

Discharges in Electrodeless Vacuum- Tubes. 521 



the wire is laid an exhausted tube, C, with a tinfoil cap*, T, 

 at one end ; T is connected to the outer coating of the jar 

 nearest to it. The object of this is to utilize the electrostatic 

 effect and make the tube more sensitive to breakdown by the 

 electromagnetic one. When the jars discharge, a straight 

 luminous band is observed in the tube directly over A B. 



If the tube C be now moved towards the jars, even by a 

 very small amount, a closed circuit discharge will be obtained. 

 There is apparently, then, a tendency for the luminous dis- 

 charge to form a closed circuit whenever possible ; and it 

 seems probable that even when the discharge is apparently 

 not closed, as in the case of the spiral or the straight line, 

 the electric stress acting in the rarefied gas takes the form 

 of a closed circuit, but is only intense enough to produce 

 sharp luminosity close to the wire f . To further test the ques- 

 tion an unclosed ring tube was made, and when it was placed 

 inside a coil of wire no trace of a single luminous band could 

 be seen J. A small glass tube was also bent so as to form a 

 spiral of four turns, and exhausted. A wire following the 

 spiral was attached to it, but this also gave no trace of 

 luminous discharge. 



Magnetic Effects of Discharge. 



The ring discharge in a bulb or closed circular tube acts 

 like a metallic circuit as far as magnetic effects are concerned. 

 This may easily be shown by the following experiment. 



A coil of three or four turns of wire has a similar one wound 

 with it to form a secondary ; the latter is connected to a 

 third coil, in which is placed an exhausted bulb. The first 

 coil is connected to the outside coatings of the jars (fig. 1). 

 The spark-gap can be adjusted so that a fairly bright ring is 



rp 



* It is not always necessary to use this cap, as, if the exhaustion is 

 high enough to give green phosphorescence of the glass, with the two 

 half-gallon jars in series, the di- charge can be obtained without the cap. 

 With another tube of lower vacuum the author finds the cap necessary. 



t That is, the return part of the discharge is so diffused and feebly 

 luminous as to easily pass unnoticed in comparison with the sharp and 

 brilliant luminosity close over the conductor. The same applies to the 

 spiral discharge, each turn of the spiral probably forming a closed circuit 

 by itself. 



% On afterwards repeating this experiment the author obtained a dis- 

 charge in parts of the tube, and with half-gallon jars in the whole tube. 

 The discharge, however, was a closed one, as there were two distinct 

 bands in the tube, one on the side next to the coil and the other on the 

 side farthest away from it. This is what might be expected if the 

 magnetic induction be sufficiently strong. 



Phil. Mag. S. 5. Vol. 35." No. 217. June 1893. 2 



