illustration of the Induction of Electric Currents. 31 



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the two bights carry similar pendent pulleys, 

 C, D, from which again hang weights, E, F. 

 The weight of the cord being negligible, the 

 system is devoid of potential energy ; that 

 is, it will balance, whatever may be the ver- 

 tical distance between C and D. 



Since either pulley A, B may turn inde- 

 pendently of the other, the system is capable 

 of two independent motions. If A, B turn 

 in the same direction and with the same 

 velocity, one of the pendent pulleys C, D 

 rises, and the other falls. If, on the other 

 hand, the motions of A, B are equal and op- 

 posite, the axes of the pendent pulleys and 

 the attached weights remain at rest. 



In the electrical analogy the rotatory 

 velocity of A corresponds to a current in a 

 primary circuit, that of B to a current in a 

 secondary. If when all is at rest the rota- 

 tion of A be suddenly started, by force applied 

 at the handle or otherwise, the inertia of the 

 masses, E, F, opposes their sudden move- 

 ment, and the consequence is that the pulley B turns back- 

 ivards, i. e. in. the opposite direction to the rotation imposed 

 upon A. This is the current induced in a secondary circuit 

 when an electromotive force begins to act in the primary. 

 In like manner, if A having been for some time in uniform 

 movement suddenly stops, B enters into motion in the direc- 

 tion of the former movement of A. This is the secondary 

 current on the break of the current in the primary circuit. 



It must be borne in mind that in the absence of friction 

 there is nothing to correspond with electrical resistance, so 

 that the conductors must be looked upon as perfect. The 

 frictions which actually enter do not follow the same laws as 

 electrical resistances, and only very imperfectly represent 

 them. However, the frictions which oppose the rotations of 

 A and B have a general effect of the right sort; but the 

 rotations of C and D, corresponding to dielectric machinery, 

 should be as free as possible. 



The effect of a condenser, to which the terminals of one of 

 the circuits is joined, would be represented by a spiral spring 

 (as in a watch) attached to the corresponding pullev, the 

 stiffness of the spring being inversely as the capacity of the 

 condenser. The absence of the spring, or (which comes to 

 the same thing) the indefinite decrease of its stiffness, corre- 

 sponds to infinite electrical capacity, or to a simply closed 

 circuit. 



