584 Professor Sir J. J. Thomson [April 2, 



we see that the density of the negative electricity at the head of the 

 striation is about \ of an electrostatic unit per c.c. The density of 

 the positive electricity in the other portions is very much less than 

 this. With Wehnelt electrodes there is frequently only a small 

 potential-difference between corresponding points in adjacent stria- 

 tions : in some cases this difference was only 2 • 7 volts. 



The changes in the electric force are much more abrupt at low 

 pressures than at high ones ; though there is always a large increase 

 in the force at the bright head of the striation. I have not observed 

 the existence of the negative forces when the pressure was more than 

 a fraction of a milUmetre of mercury. 



I have found other cases in which the negative forces are even 

 more pronounced than those I have already considered ; perhaps the 

 most striking of these is one where the anode and cathode are con- 

 nected together and with earth by stout metallic connexions, so that 

 the two are at the same potential, and therefore the average negative 

 force between them is as great as the average positive force. The 

 anode is perforated by a very fine hole, and through this hole a stream 

 of Canal rays, i.e. positively electrified particles, passes into the tube : 

 this produces when the pressure is suitable a fully developed discharge, 

 with striations, Faraday dark space, a well-developed negative glow 

 and dark space ; and in spite of the anode and cathode being at the 

 same potential there is in this case the normal cathode fall of about 

 300 volts at the cathode : the negative forces in a tube of this kind 

 must be very considerable, as they have to balance the cathode fall. 



The heaping up of tlie negative electricity at the head of the stria- 

 tions seems to me to be the most important factor in the production 

 of striations. 



This concentration of the negative electricity at regular intervals 

 along the discharge may be explained as follows. Consider a stream 

 of negative corpuscles projected from the neighbourhood of the cath- 

 ode with considerable velocity : they will collide against the molecules 

 of the gas, and thereby lose velocity : if the electric field acting on 

 them is not sufficiently intense to restore the velocity lost by the colli- 

 sions, the corpuscles will lose velocity as they travel through the gas, 

 thus the corpuscles in the rear will gain on those in front, and there- 

 fore the density of the corpuscles and therefore of the negative elec- 

 tricity will be greater in the front, and by the equation --— = 4 tt p, 



ct X 



when X is the electric force, x the distance from the cathode, 

 and p the density of the electricity, the electric force will increase 

 rapidly in consequence of this concentration. This increase in the 

 force will increase the velocity of the particles in front. If the in- 

 crease in velocity is not sufficient to make the corpuscles ionize the 

 gas by collision, the congestion will be relieved by the gradually in- 

 creasing velocity of the corpuscles in front, and there will be no 

 periodicity either in the density of the electricity or the electric force. 



