14 .TACOBI ON THE APPLICATION OF ELKCTUO-M AON KTISM 



Sciences of Berlin, stated to me that he had not obtained the 

 least development of magnetism by means of a magneto-electric 

 helix, which he had introduced into a piece of a giin-barrel. I 

 was then of the opinion of this philosopher, that this negative result 

 must be attributed to some accidental property of the iron ; the 

 more so, as a helix which he had coiled I'ound the barrel had 

 not produced any considerable effect. M. Dove does not appear 

 to have pubhshed any additions to this isolated experiment. 

 Altogether independent of this previous experiment, M. Parrot 

 has made the discoveiy which he has pubUshed in his memoir, 

 {Bulletin scientifique, No. IG) that a magneto-electric helix, placed 

 in the interior of a hollow cylinder, does not communicate any 

 magnetic force to the soft iron. This coincidence of observations 

 does not allow of the admission of any accidental state of the iron 

 which has been exposed to the influence of the current. We 

 might probably ai'rive at the explanation of this curious fact, by 

 following up the electro-dynamic views of M. Ampere on the 

 constitution of magnets. Regarding the magnets as an assem- 

 blage of elementary wires, parallel to the axis, and surrounded by 

 electric currents, which are all in the same direction relatively to 

 the axis, M. Ampere is obliged, in order to comj)ly with known 

 facts, to adopt a covered surface, which represents the total effect 

 of these isolated currents, or of those solenoids which may be 

 substituted in their stead. In conformity with experiment, it is 

 unnecessary to take any notice of the inner cmTents, as they for 

 the most part destroy each otiier. The direction of the magneti- 

 zation is determined by that of the currents ; so that if we suppose 

 the magnet to be directed by the action of the earth, the currents 

 proceed from east to west in the upper part, and from west to 

 east in the lower part. This is merely a small omission which 

 has escaped this illustrious philoso]iher, if he has not specially 

 added that, in order to be able to replace the elementary effects by 

 the magnetic covering, it is absolutely necessary to admit also the 

 condition, that the enveloped currents have, relatively to the axis 

 of the whole body, the same chrection as the elementary currents 

 have relatively to the axis of the wires. It is this condition, how- 

 ever, which constitutes the distinctive character of a magnetic sur- 

 face. For it suffices to look at fig. 1. in PI. I. to judge of the 

 discordance of these directions relatively to the inner surface, and 

 of their concordance relatively to the external surface. The expe- 

 riments of Mr. Barlow, and the calculations of M. Poisson, to 



