ELECTRO-MAGNETISM. 



54 



steel wire or needles ; and the effect 

 was communicated to a distance of five 

 inches above or below laterally from the 

 wire, through water, or thick plates of 

 glass or metal electrically insulated. 



(153.) The efficacy of electro-mag- 

 netic induction is, as might be expected, 

 greatly increased by employing a helia- 

 cal coil of wire, and placing the needle 

 or bar to be magnetized in the axis of 

 the helix, in the situation represented in 

 fig.M. Mr. Arago first employed this 

 method, and was enabled to produce the 

 maximum effect on the needle almost in- 

 stantaneously. It is not necessary, how- 

 ever, that the bar to be magnetized should 

 be exactly in the axis of the helix, as it 

 may lie in any situation within it, or be 

 inclosed in a tube of glass, or of any 

 other material which is not a good con- 

 ductor of electricity. Such a tube will 

 also be convenient as a support for the 

 coils of the wire, as well as for admitting 

 of the introduction of different needles in 

 succession. The needle should not be 

 allowed to remain beyond a moment in 

 the tube, for the magnetizing effects of 

 the helix are produced nearly instanta- 

 neously ; and it sometimes happens that 

 if the needle be left there a few minutes, 

 the polarity it had at first acquired be- 

 comes impaired, or contused, and even 

 occasionally destroyed. 



(154.) If a long steel wire be placed 

 in the axis of a helix, the direction of 

 the turns of which change at different 

 points, the wire will be found to have a 

 number of consecutive points, corre- 

 sponding to those at which these changes 

 take place. 



(155.) Mr. Watkins observes that the 

 needle to be magnetized, if it be not 

 very hard, need not have its whole 

 length inserted into the glass tube ; for 

 if held in the hand so that only half of 

 it is within the helix, it will become 

 magnetic equally with one that has 

 been wholly acted upon; because the 

 portion of the needle that has received 

 the magnetism communicates it to the 

 other portion. When a small part of a 

 needle, very highly tempered, is intro- 

 duced into the glass tube, the induced 

 magnetism will be found to extend to 

 about twice the length of the part so 

 introduced. 



(156.) A very powerful temporary 

 magnet may be obtained by bending a 

 thick cylinder of soft iron into the form 

 of a horse-shoe, and surrounding it with 

 a coil of thick copper wire, secured from 

 communication among its several parts 



by a covering of silk, or other non-con- 

 ducting material. When the wire is 

 made, ptart of the galvanic circuit of a 

 battery, even of moderate power, the 

 iron is rendered powerfully magnetic, 

 and will lift up a very heavy weight by 

 means of a piece of iron applied to its 

 poles, which act precisely like those of 

 a horse-shoe magnet. Fig. 95 exhibits 

 an arrangement of this kind ; W w being 

 the two ends of the wire, coiled round 

 the iron to be magnetized, and bent so 

 as to dip into the cups P and N, for 

 forming connexions with a battery. 



Fig. 95. 



(157.) This experiment has been 

 made upon a very large scale by Pro- 

 fessor Moll with an apparatus con- 

 structed by Mr. Watkins*. It con- 

 sisted of a cylinder of soft English 

 iron, an inch in diameter, bent into 

 the form of a horse- shoe, the interval 

 between the ends being eight inches 

 and a half. The copper wire forming 

 the spiral was one-eighth of an inch in 

 diameter, and made eighty-three convo- 

 lutions; the weight of the whole was 

 five pounds. A connecting piece of iron 

 was placed in contact with the two ex- 

 tremities of the horse-shoe ; and the 

 ends of the spiral wire dipped in mer- 

 cury, so as to form a voltaic circuit with 

 a simple battery, consisting of a zinc 

 plate, which exposed a surface of eleven 

 square feet to a very diluted mixture of 

 sulphuric and nitric acids, in a copper 

 cell. In the first experiment the ap- 

 paratus sustained, first, fifty pounds, 

 and afterwards, with care, seventy-six 

 pounds, by the magnetism induced upon 

 it. 



(158.) When the weight suspended to 



* Bibliotheque Universelle, 1830, p. 19. 



