216 Intelligence and Miscellaneous Articles. 



receiving machine. We verify this reversal of the polarities of the 

 inductors by placiug near them a simple compass, the needle of 

 which suddenly turns half round at each change of magnetization 

 of the inductors. 



Now we ascertain that these movements of the compass-needle 

 coincide with those of the galvanometer ; we may then be sure that 

 the two phenomena are connected the one with the other, and so 

 intimately that the one must be the consequence of the other. 



But this explains nothing. Let us frame an hypothesis, and 

 suppose for a moment that the receiving magnetoelectrical machine 

 can, from some cause wliicli we will not investigate, receive periodi- 

 cally an increase of velocity. Under the conditions of this hypo- 

 thesis our receiving magnetoelectrical machine, instead of continuing 

 to revolve under the action of the current to which it was at first 

 submitted, would, in virtue of its increased velocity, give rise to a 

 current of its own, which in its turn would traverse the dynamo- 

 electrical machine. As this current would have the opposite 

 direction to that proceeding from the generating dynamoelectrical 

 machine, it would reverse the polarities of the inductors and give 

 rise to a new current having the same direction as itself, which in 

 its turn would reverse the direction of rotation of the receiving 

 machine. 



We have seen above that these effects are shown by the galva- 

 nometer and the compass ; but if our hypothesis is true, this phe- 

 nomenon will no longer be produced when, by any means whatever, 

 we prevent the receiving magnetoelectrical machine from increasing 

 its velocity : for this purpose the application of a brake suffices. 

 Now, as soon as the brake comes into play the preceding effects 

 disappear : the rotation of the machine continues constantly in 

 the same direction ; the needles of the galvanometer and the com- 

 pass remain motionless. 



What are we to conclude from this ? Nothing, except that we 

 are confronted by a scientific paradox, the explanation of which 

 will come, but which does not cease to be interesting. 



The experiment is very easy to carry out ; it succeeds as often 

 as we will. Nevertheless it is necessary to say that with a gene- 

 rating machine such as the ordinary Gramme, the inductors of 

 which are of cast iron, the experiment is more delicate and requires 

 certain conditions of velocity, though very simple after all. We 

 believe that this is due to the nature of cast iron, the residual 

 magnetism of which offers a certain amount of resistance to the 

 reversing current proceeding from the receiving magnetoelectrical 

 machine. On the other hand, any machine with a soft-iron induc- 

 tor, taken as generator, permits the first attempt to be successful, 

 without any precaution or care. 



We usually employ for the experiment a Siemens dynamoelectric 

 machine with continuous currents as generator, and a small labo- 

 ratory Gramme with ordinary permanent magnet as receiver. — 

 Comptes Rendus de VAcademie des Sciences, July 26, 1880, t. xci. 

 pp. 226, 227. 



