50 



ELECTRO-MAGNETISM. 



south ; the arrow-heads at the end of the 

 lines shew the direction of the current 

 in the wire ; and the short arrows pro- 

 ceeding laterally from the middle, the 

 direction of the motion induced in the 

 wire. 



(139.) It is evident that, in all these 

 cases, the wire is moving in obedience 

 to the same law, which produces the 

 revolution of a wire round a magnetic 

 pole in Mr. Faraday's first experiment 

 on magnetic rotations, already described 

 ($ 59). It is a direct and neces- 

 sary inference from this law, that 

 were the two troughs of mercury 

 continued to ever so great a length, 

 and even were they carried round 

 the globe in a circle round the acting 

 magnetic pole of the earth, the wire 

 would continue to move along them, 

 and after describing the whole circle, 

 and returning from the point at which 

 it had set out, would resume its course, 

 and perform perpetual revolutions. In 

 the very limited space compatible with 

 actual experiment, the wire appears to 

 move in a plane ; but theory shews that 

 it is in reality a small portion of a cy- 

 linder, of which the radius is the dis- 

 tance of the magnetic pole of the earth 

 from the wire. It is amusing to compare 

 this incipient revolution of the wire with 

 the complete rotations effected in expe- 

 riments with artificial magnets ; and, 

 considering it as part of a similar expe- 

 riment upon a much vaster scale, to 

 view the wire as setting out on its 

 voyage of circumnavigation of the globe, 

 although it is in the next moment ar- 

 rested in its progress. 



(140.) It is also a consequence dedu- 

 cible from the same law, that the force 

 by which the horizontal conducting wire 

 is urged is the same in all azimuths. 



(141.) A real rotation, visible in all 

 its course, may however be exhibited, 

 as the effect of terrestrial magnetism. 

 This has been accomplished also by Mr. 

 Faraday, who, reflecting that in the ex- 

 periment of rotation round the pole of a 

 magnet, the pole is perpendicular to but 

 a small portion of the wire, and more or 

 less oblique to the rest, thought it pro- 

 bable that a wire, very delicately hung 

 and connected, might be made to rotate 

 round the line of dip by the earth's 

 magnetism alone ; the upper part being 

 restrained to a point, in the line of the 

 dip, and the lower being made to move 

 in a circle surrounding it. With this 



view, a piece of copper wire, about 0.018 

 of an inch in diameter, and six inches 

 long, well amalgamated all over, was 

 hung by a loop to another piece of the 

 same wire, at W, see fig. 90, so as to al- 

 low of very free motion ; and its lower end 

 w, was thrust through a small piece of 



90. 





cork, in order to render it buoyant when 

 placed on mercury. A glass basin, ten 

 inches in diameter, was tilled with pure 

 clean mercury, and a little dilute acid 

 poured on its surface. The thick wire 

 which communicated with one of the poles 

 of the voltaic battery was then hung 

 over the centre of the glass basin, and 

 depressed so low, that the thin moveable 

 wire, having its lower end resting on the 

 surface of the mercury, made an angle 

 of about 40 degrees with the horizon. 

 On the circuit through the mercury 

 being completed, the wire immediately 

 began to move and rotate, and continued, 

 whilst the connexions were preserved, 

 to describe a cone, which though its axis 

 was perpendicular, had, evidently, from 

 the varying rapidity of its motion, re- 

 lation to a line W D, parallel to the 

 dipping-needle, as being that of the force 

 by which it was actuated. The direction 

 of the motion was, of course, the same 

 as that communicated in the expe- 

 riment described $ 64, when a south 

 pole is placed beneath the apparatus. 

 If the centre from which the wire hung 

 was elevated, until the inclination of 

 the wire was equal to that of the dip, 

 no motion took place when the wire was 

 parallel to the dip ; and if the wire was 

 less inclined than the dip, the motion in 

 one part of the circle capable of being 

 described by the lower end was reversed ; 

 results that necessarily follow from the 



