1891.] 



on Electro-magnetic Repulsion, 



307 



trically over an alternating pole, the action of the pole is one of pure 

 repulsion on the disc, which, however, causes no rotation in it. 



Fig. 10. 



Electro-magnet with shaded poles causing a copper disc placed between 



the jaws to revolve. 



When a copper sheet is so placed as to shield or " shade," as 

 Prof. Thomson calls it, part of the magnetic pole, currents are 

 induced both in the fixed plate 



and in the movable one. The Em. 11. 



fixed disc shields part of the 

 other from the induction of the 

 pole, and hence causes the in- 

 duced currents in that plate and 

 disc to be so located that they 

 are in positions to cause con- 

 tinual attraction between one 

 another and continuously pull 

 round the movable disc into 

 fresh positions, so creating re- 

 gular rotation. This principle 

 of " shading " a pole is em- 

 ployed in constructing the polar 

 coils of the magnet used in our 

 experiment a moment ago, and 

 the experiments present us with 

 a form of self-starting alter- 

 nating-current motor, although not perhaps a very efficient one in 

 the technical sense. This principle of " shading " a portion of a 

 conductor from the inductive action of the pole, and so causing the 

 eddy currents in it to be located in a portion of its surface and to 



Eevolution of a shaded copper plate held 

 over an alternate-current magnetic pole. 



