136 Prof. J. J. Thomson on the Velocity of 



Very striking examples of this effect have been given by 

 Hittorf and others. Hittorf (Wied. Ann. xxi. p. 90) describes 

 an experiment in which when the electrodes were very near 

 together the discharge passed along a curved path of very 

 much greater length, leaving the short space between the 

 electrodes free from glow. In Lehmann's Molekularphysik, 

 vol. ii., many illustrations of the discharge between electrodes 

 very near together are given. Some of these are very re- 

 markable, the space immediately between the electrodes being 

 free from glow, while the discharge when the electrodes 

 are plane passes between the faces of the electrodes which are 

 most remote from each other. 



The view we have taken of the electric discharge requires 

 the molecules to be split up for a finite distance along the 

 line of force: the electromotive intensity has to maintain over 

 this length the value required to split up the molecules ; 

 thus, if the field of force is not uniform, it will require a 

 greater electromotive intensity at the surface of the electrode 

 to produce a spark than if the field were uniform. However, 

 at ordinary pressures YT is so small that it would be very 

 difficult to produce a sufficiently rapid variation of the field 

 to make this effect felt. 



The striations on this view of the electric discharge may be 

 regarded as forming little electrolytic cells, the beginning 

 and end of a stria corresponding to the electrodes of the cell. 

 Let us consider one of these cells. Suppose the unit of 

 electricity passes through it, then the work done by the elec- 

 tric field is FX, if F is the electromotive intensity, and A the 

 length of the stria. The passage of this unit of electricity 

 will be accompanied by definite chemical changes in the 

 space between two striations. Let w be the increase in the 

 potential energy due to these changes, then, if there is no 

 increase in the kinetic energy of the gas, we must have 



Thus, when the chemical change is the same, the electro- 

 motive intensity varies inversely as the distance between two 

 strise. Thus, if we can diminish in any way the distance be- 

 tween the strise without altering the chemical action, the 

 electromotive intensity required to force electricity through 

 the gas will be increased. If we have gas at the same 

 pressure in tubes of different diameter, after the exhaustion 

 has passed a certain point, the distance between the striae will 

 be greater in the wide tubes than in the narrow ones, so that 

 the resistance offered to the discharge ought to be greater 

 in the narrow tubes than in the wide ones. Hittorf (Wied. 



