NATURE 739 



from the first conductor to the second, but they did not know it 

 returned from the second to the first. All currents of this kind were 

 therefore considered by Ampere to be open currents for instance, 

 the currents of discharge of a condenser; he was unable to experi- 

 ment on them, their duration being too short. Another kind of open 

 current may be imagined. Suppose we have two conductors A and 

 B connected by a wire AMB. Small conducting masses in motion 

 are first of all placed in contact with the conductor B, receive an elec- 

 tric charge, and leaving B are set in motion along a path BNA, carry- 

 ing their charge with them. On coming into contact with A they 

 lose their charge, which then returns to B along the wire AMB. Now 

 here we have, in a sense, a closed circuit, since the electricity describes 

 the closed circuit BNAMB ; but the two parts of the current are quite 

 different. In the wire AMB the electricity is displaced through a 

 fixed conductor like a voltaic current, overcoming an ohmic resistance 

 and developing heat; we say that it is displaced by conduction. In 

 the part BJSTA the electricity is carried by a moving conductor, and 

 is said to be displaced by convection. If therefore the convection cur- 

 rent is considered to be perfectly analogous to the conduction current, 

 the circuit BNAMB is closed; if on the contrary the convection cur- 

 rent is not a " true current," and, for instance, does not act on the 

 magnet, there is only the conduction current AMB, which is open. 

 For example, if we connect by a wire the poles of a Holtz machine, 

 the charged rotating disc transfers the electricity by convection from 

 one pole to the other, and it returns to the first pole by conduction 

 through the wire. But currents of this kind are very difficult to pro- 

 duce with appreciable intensity; in fact, with the means at Ampere's 

 disposal we may almost say it was impossible. 



To sum up, Ampere could conceive of the existence of two kinds of 

 open currents, but he could experiment on neither, because they were 

 not strong enough, or because their duration was too short. Experi- 

 ment therefore could only show him the action of a closed current 

 on a closed current or more accurately, the action of a closed cur- 

 rent on a portion of current, because a current can be made to de- 

 scribe a closed circuit, of which part may be in motion and the other 

 part fixed. The displacements of the moving part may be studied 

 under the action of another closed current. On the other hand, Am- 

 pere had no means of studying the action of an open current either 

 on a closed or on another open current. 



1. The Case of Closed Currents. In the case of the mutual action 

 of two closed currents, experiment revealed to Ampere remarkably 

 simple laws. The following will be useful to us in the sequel : 



(1) // the intensity of the currents is kept constant, and if the 

 two circuits, after having undergone any displacements and deforma- 

 tions whatever, return finally to their initial positions, the total work 



