upon the Electric Current. 3 



of the direction of the current, the stream in the same portion 

 lying betweer. the two given points is, in the one case, depressed 

 towards the sui'face, in the other drawn away from it. It is 

 only in the first case that a proper current can exist. In the 

 second case, as well as generally, where the terminations of the 

 stream are not points of the curve described, the magnet pre- 

 vents the formation of a current*. 



80. In the theoretical consideration of Law I. we encounter 

 three cases, which are also again met with in experiment. 



(1) The electric discharge takes place between two given fixed 

 points. This is the case of Davy's arc between carbon-points, 

 which at first are in contact, and are then gradually separated 

 from one another and held at a fixed distance apart. If no dis- 

 turbing influence is at work, the path of the electric discharge 

 in air, as in a rarefied atmosphere, is the straight line connecting 

 the two carbon-points, or the metallic points which may replace 

 them. The glowing particles which are carried over and which 

 may be recognized in the specti'um, are to be considered, in part 

 at least, as conductors of the current. In long tubes, in which 

 the gas is in a condition of maximum rarefaction, no such par- 

 ticles are transported when Ruhmkorfi"'s apparatus is discharged 

 through it : in such case the rarefied gas alone is the bearer of 

 the current ; for the spectra obtained characterize each gas 



* In order to illustrate as graphically as possible that electric light, first 

 observed by me, which collects in magnetic curves, I imagined the ex- 

 istence of perfectly flexible, infinitely fine magnetic threads (47 to 49). It 

 is to be noted that such a thread, when rigidly held in one of its points, 

 will remain in equilibrium under the influence of the magnet when it 

 assumes the form of the magnetic curve passing through that point; 

 just as is the case also with a linear, perfectly flexible, electric conduc- 

 tor. In the first case we immediately recognize the force which gives 

 form and position to the magnetic thread of arbitrary form and posi- 

 tion, when equilibrium is established. In the second case, however, 

 the circumstances are otherwise. If, for instance, we imagine moveable 

 rectilineal conductors proceeding radially in all directions from a given 

 point, and which are subjected to the action of a given magnetic pole, we 

 may suppose all these conductors to be distributed upon conic surfaces 

 whose common axis is the straight line connecting the given point with 

 the given ])ole. All such conic surfaces rotate uniformly around their 

 common axis. If we take the case of an arbitrary magnetic action and 

 supj30se the conductors to be infinitely small, all the conductors rotate in 

 conic surfaces whose common axis is the tangent of the corresponding 

 magnetic curve at the given point. That conductor alone which follows 

 the course of this curve, remains at rest without the other conductors 

 being forced into this position. The difficulty of the question consists in 

 our being obliged to consider the current not as already formed, but in the 

 course of formation ; and although we have no definite notion as to the 

 formation of a current in general, we know at least so much, that under 

 the magnetic influence it can only shape itself according to the magnetic 

 curve. 



B2 



