ELECTRO-MAGNETISM. 



preferable to suspension by a horizontal 

 axis, as the former occasions less fric- 

 tion. Thus Mr. Watkms finds, that the 

 attractions and repulsions of wires trans- 

 mitting voltaic currents are exhibited 

 with more facility when they are sus- 

 pended vertically, their two ends termi- 

 nating respectively in upper and lower 

 cisterns, than when turning horizontally, 

 as in the apparatus exhibited in the pre- 

 ceding figure. 



These attractions and repulsions 

 may also be exhibited with a very fee- 

 ble current of electricity, by means 

 of the gold-leaf galvanoscore, already 

 described, 1-27. By removing the 

 magnet, and inserting within the instru- 

 ment a thick wire, inclosed within a 

 glass tube, parallel and near to the gold 

 leaf, a strong current may be passed 

 through the wire, at the same time that 

 a feeble current is transmitted through 

 the gold leaf, which will then exhibit the 

 attractions or repulsions of parallel cur- 

 rents, according as they are in the same 

 or in opposite directions; for these 

 actions take place equally whether the 

 two currents are obtained from sepa- 

 rate voltaic combinations, or whether 

 they are merely two portions of the 

 same current in different parts of its 

 course. 



(173.) This latter case occurs when- 

 ever a wire is coiled round in a spiral 

 or heliacal form, so as to bring different 

 portions of the same current, passing in 

 the same direction, very near to one 

 another. It occurred to the author of 

 this treatise, soon after hearing of Am- 

 pere's discovery of the attraction of 

 electrical currents, that it might be 

 possible to render the attraction between 

 the successive and parallel turns of a 

 heliacal coil very sensible, if the wires 

 were sufficiently flexible and elastic; 

 and, with the assistance of Mr. Faraday, 

 this conjecture was put to the test of 

 experiment, in the laboratory of the 

 Royal Institution. A slender harpsi- 

 chord-wire bent into a helix, being 

 placed in the voltaic circuit, instantly 

 shortened itself whenever the electric 

 stream was sent through it ; but re- 

 covered its former dimensions the mo- 

 ment the current was intermitted. It 

 was supposed that possibly some analogy 

 may hereafter be found to exist between 

 this phenomenon and the contraction of 

 muscular fibres, which seems to be 

 regulated by some properties of the 

 nervous system, not unlike those of 

 electric agency. Messrs. Prevost and 



Dumas have advanced a similar theory 

 of muscular contraction, founded on a 

 supposed distribution of nervous fila- 

 ments, through which they imagine a 

 current of electricity is sent, for the 

 purpose of determining the action that 

 precedes contraction. This theory, they 

 conceive, is supported by microscopic 

 observations ; but it is far too hypo- 

 thetical in its present form to deserve 

 serious discussion. 



(174.) The general fact of the mutual 

 action of electric currents being esta- 

 blished by these and similar experi- 

 ments, we must proceed to consider the 

 different modifications it receives by a 

 variation of circumstances regarding the 

 quality and direction of the currents. 



(175.) We possess as yet but an im- 

 perfect knowledge of the peculiar affec- 

 tions which electricity experiences when 

 in motion, and which enable it to exert 

 the singular species of action we are 

 here investigating, so different from its 

 attractive and repulsive powers when at 

 rest. These two classes of effects obey 

 laws not only different, but in some re- 

 spects of an entirely opposite nature. 

 Accumulated electricity, when not in 

 motion, acts in a degree proportioned 

 to its tension ; but the wires, which are 

 silently conducting a current of elec- 

 tricity in motion, exhibit no sign what- 

 ever of electric tension ; they produce 

 no change in the electrometer, and 

 neither attract nor repel light, bodies in 

 their vicinity. The law of action in the 

 state of rest is, that dissimilar electrici- 

 ties attract, and similar electricities re- 

 pel one another. When in motion, on 

 the contrary, it is between similar cur- 

 rents, that is, currents moving in a 

 similar manner, that attraction takes 

 place; while a mutual repulsion is ex- 

 erted between dissimilar currents. The 

 electro-statical effects of electric tension 

 cease when the atmospheric pressure 

 is removed ; but the electro-dynamical 

 effects of currents take place equally 

 whether the conductor be surrounded by 

 the air or placed in vacuo. 



(176.) Since the effects of electric 

 currents are the consequences of the 

 motion of the electricity, it is natural to 

 suppose that they will be in proportion to 

 the velocity with which it moves, as well 

 as to the quantity that is set in motion. 

 But we are in utter ignorance of the 

 real velocity with which the electrical 

 effects are propagated along a conduct- 

 ing body, during the completion of the 

 voltaic circuit: nor do we even know 



