Discharge in Rarefied Gases. 371 



One of the experimental tubes is represented in fig. 1. 

 A cylindrical tube is provided with a flat kathode placed 



at right angles to the axis of the tube, and nearly as large 

 as the section of the tube. The anode is placed close in 

 front of it, or in other cases consists of a very short wire 

 in the plane of the kathode itself. Inside the cylinder is 

 a movable partition c, consisting of a short glass cylinder ter- 

 minated by a plane surface at the end turned towards the 

 kathode. In accordance with what I have noted on former 

 occasions*, the positive light disappears for such an arrange- 

 ment of the electrodes when the exhaustion has reached a 

 certain limit, or it is confined to the immediate neighbourhood 

 of the anode ; the kathode-light, on the other hand, expands 

 to any extent if the exhaustion is sufficient, and the expansion 

 of its rays is limited only by their striking upon a solid wall. 

 Hence, when a sufficient exhaustion has been reached, we can 

 vary the expansion of the kathode-rays within wide limits by 

 sliding the piece c (by inclining the discharge-tube and tap- 

 ping it) along the tube; since its length is always equal to the 

 distance between the kathode and the end-surface of the 

 movable cylinder, which can be moved right up to the anodef. 



If now the expansion of the kathode-light is made to vary 

 in the ratio 1 : 30, the total resistance of the discharge at low 

 pressures does not alter so much as in the ratio 1 : 1'05. 

 Hence the resistance of the kathode is a vanishing quantity in 

 comparison with the resistance at the surface of the kathode. 



Hence we see that the resistance of the whole quantity of 

 gas contained in a discharge-tube becomes smaller the more 



density of the discharge at a is not changed. There is in fact a certain 

 change ; but its influence upon the phenomena considered in the place 

 referred to is so small that, upon repetition of these experiments with 

 actual constancy of density at a, results were obtained partly exactly cor- 

 responding, partly nearly corresponding. 



The examples given on p. 149 for the magnitude of the deflective 

 power in a particular case consequently represent these values at least 

 very nearly. The same method was employed (p. 131) to confirm a result 

 obtained by two other methods ; so that the result given there is not 

 affected by the failure of the experiment in question. 



* Goldstein, Phil. Mag. iv. p. 362 ; 'A new Form of Repulsion,' p. 8. 



t In order to resist blows without deformation, the anode in these cases 

 was made of strong iron or steel wire. 



