ELECTRO-MAGNETISM. 



south, and which, for the sake of illus- 

 tration, we may conceive to be insulated, 

 will be urged by their united influence 

 to move parallel to itself, in the direction 

 denoted by the arrows a, a, in the figures ; 

 that is, from right to left, if the north 

 pole be behind, and the south pole 

 before, as in jig. 52 ; and from left to 

 right, if the poles are in a contrary posi- 

 tion, as \nfig. 53. 



(83.) Several amusing experiments 

 have been contrived, in which vibratory 

 or rotatory motions of different kinds 

 are obtained by various applications of 

 this principle. 



The following is the invention of Mr. 

 Marsh. A conducting platina wire W, 

 fig. 54, is suspended by a loop from a 



Fig. 54. 



metallic hook at the lower end of ano- 

 ther wire, which is fixed to the end of 

 the arm of a stand ; and which supports 

 above the small cup P, to contain mer- 

 cury. The lower end of the platina 

 wire, which thus hangs freely, dips into 

 a small cistern of mercury, Q, formed 

 out of the wooden base, and is just mid- 

 way between the two poles of a horse- 

 shoe magnet, M, laid flat upon the same 

 base. 



The mercury in the trough is placed 

 in electrical continuity with another cup, 

 N, by means of a wire passing out from 

 the side, and supporting the cup. On 

 making a communication with the two 

 ends of the voltaic battery by means 

 of these cups, the current passing along 

 the loose platina wire, being influ- 

 enced by the magnet, urges the wire 

 either forwards towards Q, or back- 

 wards towards M, according to the 



position of the poles, and the direction 

 of the current. In either case it is 

 thrown out of the mercury; and the 

 circuit being thus broken, the effect 

 ceases, until the wire falls back again 

 by its own weight into the mercury; 

 when the current being re-established, 

 the same influence is again exerted, 

 the phenomenon is repeated, and the 

 wire exhibits a quick succession of vi- 

 bratory motions. 



(84.) This reciprocating movement of 

 the wire may be converted into one of 

 rotation, by adapting, as proposed by 

 Mr. Barlow, a spur-wheel, as shewn in 

 fig. 55, to the lower part of the upright 

 wire, which must then be firmly fixed 

 to the arm of the pillar. The wheel, 

 being constructed so as to turn round 

 freely, will revolve with great rapidity 

 as soon as the contacts are made with the 

 battery : for this purpose, however, the 

 wheel must dip so far into the mercury, 

 as that each of the rays shall touch the 

 surface before the preceding ray has 

 quitted the mercury. The direction of 

 the motion depends, of course, on the 

 same circumstances as were before men- 

 tioned : Mr. Barlow observes, however, 

 that in general the experiment succeeds 

 best when the wheel revolves inwards. 



Fig. 55. 



(85.) But it is not necessary to divide 

 the wheel into rays in order to produce 

 the effect above described ; for a cir- 

 cular metallic disc substituted for the 

 spur-wheel will revolve equally well, 

 when it is traversed by an electrical 

 current passing into mercury between 

 the poles of a horse-shoe magnet. For 



