SKIN EFFECT 



27 



ring is fixed, the sheet will move from right to left i.e. from the 

 " unshaded " to the " shaded " portion of the magnetic pole. Since, 

 however, the conducting sheet is continuous, as it moves successive 

 portions of it come into the position of the chain-dotted line, and so 

 the pull is maintained during the motion. 



A small motor constructed on this principle is shown in Fig. 18. 

 The core of the magnet consists of a number of steel or soft-iron 

 stampings of rectangular 

 shape with an opening in 

 the middle, and around 

 one limb of the core is 

 wound the exciting coil. 

 The stampings are cut 

 across the middle of the 

 opposite limb of the core, 

 so as to forin a narrow 

 air-gap. Around one half 

 of one of the poles is 

 placed the " shading " coil, 

 which consists of a simple 

 band of thick copper en- 

 circling half the pole- 

 piece. A suitably mounted 

 aluminium disc is free to 

 rotate between the poles, 

 and corresponds to the 

 " conducting sheet " of 



Fig. 17 (fc). From what FIG. 18. Shaded-pole Motor. 



has been said, it is evident 



that the direction of rotation will be that indicated by the arrow.* 



ALUMINIUM DISC 



COPPER BAND 



CORE STAMPINGS 



13. Skin Effect 



In the case of a conductor of large cross-section conveying a 

 current, there will be an appreciable difference between the magnetic 

 flux linked with the surface layers of the conductor and the flux 

 linked with its central portion. For if we imagine the conductor 

 split up into a large number of small parallel filaments, then the 

 current along any filament at a considerable depth below the surface 

 will be linked not only with the lines external to the conductor, but 

 also with an appreciable number of lines in the substance of the 



* The form of motor sketched in Fi<*. 18 is used by Messrs. Brown, Boverie & Co., 



of Switzerland, in the relays employed by them in connection with their automatic 

 maximum-current switches. 



