ELECTRO-MAGNETISM. 



fo Ibis direction ; that is, perpendicular 

 to the position of the dipping needle. 

 (133.) The following: was the appa- 

 ratus, by which Ampere succeeded in 

 exhibiting the effect now described. A 



Fig. S3. 



wire bent in the shape of a rectangle 

 R R, fig. 85, is supported by a tube of 

 wood, T T, passing directly across the 

 middle of its longest sides, and serving 

 as an axis. The two shorter sides or 

 ends of the rectangle are supported by a 

 lijjht wooden beam, B, in the form of a 

 lozenge, the middle of which is perfo- 

 rated by the tube just mentioned. One 

 end of the wire, W, which forms the 

 rectangle, is fixed to a steel pivot, which 

 turns horizontally on a small metallic 

 plate fixed upon the top of an upright 

 metallic pillar, rising from the side of 

 the basis of the apparatus. A little 

 mercury is laid upon the plate in order 

 to render the contact more perfect. The 

 wire, after it has completed a circuit of 

 the rectangle, and returned to the same 

 point W, where it had commenced it, is 

 bent so as to pass through the tube, and 

 to come out at the other end, w, where 

 it terminates in another steel point, 

 turning in a like manner upon a me- 

 tallic plate, fixed on the top of a pillar 

 on the other side of the apparatus. The 

 lower ends of both pillars, where they 

 are fixed to the stand, are continuous 

 with wires supporting cups with mercury, 

 P and N, in the usual manner. On esta- 

 blishing a communication between the 

 Voltaic battery and the cups, the electric 

 current will ascend in the pillar which 

 is next to the positive pole of the bat- 

 tery, and circulating along the rec- 

 tangular wire, will pass out by its other 

 extremity, descend by the other pillar, 



and make its exit through the cup on 

 that side. As the rectangle is at perfect 

 liberty to move around the axis formed 

 by the two points by which it rests on 

 the plates, and this axis being hori- 

 zontal, it will be limited to a vertical 

 motion. If the axis of motion be placed 

 so as to be at right angles to the mag- 

 netic meridian, and the moveable part 

 of the apparatus be exactly balanced, 

 so as to retain any position in which it 

 may be placed, then on directing the 

 electric current through the wires, the 

 rectangle will, after a few oscillations, 

 place itself steadily in the plane of the 

 magnetic equator; that is, in a plane 

 perpendicular to the line of the dip; 

 being the exact position which the 

 theory would assign to it. On reversing 

 the direction of the current, the mag- 

 netic polarity of the wire becomes im- 

 mediately reversed, and turns completely 

 round, so as still to place itself in the 

 same plane as before, but with its faces 

 turned in opposite directions to those 

 they before assumed. 



(134.) It is evident that, by adopting 

 a similar mode of suspension, a voltaic 

 magnet, formed by a heliacal coil of wire, 

 as described in 105, would exhibit the 

 phenomena of the dipping needle, as 

 completely as a magnetized needle. 



(135.) Thus has the analogy between 

 the action of terrestrial magnetism on 

 wires conducting an electric current, and 

 magnetized needles, been completely 

 established. We have next to inquire 

 whether a straight wire is affected by 

 the earth in the same manner as it 

 would be by the corresponding pole of 

 a magnet placed near it. In order to 

 make this comparison, we must first 

 clearly deduce from the theory formerly 

 laid down, what effects are to be ex- 

 pected on a straight conducting wire 

 from the magnetism of the earth, or 

 what is equivalent to it, from a south 

 magnetic pole, acting at an indefinite 

 distance, in the direction of the line of 

 the dip. The electro-magnetic force 

 being tangential, is exerted at right 

 angles to this direction, which is that of 

 the line connecting the wire with the 

 magnetic pole, or origin of the force. 

 Its action upon a current, whether as- 

 cending or descending, which moves in 

 this exact line, or the line of the dip, is 

 reduced to nothing : and it must act 

 with greatest intensity upon a current 

 which moves in a direction perpen- 

 dicular to the line of the dip. Now, 

 in order that a straight wire may be 



