ELECTRO-MAGNETISM. 



has been termed by Ampere an electro- 

 dynamic cylinder, with a view to its 

 assimilation with the condition of a 

 magnetic cylinder is this ; that what- 

 ever magnetic action the turns of the 

 heliacal part of the wire may have in a 

 longitudinal direction, (that is parallel 

 to the axis,) is counterbalanced by the 

 contrary action 'of the returning wire. 

 For the direction of each of the heliacal 

 portions of the wire, that of W w, for 

 instance,^. 72, being necessarily some- 

 what oblique, and the magnetic force it 

 exerts being along M m, at right angles 



Fig. 72. 



w 



current through them, ceasing the in- 

 stant that current is arrested, and 

 capable of being suddenly reversed by 

 changing the direction of that current. 



(108.) In order to facilitate the com- 

 parison of the properties of Voltaic 

 magnets with those of ordinary magnets, 

 it will be found convenient to adapt 

 them to the simple floating galvanic 

 apparatus devised by M. de la Rive. 

 Such is the one represented mjfg. 73. 



Fig. 73. 



to that direction, the whole of the force 

 is not exerted in the direction of the 

 axis A X, but only that part of it repre- 

 sented by C/, while another part, C e, 

 is directed at right angles to the axis. 

 But that portion of the straight wire 

 which passes along the axis, and cor- 

 responds in its length to the interval 

 between the two adjoining spiral turns, 

 exerts a force C d, precisely equal, and in 

 an opposite direction to G e. These two 

 forces, therefore, exactly destroy one 

 another; and there remains only the 

 force C/, in the direction of the axis. 



(107.) Experiment has fully con- 

 firmed the accuracy of this theoretical 

 deduction: and the heliacal arrange- 

 ment just described is found to be a 

 tolerably exact representation of what 

 may be conceived to be a simple or 

 elementary magnetical filament ; for it 

 has opposite poles at the two ends, the 

 one being north, the other south. It 

 obeys the action of magnets that may 

 be presented to it, being attracted and 

 repelled, and assuming determinate po- 

 sitions with respect to the poles of the 

 magnet, just as if it were itself a magnet, 

 of which, indeed, it appears to possess 

 all the essential properties, and for 

 which it may be substituted in almost 

 every form of experiment. It is hardly 

 necessary to observe that the polarity of 

 these Voltaic magnets, as we may call 

 them, is entirely of a conditional nature, 

 dependant on the passage of the electric 



Both the ends of the wires are here 

 made to descend through the cork, the 

 one being soldered to the zinc, and the 

 other to the copper plates; and the 

 whole being enclosed in a glass cylinder 

 adapted for floating it in water. 



(109.) A very simple apparatus act- 

 ing on the same principle, is that of 

 Professor Vanden Boss, represented in 

 fig. 74. It consists of a plate of copper 

 about an inch square, and a similar 

 plate of zinc, placed parallel to the for- 

 mer, their contact being prevented by a 

 small piece of cork interposed between 

 them. To the upper part of one of 

 these plates a slender brass wire is at- 



Fig. 74. 



