Arc constriction in lamps containing mercury and iodine

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Abstract
Quantitative calculations indicate that the mechanism by which iodine causes constriction of arcs in mercury vapor lamps is the emission of molecular radiation from HgI. Calculated arc temperature profiles from the energy balance equation for various mixtures of mercury and iodine are found to agree with measured profiles when allowance is made for the emission and self‐absorption of radiation from all atomic and molecular species. Approximate emission coefficients for HgI molecular radiation and atomic iodine radiation have been determined as a function of temperature. Molecular radiation emitted from the plasma at temperatures between 2500 and 4500 °K is found to be the major cause of arc constriction for plasma pressures of the order of a few atmospheres.