CHAPTER VII 



ELECTROLUMINESCENCE 



Introduction 



THE WORD " electroluminescence " is usually applied to the light 

 resulting from flow of current through partially evacuated tubes 

 of gas— neon, argon, nitrogen, mercury vapor, etc.— which glow in 

 various colors.^ This phenomenon is commonplace today. Indeed, 

 the present widespread use of neon lights as advertising signs gives 

 little clue to the chance discovery and many steps which led to 

 practical application.^ 



Sometimes light appears when electric currents are passed through 

 certain solutions, mostly from changes occurring at the electrodes, 

 such as the " electrolytic flames " when halides (especially sodium 

 bromide solutions) are electrolyzed at mercury anodes,^ or the light 

 from the aluminium anode of a carbon-aluminium rectifier.* Such 

 light emissions are more properly called galvanoluminescences 

 rather than electroluminescences and actually belong in the cate- 

 gory of chemiluminescence. Light may also be observed at elec- 

 trodes in a solution of certain chemiluminescent compounds, the 

 result of hydrogen peroxide or oxygen formation.^ , 



Electroluminescence as used in this history, that is, light in gases 

 due to an electric discharge, requires no vacuum tube. It is very 

 common and occurs in the air when clothing or fur or other insula- 



^ When a beam of electrons (cathode rays) strikes a solid or liquid, the light 

 emitted is sometimes called an electroluminescence rather than a cathodoluminescence. 



" Within the last few years, the word " electroluminescence " has been used for a 

 new method of exciting specially prepared phosphors, by which the luminescent 

 material forms a sheet of dielectric, between a metallic and a transparent conductor 

 in the form of three thin plates. The whole acts as a condenser and a brief flash of 

 light appears whenever a potential is applied to or discharged from the conducting 

 plates. If an alternating current field is used for excitation the light appears con- 

 tinuous to the eye and large areas may be rendered light-emitting. Such a device has 

 been called a panelescent lamp. 



' See Bancroft, W. D. and H. B. Weiser, Flame reactions. Jour. Physical Chem. 18: 

 213. 281, 762; 19: 310, 1913-1915. 



♦Discovered by Ferdinand Braun (1898). A luminous phenomenon accompanying 

 electrolysis of acid water with platinum electrodes at high current densities has been 

 described by N. P. Sluginov (1880) in Russian, translated in the Jour, de Physique 

 Theor. et Appl. (Series 2) 3: 465-466, 1884. The light is practically an arc, with a con- 

 tinuous spectrum containing some lines of hydrogen and of platinum. See also S. 

 Anderson, Jour. App. Phys. 14: 601-609, 1943, for explanation of anode light. 



= See E. N. Harvey on luminol (Jour. Phys. Chem. 33: 1456-1459, 1929) and on luci- 

 ferin (Jour. Gen. Physiol. 23: 275-284, 1923) . 



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