The Discharge of Electricity through Gases. 5i$ 



It is interesting to gain some idea of the charges in absolute 

 units which are involved, and of the number of charged ions taking 

 part in the discharge according to our theory. The electrification at 

 the surface according to (2) is V /e 2 /47r?; 3 . In a series of experiments 

 in which V varied from 672 to 1330 volts, and the current between 

 400 and 3000 microamperes, /c 2 Y was found to vary between 3 X 10 12 

 and 10 13 C.G.S. units. Taking the highest value, we find p = 10 -9 

 C.G.S. electromagnetic units, or about 30 electrostatic units. From 

 this I calculate that the mass of electrified atoms per nnit volume is 

 7 X 10~ 13 , on the supposition that the charges are carried by 

 nitrogen-atoms. The density of nitrogen in the experiment which 

 forms the basis of this calculation is 5 x 10 ~ 7 , so that even close up 

 to the electrode the surplus of positive ions is very small compared 

 to the number of atoms and molecules present, only about one molecule 

 in a million being decomposed. Further away from the kathode the 

 relative amount of charged to uncharged particles is considerably 

 smaller still. The volume-density, which is derived from calculation, 

 gives us, of course," the difference only between the sum of the posi- 

 tive and the sum of the negative charges. The value of this excess of 

 positive charges cannot therefore be taken as a measure of the total 

 number of ions present, except perhaps close up to the electrode. 

 In how far the positive charges in the polarising layer and the 

 negative charges projected away from the kathode are alone sufficient 

 to account for the whole current, cannot be decided at present. The 

 mutual repulsion of the particles within the atmosphere of positive 

 particles which surrounds the kathode explains at once the tendency 

 of the glow to spread all over the surface of the electrode. Other 

 important facts, such as the effects of an anode near the kathode in 

 driving away the glow, will also find their natural explanation. The 

 spreading of the glow over the negative electrode is the cause of a 

 series of peculiar differences which appear at the two poles even 

 in the case of discontinuous discharges. The differences observed 

 in Lichtenberg's figure and in Priestley's rings, according as the dis- 

 charging point is positive or negative, seem to me to be readily ex- 

 plained by the atmosphere of positive ions which always tends to 

 spread all over the kathode, while the discharge from a positive elec- 

 trode is confined to the points of maximum electric density. It will 

 lead me too far to enter into these questions here ; but one objection 

 must be met : why does this positive electrification not make itself 

 apparent by electrical action in the space outside ; and why do elec- 

 trified bodies outside not act on the glow ? 



As regards the first question, the charge is not really a large one. 

 The whole quantity of electricity in the glow per square centimeter 

 cross-section would, according to the formula, be — 



