30 Prof. E. Edlund on the Path of Electrical Induction- and 



rent to produce the strongest disjunction-current need not be 

 ascribed to any special property of it, as it is sufficiently ex- 

 plained by the fact that the formation of sparks commences with 

 this current. When the poles are in an enclosed space, from 

 which the air can be exhausted, the electromotive force of the 

 disjunction diminishes in proportion as the air is rarefied. 

 Finally the electromotive force of induction, which does not de- 

 pend on the density of the layer of air traversed by the spark, 

 begins to be greater than the former, and then the deflection 

 of the magnetic needle mainly depends upon the induction- 

 currents. 



Professor Riess* has shown, by means of the electric valve which 

 he has devised, that when this is inserted in the path of an induc- 

 tion-current, the following relations take place when the density 

 of the air and the position of the valve are altered. When the spark 

 is formed under a pressure of one atmosphere, there is obtained 

 on a galvanometer inserted in the circuit a deflection in the same 

 direction as that which would be obtained with *the second induc- 

 tion-current. Here, as regards the direction of the deflection, 

 it is immaterial whether the second induction-current goes from 

 the disk to the point, ox vice versa. When the current in ques- 

 tion goes from the disk to the point and the air is gradually 

 exhausted from the valve, the deflections of the magnetic needle 

 are always in the same direction, but their magnitude gradually 

 diminishes at first, increasing again on subsequent rarefaction. 

 When, on the contrary, the valve is so applied that the second in- 

 duction-current goes from the point to the disk, the deflection 

 diminishes more rapidly with the rarefaction, and afterwards 

 changes to a deflection towards the opposite side, which increases 

 when the rarefaction is increased. 



These details could not well have been sufficiently ex* 

 plained before the discovery of disjunction-currents; but now 

 the explanation follows spontaneously. The deflection obtained 

 when the valve was full of air did not arise, as has been hitherto 

 assumed, from the second induction-current, but from the dis- 

 junction-current, which is caused by the first induction-current. 

 When the air is rarefied, the disjunction-current becomes feebler, 

 and the induction-currents begin to have more and more effect ; 

 at last they determine the direction of the deflection. Now, from 

 the results of experiments 27 and 28, we know that the induction- 

 current can traverse the spark most readily when it goes from the 

 disk to the point. If, therefore, the valve is so applied that the 

 second induction-current goes from the disk to the point, the di- 

 rection of the deflection must remain unaltered when the air is 



* Abhandlungen uber die Lehre von der Reibungs-Electricitat. Berlin, 

 1867, p. 316. Pogg. Ann, vol. cxx.p, 513. 



