314 Professor J, A. Fleming [Marcli 6, 



a light iron-rimmed wheel, free to turn in the centre. The actions 

 just explained drive the wheel round, when the magnet coils are 

 traversed by an alternating current. The iron wheel carries on its 

 shaft a set of mica vanes, which retard the wheel by air friction. 

 Under the opposing influences of this retardation and the electro- 

 magnetic rotation forces, the wheel takes a certain speed corres- 

 ponding to different current strengths in the magnetic coils, and 

 hence the total number of revolutions of the wheel in a given time, 

 as recorded by a counter, serves to determine the total quantity of 

 alternating current which has passed through the meter. A motor 

 (Fig. 17) working a fan is also here exhibited, the operation of which 

 depends on the same facts. In the case of the motor the iron-rimmed 

 wheel has its tyre closed with copper sheet to aid the action. 



§ 16. The rotation of iron discs can be shown also by means of a 

 badly-designed transformer. If a closed laminated iron ring (Fig. 18), 



like the one before me, is wound with a 

 Fig. 18. couple of conducting circuits, such an ar- 



rangement constitutes a transformer. If 

 these two circuits are wound on opposite 

 sides of the iron ring, the previous explana- 

 tions will enable you to perceive that the 

 arrangement will be productive of great 

 magnetic leakage across the iron circuit. 

 In designing transformers for practical work, 

 one condition amongst others which must be 

 held in view is to so arrange the conductive 

 and magnetic circuits that a great magnetic 

 leakage of lines of force across the air does 

 not take place. If, however, this leakage 

 exists, it indicates that the secondary circuit 

 ^th^^'ttlldiro^^Snt^au^^^ ^^ ^^* getting the full benefit of the induc- 

 rotation oraTlrSrdfs? *ion created by the primary. To detect it 

 placed near the secondary we have merely to hold near the iron circuit 

 coil. a little balanced or pivoted iron disc, and 



if it is set in rapid rotation, as you observe 

 in this case, it indicates that there are laterally-moving lines of 

 magnetic force outside the iron, which have escaped from the iron in 

 consequence of the back magneto-force of the secondary circuit. 



§ 17. Time would fail me if I were to attempt to enlarge on the 

 practical applications of the scientific principles which these experi- 

 ments disclose to us. They are a fertile field both for the investigator 

 seeking to add to the sum total of existing knowledge, or to the 

 inventor in search of applications in electrical technology for such 

 acquired facts. Ingenious minds, and that of Prof. Elihu Thomson 

 foremost amongst them, are busy in seeking to turn these facts to 

 account in the construction of alternating current motors. 



One of the simplest of these is shown in principle in the diagram 

 now on the screen (Fig. 19). The coils C are traversed by an alter- 



