IG JACOBI ON THE APPLICATION OF EI.KC: THO-M AtiNKJ'ISM 



magnetic, unless the centre C of the vohaic circuit is situated on 

 the same side as the bar, or that this latter is placed at the interior 

 of the circuit. The following experiment may sene to confirm 

 this opinion. 



A wooden box, .4 5 CD (PI. I. fig. 3), 3 feet in length, and ] ^ 

 foot broad, was surrounded with six convolutions of brass wire. 

 The ends of this wire being placed in contact with a single 

 voltaic pair, a small bar of soft iron, placed in M, was suffi- 

 ciently magnetized to attract a quantity of iron filings. But 

 in placing this same small bar on M, there was no trace of mag- 

 netism even on emplojang the powerful batter}- of 16 pairs. This 

 simple experiment appears to me to be sufficiently imjiortant to 

 render the facts intelligible. We might thus lay down the general 

 rule ; that if we considered any body magnetized by induction, 

 there will be a development of magnetism only at the surfaces, whose 

 rays of cm^ature traverse the body, or bisect it according to its 

 thiclcness. If the rays are obliged to be prolonged in order to bisect 

 the body, there will be no magnetism evident. Moreover, the 

 currents must always be so directed, that the body shall be situated 

 in the interior, and surrounded by the circuit. When a plate of 

 soft iron, or of sheet-u'on, is bent in any way whatsoever, and is 

 exposed to the influence of electric cuiTcnts, the curve of inflec- 

 tion yi and B (PL I. fig. 4) are at the same time the Umits of the 

 developed magnetism. The iron filings will attach themselves only 

 to the extremities, or convex surfaces, as indicated by the figure. 

 I did not consider it necessaiy to make tliis experiment, for there 

 can be no doubt of its being the case. This manner of consi- 

 dering these relations also explains why soft iron employed to 

 complete a voltaic circuit, acts like all other metals, and is not 

 magnetized transversally or accorcUng to the axis of the cuiTents, 

 A plate of sheet-iron F (PI. I . fig. 5 ), of half a line in thickness, and 

 cui-v^ed as shown in the figure, was soldered to a voltaic pair of 

 3^ inches in length; on plunging it into acidulated water, a 

 testing needle was slightly aflected, but not more than by a cur- 

 rent of the same intensity which had been passed through a 

 conjunctive wii-e of copper. 



36. 



M. PaiTot seems to be of opinion, that an interior helix not 

 only adds nothing to the development of magnetism, but that it 

 even exercises some weakening action, whatever may be the di- 

 rection of the current. In truth, this would be verv remarkable ! 



