Discharges in vacuo. 



123 



issuing from the point touched, so soon as the exhaustion is 

 sufficient for them to reach the opposite wall of the tube. 



In what follows, I propose to describe experiments with the 

 induced currents obtained in a high vacuum when the glass 

 wall is touched with a conductor at any point. 



I observe, first of all, that these induced currents may be 

 obtained in any tube which has been exhausted so far that the 

 glass wall in the neighbourhood of the kathode fluoresces 

 under the influence of the negative light, and that these cur- 

 rents may be obtained by touching the tube with a conductor 

 at any point whatever. A Geissler's tube marked SiCl showed 

 these currents not only in the neighbourhood of the kathode- 

 space, but also in the positive space, whilst on touching any 

 point of the glass wall a fluorescent surface appeared on the 

 opposite side of the tube. It is advantageous to include an 

 air-spark in the circuit ; the longer this spark is, the more 

 distinctly do the induced currents appear. 



In all the tubes in which the vacuum is more nearly com- 

 plete (for example, in Crookes's bulbs) these induced currents 

 are much more intense. 



Rectilinear Propagation of the Induced Currents. 



For these experiments I used, first of all, a vacuum-tube 

 such as was employed by Crookes to show that an opaque body 

 placed in the path of the rays issuing from the negative elec- 

 trode throws a shadow on the posterior glass wall. The tube, 

 constructed by Grotze, differed from that employed by Crookes 

 by having the cross suspended upon an aluminium wire which 

 crossed the tube, and upon which it could be made to slide 

 into a lateral enlargement of the 

 tube (fig. 1). When sparks 

 from the coil are passed through 

 this tube, a being connected with 

 the negative pole and b with the 

 positive pole, and a spark-length 

 of 6 to 8 centims. being included 

 in the circuit, brilliant fluores- 

 cence was observed on the part 

 of the tube opposite the point 

 touched, while at the same time 

 the light causing this fluorescence 

 could be seen in the tube itself, 

 the rays spreading out from the 

 inner surface of the tube at the 

 point touched to the opposite 

 surface. 



If, for example, tha tube were 



Fig. 1. 



X 



