1821.] Historical Sketch of Electro-magnetism. 289 



very considerable, and M. Moll draws the conclusion, that the 

 apparatus with cells produces intense chemical effects while its 

 mao-netic action is very little; while the single plates have 

 scarcely any chemical effect, though their magnetic power be very 

 great. 



M. Moll also remarks, that the positive and negative state of 

 his single plates is in opposition to the states of a battery of 

 many plates, inasmuch as the zinc pole is negative, and the 

 copper pole positive. He found the positions of the magnetic 

 needle near his wire the reverse of what it was near the wire of 

 a battery of many plates ; for when the wire extended from the 

 zinc pole, north, to the copper pole, south, a needle placed beneath 

 turned towards the west instead of the east, and above in the 

 contrary direction. The cause of this difference will be readily 

 seen, when it is remembered that M. Moll was using the wire that 

 connected the zinc and copper plates of the same pair, and not 

 of different pairs ; so that, in fact, his connecting wire was in the 

 inverse direction of those used to connect between the poles of a 

 battery with four or more plates. Hence the deduction, that the 

 plates in the single pair battery are in an opposite state to those 

 of the battery of many plates, is probably premature. 



In endeavouring to increase the size of the plates, and yet 

 preserve the instrument in a portable form, the plates were, at 

 the suggestion of Lieut.-Col. Offerhaus, bent into a spiral form 

 one within the other. This arrangement has been adopted some 

 time since by Dr. Hare, of Philadelphia. The effects with this 

 instrument were very powerful, and similar to those described 

 above. 



Though possessing such powerful apparatus, M. Moll could 

 not succeed in magnetizing a needle, except by making the con- 

 necting wire form more or less of a spiral around it ; and con- 

 cludes from his experiments, that it is absolutely necessary that 

 such spiral should exist to give magnetism. There can, however, 

 be no doubt, particularly from the experiments of Sir H. Davy, 

 that such spiral is not necessary, since a straight discharge 

 transverse to a needle magnetized it, though many inches off. 

 Neither does M. Moll admit of the magnetization of needles on 

 the outside of spirals. 



Some experiments made by M. Pictet, April 7, at Florence, 

 confirmed the results obtained by M. Oersted. 



M. Poggendorf's galvano-magnetie condenser is simply a 

 helix placed perpendicularly, and having the ends of the wire 

 forming it attached to a zinc and a copper plate, which are 

 placed in dilute acid. When an unmagnetized needle is raised 

 on a pivot into this helix so as to be perpendicular to its axis, 

 it is said soon to become magnetic whatever may be the nature 

 of the helix, or however connected with the plates, and to point 

 north and south. It is to be observed, that the needle is not in 

 this case, as in all the previous experiments, in, or parallel to, the 



New Series } vol. ii. u 



