Electroluminescence 303 



brilliant luminescence of gases and " Leuchtfarbe " and the small 

 amount of energy used, suggesting that such devices might be the 

 " Luminescenzlampe " of the future. 



The experiments are reminiscent of the fluorescent lamp and of 

 the newest light, the " panelescent lamp," in which special phos- 

 phors are excited by the rapidly changing potentials of condenser 

 plates (see Introduction) . In 1896 Wiedemann and Schmidt, who 

 had discovered the green fluorescence of sodium and the red of 

 potassium vapor in the light from an arc lamp, noted the same 

 colors when electrodeless tubes containing these vapors were sub- 

 jected to oscillating discharges. Thus the fundamental discoveries 

 had been made, and the twentieth century was to see the gas dis- 

 charge tube undergo rapid commercial development. 



NEW TYPES OF RAYS 



The attempts of the older investigators to obtain more perfect 

 vacua bore fruit in the middle of the nineteenth century. As the 

 pressure in a vacuum tube decreased, new types of rays were found 

 to appear, revealed by the nature of the limiinescence they produced. 

 The names of Pliicker, Hittorf, Goldstein, Crookes, Lecoq de Bois- 

 baudran, Lenard, Thomson, and E. Wiedemann are associated with 

 cathode ray discoveries, while Goldstein, W. Wien and G. C. Schmidt 

 investigated anode ray phenomena. 



Among these men. Sir Wm. Crookes (1832-1919) paid particular 

 attention to luminescence. Beginning in 1874, he carried out his 

 series of studies on the discharge of electricity in vacuum tubes 

 containing different pressures of gas. He paid particular attention 

 to the cathode glow, the dark spaces in the tube, and the streams 

 of " radiant matter " (cathode rays or electrons) which he thought 

 were molecules of the residual gas repelled from the cathode. 

 Crookes' name was applied not only to the tubes but to one of the 

 characteristic regions of the tube, the Crookes dark space, called the 

 Hittorf dark space in Germany. As we have seen, another region, 

 the Faraday dark space, honored a second early investigator of elec- 

 trical discharges in vacuum tubes, Michael Faraday (1791-1867), 

 who later became more noted for discovery of the laws of electrical 

 induction. Perhaps the name of J. J. Thomson, who did so much 

 to elucidate the nature of the electron, should be applied to a third 

 region of the tube. 



After cathode rays were established as a definite new phenomenon 

 of vacuum tubes, the part they played in the luminescence of the 

 tube became a matter of discussion, leading H. Hertz (1883) to 

 remark that " The light of gases in the glow-discharge is no phos- 



