 force developed between these currents in the disc and the 

 magnetic field producing them will be such that the disc tends 

 to follow the magnet, as shown in the figure. 



To illustrate this more in detail, consider Fig. 217 (a), (6), and (c). 

 In (a) , the north pole of the rotating magnet is shown as moving 

 in a counter-clockwise direction. The conductor beneath the 

 magnet also moves in a counter-clockwise direction, but more 

 slowly than the magnet. Therefore, the relative motion between 

 the magnet and the conductor is the same as if the magnet were 

 stationary and the conductor moved in the clockwise direction. 

 This relative motion of the magnet and the conductor is illus- 

 trated in Fig. 217 (6), where the north pole is shown as being 

 stationary and the conductor is moving from right to left. 

 Applying Fleming's right-hand rule (see Vol. I, page 218), the 

 direction of the induced current is toward the observer. The 

 lines of force about the conductor, due to its own current, are 

 therefore counter-clockwise and the resultant field is found by 

 combining the conductor field and the field produced by the 

 magnet. The appearance of this resultant field is shown in Fig. 

 217 (c) (also see Vol. I, page 309). As the magnetic field is in- 

 creased in intensity to the left of the conductor and reduced in 

 intensity to the right of the conductor, there is a force developed 

 which urges this conductor from left to right. That is, the con- 

 ductor tends to follow the magnet. Actually, the magnet rotates 

 in a counter-clockwise direction. Therefore, the disc rotates in 

 the same direction but at a speed less than that of the magnet. 



The disc can never attain the speed of the magnet, for were it 

 to attain this speed, there would be no relative motion of the 

 disc and the magnet and, therefore, no cutting of the disc by the 

 magnetic flux. The disc current would then become zero and 

 no torque would be developed, which would result in the disc 

 speed becoming less than that of the magnet. Because the disc 

 can not attain the speed of the magnet, there must always exist 

 a difference of speed between the two. This difference of speed is 

 called the revolutions slip. 



