288 Mr. Faraday on Magneto-electric Induction. 



rents of electricity may be obtained by means of the re- 

 volution of a cylinder (F. 219.), or of a disc in connection 

 with a magnet (F. 218. ), or of the magnet itself (F. 220.), dis- 

 proves each of them. ONE LAW, which includes all the ef- 

 fects, is given in my memoir (F. 114. 116. &c.), and it simply 

 expresses the direction in which the moving conductor in- 

 tersects the magnetic curves. This law of direction being 

 given, I endeavoured to recapitulate the whole in the terms 

 that I shall here repeat (F. 118.). 



" All these results show that the power of inducing electric 

 currents is circumferentially excited by a magnetic resultant 

 or axis of power, just as circumferential magnetism is de- 

 pendent upon and is exhibited by an electric current." 



I have quoted this passage of the Italian physicists at full 

 length, because it contains nearly all our points of difference, 

 both as to fact and opinion, concerning this part of the sub- 

 ject. Having thus shown all the errors included in it, I shall 

 endeavour to be more concise, while I exhibit, with the assist- 

 ance of the galvanometer, such others, derived from them, as 

 are dispersed over the remainder of the memoir. It is in- 

 deed curious to remark, how, with galvanometrical indica- 

 tions generally correct, these gentlemen have suffered them- 

 selves to be led astray under the influence of preconceived 

 opinions. For example, at page 287288, and in fig. 2. 

 plate iii. is shown the result of an examination by the gal- 

 vanometer of the currents in a revolving disc. These cur- 

 rents are indicated nearly correctly by means of arrows ; but 

 the two consequences deduced from them agree with the theory 

 enunciated, and are diametrically opposed to the facts. 



" The immediate inspection of the arrows which mark the 

 currents in the two regions of the disc (fig. 2.) leads to one 

 of these consequences (p. 287.) an d it is that a system of cur- 

 rents is developed upon the parts that enter contrary to those 

 produced on the other side. The other consequence arises from 

 comparing the currents produced upon the disc with the cur- 

 rents of the producing cause, and it is that the direction of 

 the currents upon the parts that enter is contrary to that of the 

 producing currents, while on the other side the direction in the 

 two systems is identical" 



But I showed in my first memoir (F. 119.)s that "when 

 a piece of metal is passed either before a single pole, or be- 

 tween the two opposite poles of a magnet, or near electro- 

 magnetic poles, whether ferruginous or not, electric currents 

 are produced across the metal transverse to the direction of 

 motion. This fact is proved by means of wires (F. 109.), 

 plates (F. 101.) and discs (F. 92. &c.) ; and in all these cases 



