On a Theory of the Striated Discharge. 981 



exist (a) if the negative ions in the arc are charged atoms 

 moving with a mobility given by the recognized mobility 

 formula of Wellisch, or (6) if the negative ions are electrons 

 moving with a mobility of 1*5 x 10 6 cm. /sec /volt/cm., a value 

 far in excess of that to be expected from measurements at 

 lower temperatures. 



That there are objections to both theories there is no 

 doubt ; but the matter can only be definitely settled when a 

 method is devised for measuring the mobility of the negative 

 ions while they remain in the arc itself. 



CIV. On a Tlieory of the Striated Discharge. 

 By Sir J. J. Thomson, O.M., F.R.S.* 



OF the many and varied phenomena connected with the 

 Discharge of Electricity through Gases, few are more 

 striking than the regular and rhythmic alternations in 

 luminosity in the positive column called striations. These 

 have been the subject of many investigations; and some idea 

 of their beauty and variety can be got from the excellent 

 plates given by De la Rue and Mtiller in a series of papers 

 in the ' Philosophical Transactions ' beginning 1878, Pt. 1. 

 The alternations in luminosity along the discharge are 

 accompanied by alternations in the electric force, and an 

 explanation of the alternations in the force would go far 

 in explaining the alternations in luminosity. 



I gave in my ' Discharge of Electricity through Gases ' 

 reasons why there should be alternations in the electric 

 force. The object of this paper is to endeavour to reduce 

 these general considerations to a definite mathematical form 

 and to see whether they lead to results which are in accord- 

 ance with experience. 



The differential equations which represent the variations 

 of the electric force along the discharge are not linear, and 

 are so intractable that solutions have only been obtained for 

 special cases. This is so even when there is only one kind 

 of positive and one kind of negative ion and when the 

 pressure of the gas is so high that the velocities of the ions 

 at any point may be taken as proportional to the electric 

 force at that point. The conditions are even more com- 

 plicated when the pressure of the gas is as low as that in the 

 striated discharge, when the mean free path of an electron 

 is several millimetres, and when its velocity at any point 

 will depend not merely upon the electric force at that 

 * Communicated bv the Author. 



