1891.1 



on Electro-magnetic Bepulsion. 



308 



may be illustrated meclianically thus :— Three light wooden laths 

 are connected together by a flat steel band, and the system 

 hung up by a string like a pair of astatic magnetic needles. 

 (Fig. 6). If we take hold of the upper rod and move it backwards 

 and forwards on a horizontal plane, compelling it to execute harmonic 

 oscillations, the upper and lower rods move together synchronously. 

 Let the upper rod symbolise impressed electromotive force, and the 

 lower rod resulting current. Under present circumstances, these two 

 move always in step with one another. Next load down the middle 

 rod or connecting mechanism by means of two lead weights, and give 

 it inertia. On again taking hold of the upper rod and causing it to 



Fig. 7. 



Diagram showing the inequahty of the attractive and repulsive impulses in the 

 case of an inductive circuit when held in an alternatiug magnetic field. 



execute forced harmonic oscillations, the lower rod no longer vibrates 

 in tune with it. It lags in phase behind the top one, and we thus 

 illustrate, though not perhaps by a perfect mechanical analogy, the 

 effect of electric inertia in the intermediate mechanism in causing a 

 lag of current behind impressed electromotive force. 



§ 7. Returning for an instant to the diagram we considered just 

 now, we must correct it to make it fit in with the facts of nature, and 

 we must represent the periodic curve which stands for the fluctuations 

 of the induced current in the ring as shifted backwards or lagging 

 behind the curve which represents the electromotive force in the 

 circuit brought into existence by the fluctuating magnetic field. 

 Making this change in our diagram (Fig. 7), and forming as before a 

 force curve to represent the impulses on the ring, we now find that 

 owing to the " lag " of the secondary current, one set of the impulses, 



X 2 



