Experiments in Electric and Magnetic Fields. 69 



end will be seen to be brighter than the other, the current 

 parsing from the dull end to the bright end. 



Not only is this effect visible with a straight tube, but on 

 rotating an exhausted bulb in the field a stationary luminous 

 region will be seen (see fig. 4, b). 



Varying Electric Field. 



Instead of rotating the tube a similar effect is produced by 

 varying the field in which the tube is placed. 



The tube should be placed with its longer axis in the direc- 

 tion of the lines of force. The plates in this case may either 

 be charged and discharged by an influence-machine and dis- 

 chargers, the latter sparking, or by an induction-coil, with or 

 without sparking. 



When an induction-coil is used, very long tubes will glow 

 when brought near. 



Set 2. Magnetic Field. 



The authors were of the opinion that, by taking advantage 

 of the action of the Geissler tube, one might show Hertzian 

 phenomena to fairly large audiences. They therefore substi- 

 tuted a Geissler tube for the spark-gap in the resonator. 



This was very successful with a Leyden-jar oscillator (fig. 5). 

 The oscillator and resonator consisted of two similar small 

 Ley den jars for capacity, and a single convolution of thick 

 copper wire for the conductor-circuit. The Geissler tube was 

 placed as a shunt to part of the conductor-circuit of the 

 resonator (see fig. 5). 



But although they designed special tubes, they were unable 

 to get good results with the ordinary forms of Hertz oscil- 

 lator and resonator. 



They next made a resonator of the following form : — 



Two metal plates, a and b (fig. 6), give capacity _, and the 

 wire self-induction. An electrodeless exhausted tube was 

 placed as dielectric between the plates. When a fairly large 

 Hertz oscillator was used, this tube glowed brightly, even 

 when taken to considerable distances. 



Glow produced in varying Magnetic Fields. 



Instead of, as in the last set of experiments, having a 

 secondary composed partly of a metallic conductor and partly of 

 a vacuum-tube, the whole circuit may be composed of rarefied 

 gas. It is evident that, in order to generate a current in such 

 a circuit, a large induced E.M.F. would be required ; this 

 would necessitate either a very strong magnetic field, or one 

 which changed very rapidly. The former was out of the 

 question, the latter was obtained by taking advantage o( 



