ELECTRO-MAGNETISM. 



Wished another great division of the 

 science ; and the law on which it is 

 founded is no less prolific in its con- 

 sequences, and important in its applica- 

 tions, than that we have hitherto heen 

 occupied in investigating. To this 

 branch of the subject Ampere has given 

 the name of ELECTRO-DYNAMICS. 



(170.) When two conducting wires 

 are suspended or supported in such a 

 manner as to be capable of moving-, 

 either towards or from one another, at 

 the time that electric currents are pass- 

 ing through them, they manifest a mu- 

 tual attraction or repulsion, according 

 as the currents are moving in the same 

 or in opposite directions in the two 

 wires. This action is variously modi- 

 fied, when the relative inclinations and 

 positions of the currents are varied. 



(171.) AVe shall begin by considering 

 the simplest case, which is that in which 

 the two currents are running in parallel 

 directions. The attraction or repulsion 

 of currents, under these circumstances, 

 admit of being exemplified in a great 

 number of ways, according to different 

 modes in which the conducting wires 

 are suspended and rendered moveable. 

 Thus the wires in the apparatus de- 

 scribed 143 and 144, figures 92 and 

 93, may be employed for that purpose, 

 by bringing either the vertical or the 

 horizontal branches sufficiently near a 

 straight wire, through which an electric 

 current is also passing. 



The following is also an apparatus 

 for the direct exhibition of this pheno- 

 menon. T,/g-. 96, represents a rectan- 

 gular table from which arise four up- 

 right pillars supporting two cross pieces 

 of wood, having a row of holes for re- 

 ceiving four cups, two on each piece, the 



Fig. 96. 



distances of which from each other may 

 by means of these holes be varied at 

 pleasure. Short wires, a, a, a, a, proceed 

 horizontally from the bottoms of the 



cups, and serve as pivots round which 

 the two wires, W, w, bent twice at 

 right angles, are made to turn at the 

 upper part of their vertical branches, 

 having small holes drilled through them 

 for that purpose. These wires, thus 

 hung freely upon their pivots, carry on 

 their upper ends small weights, which, 

 bringing the centres of gravity as nearly 

 as possible in coincidence with the points 

 of suspension, enable them to be moved 

 by a very slight force. Conducting 

 wires, proceeding from a voltaic battery, 

 arc then inserted into the cups pre- 

 viously filled with mercury, in such a 

 manner that the galvanic current shall 

 pass in the same direction through both 

 the parallel wires; the moment this is 

 done, the wires move towards each 

 other, even from a distance of several 

 inches, exhibiting a powerful mutual 

 attraction. When the currents are trans- 

 mitted in directions opposite to each 

 other in the two wires, which they may 

 be made to do by transposing the com- 

 municating wires inserted into the cups 

 leading to one of the moveable wires, 

 while the others are left as before, the 

 moveable wires immediately recede from 

 each other, manifesting a repulsion as 

 powerful as the attraction was in the 

 former case. 



(172.) The electro-magnetic forces ob- 

 tained from voltaic batteries of the ordi- 

 nary strength are so feeble when com- 

 pared with the force of gravity, that, in 

 devising experiments for exhibiting their 

 action, it becomes necessary, in order to 

 succeed, so to contrive the apparatus as 

 that the parts to be moved by these 

 forces may be as light as possible, and 

 be also suspended in such a way as to 

 occasion ihe smallest amount of friction. 

 Attention should be given not to en- 

 cumber the conductors, or the magnetic 

 bars which are to be set in motion, with 

 any superfluous materials capable of 

 adding to their weight ; and we should 

 avoid such dispositions as require them 

 to move in opposition to their own gra- 

 vity. The surfaces which are intended 

 to move through mercury should be as 

 much as possible reduced ; not only on 

 account of the friction which takes place 

 between the solid and the fluid, but also 

 because the surface of the mercury ra- 

 pidly oxidates, and the film of oxide thus 

 formed opposes considerable resistance 

 to the motion of a solid body, and greatly 

 impedes the mechanical action of the 

 apparatus. In general, it will be found i 

 that a vertical suspension by a point is j 



