286 Jlistorical Sketch of Electro-magnetism. [Oct. 



compared with the positions deduced from M. Oersted's experi- 

 ments, fig. 1, 2, and 3, it will be found that in two of the cases, 

 those pointed out by M. Von Buch, it was necessary a half revo- 

 lution of the needle should take place to bring it into a state of 

 equilibrium with the wire in those positions. 



M. Von Buch, also, appears to have ascertained the effect of 

 common electricity in producing magnetism without a- previous 

 knowledge of what had been done by others in that way, and 

 succeeded in producing the effect by a smaller power than had 

 before been used for that purpose. He found that a strong 

 discharge was not necessary, nor even a Leyden phial ; but, fixing 

 a helix between the prime conductor of a machine and another 

 insulated conductor, placing a steel needle in it, and then draw- 

 ing sparks from the latter conductor, the needle became magnetic. 

 One single turn of a machine, with two discs 18 inches in dia- 

 meter, was sufficient to make the needle evidently magnetic. 



In Italy, many experiments relating to magnetism by electri- 

 city have been made, and which, though new at the time to 

 those who made them, had been previously made by others. A 

 series was made between Jan. 6 and 18, by MM.Gazzeri, Ridolfi, 

 and Antinori, at Florence. The results, which appear to me to 

 be most interesting, are as follows : Needles placed in helices 

 connected with the poles of the battery received their full magneti- 

 zation in one minute. Needles on the outside of the helices 

 would receive no magnetism, unless there was one or more also 

 within, and then they became magnets with their poles in oppo- 

 site directions to the poles of the inner magnet. The helix was 

 changed into a square form, by having its wire wrapped round a 

 parallelopiped ; the magnetising effect remained the same. A 

 needle and a long wire of platina were wrapped in a sheet of tin 

 foil, and that part which contained the needle introduced into a 

 spiral of copper wire ; the circuit was then made by the platina 

 wire without the copper spiral; being in connexion with either 

 pole, the needle became magnetised. A spiral of copper wire 

 with aneedle in it was placed on the surface of'abasin of mercury, 

 and the mercury then made part of the circuit : the needle 

 became feebly magnetic. Sparks from a common machine taken 

 through a helix containing a steel needle made the needle mag- 

 netic. These philosophers appear to have found that the con- 

 necting wire placed in other parts of the battery than from end 

 to end would not magnetize needles. There is, probably, some 

 mistake in this. 



M. la Borne, in repeating Arago's experiments, Jan. 8, varied 

 the use of the helix by making it of iron, and putting it round 

 the straight wire, through which an electrical discharge was 

 made. The helix in this case became the needle to be magne- 

 tised, and it was found to be a strong magnet, the poles being in 

 the positions so often referred to. Such a magnet is flexible and 

 elastic, and may be doubled, lengthened, or shortened: on 



