1890.] between Electrodes. at different Temperatures. 125 



It has been shown by other experimenters* that for arcs of vary- 

 ing length, but the same current, beyond a certain small initial length, 

 the potential difference necessary to maintain the arc is proportional 

 to the length of the arc plus a constant. This might thus be inter- 

 preted to mean that a certain proportion of the working electromo- 

 tive force of the arc was employed in detaching the carbon molecules 

 from the mass of the poles, and that the excess alone is represented 

 by the current produced in an arc of definite length. 



In the case of the incandescence lamps the hypothesis of the 

 projection of negatively charged carbon molecules from the incan- 

 descent conductor, to which the name of molecular electrovection may 

 be given, will suffice to explain all the various different effects pro- 

 duced by varying the surface, position, and distance of the metal 

 plate against which they impinge, and also the nullifying effect of 

 shielding this plate from the negative leg of the carbon. 



That this molecular discharge goes on chiefly from the negative 

 leg is additionally proved by the greater erosion which takes place 

 in the deposit of carbon on the negative leg when the carbon is 

 uniform and traversed by a continuous current. 



The hypothesis that a carbon molecule detached from an incan- 

 descent carbon surface in a high vacuum can only convey away a 

 negative charge, reconciles also the above described observed effects 

 in which a negative discharge can be made out of a hot surface of 

 carbon more easily than a positive discharge. When an electromotive 

 force is applied to two metallic terminals or electrodes sealed into a 

 good vacuum, it is well known that a certain initial electromotive 

 force has to be applied before any electric current begins to 

 flow through the gas at all. It seems conclusively proved by 

 Mr. Crookes's researches that the nature of an electric discharge 

 through a high vacuum consists in a torrent of electrified particles 

 proceeding from the negative electrode. If this is the case the 

 initial electromotive force required to begin a discharge through such 

 rarefied gas would naturally be reduced by heating the negative elec- 

 trode, so as to favour and assist the detachment of the charged mole- 

 cules of that electrode. The effect of heating the negative electrode 

 in facilitating discharge through vacuous spaces has previously been 

 described by W. Hittorf (' Annalen der Physik und Chemie,' vol. 21, 

 1884, p. 90 — 139), and it is abundantly confirmed by the above 

 experiments. We may say that a vacuous space bounded by two 

 electrodes — one incandescent, and the other cold — possesses a uni- 

 lateral conductivity for electric discharge when these electrodes are 

 within a distance of the mean free path of projection of the mole- 



* See Professors Ayrton and Perry, ' Proceedings of the Physical Society,' 

 vol. 5, p. 201. 



