1887.] 



Discharge of Electricity through Gases. 



375 



Hittorf has measured with great success the fall of potential at 

 different points of a vacuum tube through which a discharge was 

 passing, and none of his principal conclusions are affected by these 

 scruples, for he gives in his paper sufficient evidence that his method 

 is applicable to the cases he has examined. But the purposes I have 

 in view rendered a measurement of potential necessary under severe 

 conditions, in which a serious error might have been introduced by 

 assuming without verification that the potential of a metal is the 

 same as that of a gas in contact. 



Strictly speaking, the potential is always continuous as long as we 

 are dealing with finite charges, but when a layer the thickness of 

 which extends to molecular distances has its sides charged with oppo- 

 site electricity, it is customary to compare the two sides of such a 

 layer directly with each other, and neglecting the rapid variation of 

 potential within the layer, to speak of a discontinuity of potential. 

 It is in this sense that I am here speaking of a possible finite differ- 

 ence of potential between a metal and the gas in contact. 

 , The question is settled by the principal result of this paper : 



A steady current of electricity can he obtained in air from electrodes at 

 the ordinary temperature which are at a difference of 'potential of one 

 quarter of a volt only (and probably less) ; provided that an independent 

 current is maintained in the same closed vessel. 



In other words, a continuous discharge throws the whole vessel into 

 such a state that it will conduct for electromotive forces which I 

 believe to be indefinitely small, but which the sensitiveness of the gal- 

 vanometer I used has prevented me from tracing with certainty below 

 a quarter of a volt. There cannot be therefore a finite difference of 

 potential between a gas and a metal in contact greater than that 

 amount. 



Hittorf,* who has done more to clear up this subject than anyone 

 else, has found already that a current from a few cells will pass cross- 

 ways through a discharge in vacuo, but his auxiliary electrodes were 

 introduced into the discharge itself, and it was doubtful, therefore, 

 how far the results were due to the high temperature of the particles 

 carrying the luminous discharge.! I was aiming, on the contrary, at 

 placing the secondary electrodes as far from the main discharge as 

 possible, and at rendering by means of screens the two electric fields 

 as independent of each other as possible. 



I need not describe all the successive experiments in which I have 

 endeavoured to make my tests more and more severe. It will be 



* ' Wiedemann, Annalen,' vol. 7, p. 614. 



f That high temperature itself is a matter on which authorities differ. I do not 

 desire at present to commit myself to any view which seems to me to involve a 

 definition of " temperatures " under exceptional circumstances. 



