Searching for working gases for substituting mercury in melal-halide lamps, the rare gases came into focus. Besides the high vapor pressure, the main property which makes mercury so unique tin the filling tif lngh-prcssuro lamps is its large transport cross section. It allows lamp operation with low current due to the high electric field strength which can be established in mercury discharges. Therefore, as a necessary condition ihe substitute must allow a high field strength. This is of course not sufficient for uefiniriji a substitute: finally the resulting lamp must have a high luminous efficacy. As a first step, howeer. the field strength is a criterion for the selection of substitute candidates. The discharge voltage alone, taken under otherwise the same conditions, is not convincing enough because it consists of the voltage of the positive column I field strength times column length) and of the electrode sheath voltage (liSV). and" the latter does not participate in Ugh! production. Therefore, we have determined the electric field strengths in Hg, Ne. Ar. and Xe, and the corresponding HSVs were obtained as a by-product.
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