320 ACTION OF ELECTRIC CURRENTS. SECT. XXXH. 



current is flowing in a contrary direction, exhibits north- 

 ern polarity. 



The phenomena mark a very decided difference be- 

 tween the action of electricity in motion or at rest, that 

 is, between Voltaic and common electricity ; the laws 

 they follow are in many respects of an entirely different- 

 nature, though the electricities themselves are identical. 

 Since Voltaic electricity flows perpetually, it cannot be 

 accumulated, and consequently has no tension, or ten- 

 dency to escape from the wires which conduct it. Nor 

 do these wires either attract or repel light bodies -in 

 their vicinity, whereas ordinary electricity can be accu- 

 mulated in insulated bodies to a great degree, and in 

 that state of rest the tendency to escape is proportional 

 to the quantity accumulated and the resistance it meets 

 with. In ordinary electricity, the law of action is that 

 dissimilar electricities attract, and similar electricities 

 repel one another. , In Voltaic electricity, on the con- 

 ^trary, similar currents, or such as are moving in the 

 same direction, attract one another, while a mutual re- 

 pulsion is exerted between dissimilar currents, or such 

 as flow in opposite directions. Common electricity 

 escapes when the pressure of the atmosphere is re- 

 moved, but the electro-dynamical effects are the same 

 whether the conductors be in air or in vacuo. 



The effects produced by a current of electricity de- 

 pend upon the celerity of its motion through a conduct- 

 ing wire. Yet we are ignorant whether the motion be 

 uniform or varied, but the method of transmission has a 

 marked influence on the results ; for when it flows with- 

 out intermission, it occasions a deviation in the magnetic 

 needle, but it has no effect whatever when its motion is 

 discontinuous or interrupted, like the current produced 

 by the common electrical machine when a communica- 

 tion is made between the positive and negative con- 

 ductors. 



M. Ampere has established a theoiy of electro-mag- 

 netism suggested by the analogy between electro-dy- 

 namic cylinders and magnets, founded upon the recip- 

 rocal attraction of electric currents, to which all the phe- 

 nomena of magnetism and electro-magnetism may be 

 reduced, by assuming that the magnetic properties 



