26 



ALTERNATING CURRENTS 



the same region of the field), so that there will be attraction between 

 them (since conductors conveying currents flowing in the same 

 direction attract each other). Let us now suppose that the rings are 

 displaced relatively to each other as in Fig. 17 (a), in a direction 

 parallel to their planes. In Fig. 17 (a), the shaded portion represents 

 the pole of the alternating-current electromagnet, which for the sake 

 of simplicity is shown of circular shape. The attraction between the 

 rings will tend to pull them into coincidence, and there will be a 

 component of stress in a direction parallel to the planes of the rings. 

 Next, suppose one of the rings to be replaced by a conducting sheet 

 of metal, as in Fig. 17 (&) in which the dotted circle shows the 

 position of the pole and let the ring be fixed while the conducting 

 sheet is free to move. If the ring were removed, then by symmetry 

 it is clear that the currents induced in the conducting sheet would 



CONDUCTING SHEET 



(a) (I) 



FIG. 17. To illustrate Principle of Shaded-pole Motor. 



(assuming the sheet to be of large extent, so as to project well beyond 

 the polar edges) follow circular paths having their centres on the axis 

 of the magnet. But with the ring in place, it is evident that the 

 portion of the polar surface covered by the ring is subjected to a 

 greater demagnetizing action than the portion lying outside the ring, 

 and the result of this is to cause a shifting of the magnetic flux 

 (whose distribution with the ring removed would be uniform) towards 

 the right-hand "unshaded" crescent-shaped portion of the polar 

 surface. But with this shifting of the flux the currents induced in 

 the conducting sheet will also be shifted to the right, following paths 

 similar to that roughly indicated by the chain-dotted line. Now, the 

 portion of the conducting sheet forming the closed circuit indicated 

 by the chain-dotted line and the ring will behave relatively to each 

 other in the manner of the two rings of Fig. 17 (a), and since the 



