196 Mr. A. Schuster on the Disruptive Discharge 



Part II. — Theoretical. 



The attraction between electrified bodies involves a tension 

 along the lines of force in the medium which, with the usual 

 notation, is equal to KE 2 877. The specific inductive capacity 

 K of gases is nearly equal to one. and the stress is therefore 

 not much affected by the presence of the gas. It follows that 

 the stress is due to a strain in the interstellar medium, and 

 that the particles of air have nothing to do with it except in 

 so far as we consider them to be conductors which can deflect 

 the lines of force in their neighbourhood. TTe may imagine 

 the distribution of electricity on each molecule to be regulated 

 by the same law as the distribution on conductors ; and in 

 that case the forces tending to produce a passage of electricity 

 between the particles of a gas will be proportional to R 2 , but 

 we do not know how large or how small its value may be. I 

 adopt the theory that the atoms of a molecule carry the same 

 charges as the ions in an electrolyte, and that a spark passes 

 as soon as the forces are sufficient to dissociate the molecules. 

 I take this opportunity to mention that Mr. Giese has in the 

 year 1882, that is, two years previous to the paper in which I 

 first applied this theory to the continuous discharge under re- 

 duced pressure, explained by the same hypothesis some of the 

 electric phenomena of flames. The problem of finding the 

 effect of the electric field on the molecules carrying their 

 electric charges is very similar to the problems we meet with 

 in the theory of magnetism. It is usual to consider two 

 vectors inside a magnetized body, the so-called magnetic force 

 H and the magnetic induction B, which are related by the 

 equation 



B = H + 4a?I, (13) 



where I stands for the intensity of magnetization. In the 

 problems of induced magnetization I is taken to be pro- 

 portional to the magnetic force defined as the force at the 

 centre of a thin hollow cylinder inside the magnet. It would 

 be more correct to take the magnetizing force as equal to 

 the force at the centre of a space left vacant by the removal 

 of a molecule; and this force could be expressed in terms of 

 the magnetic force as usually defined and the magnetic in- 

 duction. Let the force within the space left vacant by the 

 removal of one molecule be G ; we may write 



G=H + nI, (14) 



where n is a numerical quantity, which, if the space left vacant 



