the Unipolar Induction of the Earth. 495 



from the circuit and replaced by a galvanic battery of suffi- 

 cient power. Experiment shows that the moment the circuit 

 is completed the cylinder commences to rotate in the one 

 direction or the other, according to the direction of the current. 

 In order to stop the cylinder it is necessary to oppose to the 

 motion a certain resistance proportional to the product of the 

 magnetic moment and the intensity of the current. 



The rotation of the cylinder is evidently produced by the 

 action of the magnet upon the current passing in the cylinder 

 itself. In order to make the cylinder rotate in a direction 

 opposite to that caused by the battery, a certain amount of 

 mechanical work must be done. The magnet, on the other 

 hand, remains unmoved : it may be turned in the one direc- 

 tion or the other at pleasure without meeting any resistance 

 from the current. Hence the current through the circuit, no 

 matter how produced, is able to put the magnet into rotation. 



If we remove the battery and make the cylinder turn by 

 exerting mechanical energy, a current is produced by unipolar 

 induction, and the heat produced in the circuit is equivalent to 

 the work done in causing it to rotate. If the magnet is made 

 to turn at the same time as the cylinder, experiment shows 

 that there is no change produced in the intensity or direction 

 of the induced current, and this rotation is effected without 

 consumption of any energy due to the current. But, in order 

 that it may be possible to produce a current by the displace- 

 ment of a magnet in the neighbourhood of a closed circuit, or, 

 vice versa, by motion of a closed circuit in the neighbourhood 

 of a magnet, the mechanical theory of heat requires as a 

 condition absolutely necessary that the induced current shall 

 provoke a resistance to the displacement which requires the 

 consumption of mechanical energy to overcome it. Now, in 

 the case in question, it is possible to turn the magnet about its 

 axis without consuming energy; and consequently this rotation 

 is as incapable of producing rotation in the cylinder as in the 

 fixed circuit. It is the energy consumed which gives birth to 

 the current. The rotation of the magnet about its axis is 

 therefore altogether indifferent to the induction, or, in other 

 words, the magnet produces the same inductive effect whether 

 it turns on its axis or not ; the induction depends only upon 

 the mechanical energy consumed in the rotation of the cylinder. 

 The chief error of the old theory consists, then, in the suppo- 

 sition that induction may be produced in the present case by 

 the alteration of the relative positions of the magnet and the 

 circuit, even if there is no consumption of energy in this alter- 

 ation. Now, according to the mechanical theory of heat, such 

 an admission is an absurdity, since it would follow that the 



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