70 Messrs. Rimington and Smith on Electric and 



the phenomenon of oscillatory discharge in the following 

 manner : — 



P (fig. 7), a primary consisting of a very few turns <of 

 gutta-percha covered wire, was placed in a Ley den-jar circuit, 

 as shown. As a secondary an exhausted bulb, or annular 

 tube, was placed near P. 



On sparks passing at D, a bright ring of light appeared in 

 the secondary. 



In the actual experiments a metallic secondary, S, was also 

 wound with P and exactly similar to it. On short-circuiting 

 S, the induced current in this circuit will weaken the mag- 

 netic field so as to completely extinguish the Geissler 

 secondary. 



If S is connected to a third coil this coil will behave in a 

 similar manner to P, with weaker effects ; P and S in this 

 case form a one to one transformer. 



The wire of the coil P is itself luminous when S is open ; 

 short-circuiting S will stop this effect. 



(Since the authors started their researches some of these 

 latter experiments have been shown by Prof. Thomson.) 



Appendix. 

 Theory of a Vacuum-tube in a varying Electric Field. 



When an electrodeless vacuum-tube is placed axially 

 between two plates, and a potential difference established 

 between i hem, the tube forms part of the dielectric between 

 the plates, and some of the lines of induction pass through 

 the tube. Now a rarefied gas is a very weak dielectric, and 

 readily gives way under the electric stress ; a convection- 

 current flowing through the tube (during which time the 

 gas in the tube behaves as if it were a conductor)*. 



This results in the endsf of the tube becoming charged, 

 or lines of induction from the plates terminate on the ends 

 of the tube, there being no lines in the tube itself (see fig. 8). 

 The whole of the tube will also be at the same potential. 



When the plates are discharged, the lines of induction 

 passing from the plates to the ends of the tube will now pass 

 through the tube ; the rarefied gas will again break down 

 under the electric stress, and a convection-current will pass 



* Of course the tube will not conduct after the manner of a copper 

 wire, but rather like a quantity of pith balls between two conducting 

 plates. 



t By the term " ends " is meant the inside surface of the glass at the 

 ends of the tube. 



