1891.] 



on Electro-magnetic Repulsion, 



309 



eddy currents to be delayed in phase behind the magnetising field, 

 and hence to persist into the period of reversal of that field, and so 

 produce the repulsion between the primary conducting circuit and 

 that part of the secondary conducting circuit in which the eddy 

 currents are set up. 



One more experiment in this part of the subject, before we pass 

 on to some other developments of it, shall be placed under your 

 notice. Returning to the use of the electro-magnet, in which the 

 iron circuit is all but complete, we find that when a highly- 

 conducting disc is put. between the closely approximated half- 

 shielded jaws of this electro-magnet, and an alternating current 

 employed to excite it, the conducting disc is held up in the air-gap 



Fig. 14. 



f^nf^ illf^ 



r £ ^ 



Alternating magnetised iron bar causing revolution of two iron discs held near 



its extremities. 



by reason of the electro-magnetic attraction set up between the disc 

 and the shielding polar plates. If, however, the disc has a relatively 

 poor conductivity, the attraction is not nearly so marked. A good or 

 bad silver coin can be discriminated thereby, because the good silver 

 coin has conductivity enough to be the seat of powerful induced 

 currents, but the bad coin has not. 



§ 13. Closely akin to the foregoing, but rather less easy to 

 explain, are the rotations in copper and iron discs which can be 

 caused by the approximation to them of a laminated iron bar alter- 

 nately magnetised. These actions have been carefully studied by 

 Prof. Elihu Thomson, and applied by him and others in many 

 practical devices. Across the top of this electro-magnet we place a 

 long bar of laminated iron with the plane of the lamination vertical 

 (Fig. 14). This bar is throttled at intervals by copper bands, which 



