286 Mr. Faraday on Magneto-electric Induction', 



With regard to the central arrangement, we read (p. 284.), 

 " In this case the magnet acting upon the centre of the disc, 

 the probes do not transmit any indication of a current to the 

 galvanometer, let them be placed "where they may ; and if by 

 chance small deviations should be remarked, they arise from 

 imperfect centralization, so that if this defect be corrected all 

 indications, &c. of an equivocal source immediately disappear. 

 Indeed what is the result if we employ an electro-dynamic 

 spiral which turns quite round on its own centre, always op- 

 posite to the same magnetic pole ? Absolutely nothing. Its 

 revolution is an unimportant circumstance ; for the formation 

 of the currents is wholly due to another condition, they being 

 manifested only at the moment when the spirals are brought 

 near to the magnets, or removed from them. So long as the 

 spirals are present, whether they move or not, there is no cur- 

 rent : so also there is none in the case of central rotation in 

 which the points of the disc remain constantly at the same 

 distance from the magnetic pole, by renewing thus the com- 

 bination of continued presence, to which Mr. Faraday's new 

 laws in relation to currents DO NOT ASSIGN ANY EFFECT." 



This assertion is so erroneous in every part, that I have 

 been obliged to quote the passage at full length. In the first 

 place, there is a tendency to the formation of currents of elec- 

 tricity in the revolving disc, in the case of " central arrange- 

 ment," as well as in every other case (F. 149 156.); but 

 their direction is from the centre to the circumference, or 

 vice versa, and it is to these parts that the collectors should 

 be applied. It is precisely this which renders the revolving 

 disc a new electrical machine (F. 154.), and it is upon this 

 point that Messrs Nobili and Antinori are so entirely mis- 

 taken in their two memoirs. This error is repeated through- 

 out the whole of the memoir that I am now comparing with 

 my first paper, which, if I mistake not, contains the theory of 

 Arago's phenomenon in all its parts. 



At page 284 we find, that when a helix turns upon its axis 

 concentrically with a magnetic pole, the result is absolutely 

 nothing, and that the condition of rotation is unimportant. 

 Now, though I have not made any experiments on the sub- 

 ject, I venture to assert that there will be an effort in the elec- 

 tric current to pass in a transverse direction to the helix, and 

 that the circumstance of its rotation, instead of being unim- 

 portant, is in these cases the only condition essentially re- 

 quisite for the production of currents. The helix, in fact, 

 may be considered as analogous to a cylinder which might 

 occupy its place, but to which it is very inferior, as it consists 

 of a long coil of wire. It may also be regarded as a simple 



