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Edited by 5. Z. LEWIN, New York University, New York 3, N. Y. and recombination in the gas. Under these conditions, the current is limited not by the voltage, but by the resistance in the external circuit, and a plateau occurs in the current-valtastpe chsrscteristic. If the ourrent is not allowed to increase too much. the discharee is not associated with appreciable l i g k emission; this is the region of the so-called dark discharge. However, if the current is allowed to increase significantly (e.g., >lo-"mpperes in the case to which Figure 1 refen), space charge effects, due to the buildup in concentration of positive ions left hehind when t,he faster-moving
These arlide8, most of which are to be contributed by puest aulhors, are intended to s m e the readers of lhis J O ~ N AbyLd n g aUenPion lo nau developments in the theory, design, or wailabilily of chemical laboratmy instrumentation, oi by presenting useful insights and ezplanations of topica Uurl are of prmlical importance to those who use, rn teach the use of, modern instrumentation and instrumental leehnipues.
XXI. Luminous Gas Light Sources S. Z. Lewin, New York Universify, N . Y. 3, N. Y. The specid features of Bash photolyaic inst~wnentationhave been desrribed in the preceding paper of this series, and mention was made of the general design of fli~sllt,~thesand spark ~ a p s s u i t a b l efor snrh experimentation. The present disrussion will deal with the vnriaus types of wmmerrial light sources rurrently available for both continuous and flnqh irradixlion of chemical systems. l h e principal requirements of s light source that is to be s~litahlefor photochen~icslwork are that it (a) emit, a high int,ensity of radiation in the desired rpertml range, and (b) generate this radiation in n repmdwible and contmllsble fmhion. The intensity and speel.rsl distribution are primarily determined 1,y the chemical nature of the emissive sohstsnce and t,he energy input to it. The contml of the radiation is achieved by proper design of the physical geometry and elertrir:d p:rr:lrneters and circuitry of t,he lamp.
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Ironsition region
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Luminous Gas Behavior The two main classes of photochemicil emitters in common use consist, of those bnrecl either on (a) a luminous ga?, or ( h ) and incandescent solid. The lat,ter include all tmgsten filament light sources, as well a? the so-palled zirconinm-arc lamps, and their behavior is adecpalely aacomted for by the well-known Wien, Stefm-Boltsmann, and Plsnrk hlurkLmdy radiation laws. The light emission properties of a gas roufined within an electric field are n complex function of the field intensit).. The general behavior of s w h z system is shown in Fignre 1, where t,he voltage, 1; applied across s. pair of flat copper electrodes of 10 em2 area and separated hy n distance of 50 rm, is plotted ngainst, tile current. flowing throtlgh a neon gas filling at 1 mm Hg pressure. At low voltages (below 600 volts in Figwe I), only very small, sporadic bursts of current (
