310 



Professor J. A. Fleming 



[March fi, 



form small closed secondary circuits upon it. We excite the magnet, 

 and hold near the bar an iron disc capable of free rotation ; it begins 

 to rotate rapidly, as you now see. Not only can this be done with a 

 laminated bar throttled by conducting circuits, but even a solid bar 

 of hard steel will serve the same purpose, and a couple of steel files 

 placed across the poles can cause rapid rotation in pivoted discs of 

 copper or of iron held with their edges close to the bars so alternately 

 magnetised. To elucidate this remarkable action, we must revert for 

 a moment to some fundamental facts. Here are two pa.per rings 

 interlinked, one of red, the other cf blue paper (Fig. 15). Let the red 



Fig. 15 



Electnc Circuit. 



Magnetic Circuit. 



E.M.F 

 Electric Circuit 



Magnetic Circuit. 



>M.M.F. 



Copper 



Iron 



Diagrams illustrating the symmetry in relation between electromotive force and 

 electric current, and magnetomotive force and magnetic induction. 



ring stand as a symbol for a copper or conductive circuit. Let 

 the blue ring stand for an iron or magnetic circuit. If we 

 introduce into the conductive circuit an impulsive or alternating 

 electromotive force, we are well aware that the interlinked iron circuit, 

 by increasing the self-induction of the conductive circuit, hinders 

 the change of current strength in it by introducing a hack electromotive 

 force of self-induction. Consider now the iron circuit. If we intro- 

 duce into that magnetic circuit an impulsive or alternating magneto- 

 motive force by interlinking it with some turns of a magnetising 



