6 On the Action of the Magnet upon the Electric Current. 



the ellipse mentioned, or, what comes to the same thing, which 

 are not both in the equatorial plane, in general no current is 

 formed. Only in the exceptional case that both points belong 

 to the same magnetic curve, and the arc of this curve between 

 the two pointsfalls entirely within the ellipsoid, does the first 

 case of Law I. immediately occur. 



84. A second simple example is offered in the case of a wide 

 cylindrical tube, which is laid horizontally upon the armatures, 

 and inclined at an angle of 45° to the equatorial plane. Then 

 the middle of the epipolar-magnetic curve consists of two distinct 

 spirals, which, taken separately, are brightly illuminated by 

 streams in opposite directions. 



85. The above laws may be also very beautifully illustrated 

 by means of induced currents, although not with the same 

 splendour as in the case of the direct discharge. I took for 

 this purpose (to adduce here only one instance) a simple glass 

 bulb, without electrodes, of about 80 millimetres diameter, which, 

 after exhaustion, contained only traces of a gas. If the outer 

 surface of this sphere be touched in any point with one of the 

 electi'odes of Rubmkorff's induction apparatus, a diffused light, 

 the colour of which is peculiar to the gas, spreads throughout 

 the whole sphere, from a point of the internal glass surface 

 corresponding to the point touched on the outside. If the sphere 

 be laid in the middle upon the approximated armatures of the 

 electro-magnet, this light collects to a single arc of light, corre- 

 sponding to that magnetic curve which passes through the fixed 

 point. A second electric current traverses the same path in an 

 opposite direction. If the electrode in contact with the sphere 

 be the negative one, a sti'ong positive stream proceeds from the 

 point of contact ; a weaker current returns by the same path to 

 the same point. If the touching electrode be the positive one, 

 the two currents mutually exchange their intensities ; the positive 

 stream which passes to the point of contact being the stronger. 

 If the sphere be touched simultaneously in any two points with 

 both electrodes, the two arcs of light corresponding to the two 

 electrodes appear simultaneously, independently, and apparently 

 without interfering with one another. It is only when the two 

 electrodes touch the sphere in two points of the same magnetic 

 curve, that the two luminous arcs unite to form one. 



86. If we lead the terminal point of an electrode along the 

 surface of the sphere, the luminous arc undergoes a continual 

 alteration of form and position. The length of the arc dimi- 

 nishes on approaching the epipolar-magnetic curve. From a 

 point of this curve no magnetic curve passes into the interior of 

 the sphere. Even on touching such a point with the electrode, 

 the light remains on the internal surface of the sphere, and is 



