1886.] electric discharge in a uniform electric field. 395 



be. The glass tube T was fused on to a Topler pump. With this 

 arrangement the only joints between the inside of the vessel S 

 and the outside air are mercury ones, and the vessel can readily 

 be exhausted. 



The discharge presents the following appearance as the vessel 

 is gradually exhausted. After the pressure gets below the value 

 reached with the first arrangement the glow between the plates 

 gets more and more uniform, until no difference can be perceived 

 in the intensity of the light between the plates ; as the pressure 

 diminishes a glow appears above the upper plate, of the kind to be 

 described below, and the intensity of the glow between the plates 

 diminishes; as the pressure falls still lower the glow above the plates 

 increases in intensity while that between the plates diminishes, until 

 at a pressure which I estimated at about ^ of a millimetre there 

 was no glow at all between the plates which were separated by 

 a dark space, while there was a strong glow above the upper 

 plate. 



This glow is represented in section in fig. 6. ABGD, EFGH 

 represent the glow, which is separated from the plate LMN by 



Fie. 6. 



dark spaces, which are left blank in the figure. The distance of 

 the glow from the plates, that is the width of the dark space, 

 depends upon the intensity of the discharge; by altering the screw 

 of the commutator of the coil the glow could be made to rise and 

 fall in a very striking fashion. The stronger the discharge the 

 smaller seemed the dark space. 



From one edge of the glow a bright thread of striated glow, 

 AP, extends, forming apparently the positive part of the discharge, 

 the glow being the negative ; this positive part looked like a con- 

 tinuation of the negative, there being no interval that I could see 

 between the glow and the striated discharge. This striated discharge 

 started from the place where the negative glow was farthest from 

 the glass of the bell-jar. It was extremely sensitive to the action of 

 a magnet, the point from which the discharge started being altered 

 by the magnet. The direction in which the discharge moved was 

 along the circumference of the glow, and the direction was de- 

 termined by the component of the magnetic force along the radius 

 from the centre of the glow to the point where the discharge took 

 place ; if the direction of this component was reversed the 



