10 Prof. E. Edlund on the Electrical 



ployed in these two series of investigations was a Holtz 

 machine giving a constant quantity of electricity proportional 

 to the velocity of rotation of the disk. In the first series the 

 rarefied gas was contained in a large receiver made of stout 

 sheet brass, and the discharge took place between spherical 

 electrodes of metal, of course insulated from the brass receiver. 

 For the observation of the electric light the receiver was fur- 

 nished with two apertures opposite each other and covered 

 with glass. In the second series the electrodes were also sphe- 

 rical, but were placed in balls of glass joined hermetically to 

 a glass tube the length and width of which were varied with 

 the experiments. In all the experiments the discharge ap- 

 peared discontinuous even when the gas had its maximum of 

 rarefaction ; this latter, however, was never carried far enough 

 to surpass that of the minimum of tension necessary for the 

 production of the discharge. The object of the investigation 

 was to determine the electric tension necessary under different 

 circumstances to produce an electric discharge. 



With the aid of a rotating mirror and a heliometric appa- 

 ratus ingeniously arranged by Wiedemann, a more particular 

 description of which would not be in place here, it was possible 

 to determine the time between two successive discharges. 

 The source of electricity being constant, that time was pro- 

 portional to the electric tension of the electrodes at the com- 

 mencement of the discharge. The most important results of 

 these experiments for the question which we have set ourselves 

 to elucidate here are the following: — (1) If the flow of elec- 

 tricity to the electrodes is constant, the electric tension neces- 

 sary at the electrodes for a discharge to take place increases 

 with the pressure of the gas. (2) This tension depends, at 

 equal pressure, on the nature of the gas employed; it is greater 

 for dry air than for hydrogen ; but it is independent of the 

 chemical nature of the electrodes*. (3) The experiments 

 made at a pressure of 25 millim. or above showed that the 

 necessary tension increased as the distance between the near- 

 est points of the electrodes was augmented ; still the increase 

 was less than proportionate to the augmentation of the dis- 



* That the tension necessary for the discharge was independent of the 

 chemical nature of the electrodes is in contradiction with Hittorf'a expe- 

 riment of which we shall speak further on. The difference of the result 

 obtained by Wiedemann and Ruhlmann is doubtless due to the electrodes 

 in their experiments presenting relatively large surfaces and the pressure 

 of the gas being considerable (13-8 millim. and above). It is known from 

 other experiments that the influence of the electrodes is in the inverse 

 ratio of the magnitude of their surfaces, and that it increases with the 

 rarefaction of the gas. 



