Arc temperatures have been measured, and relative electron densities have been estimated for dc arcs in Ar, Ar-CO, He, and He-CO. Two temperatures were measured: the excitation temperature, <i>T</i><sub>exc</sub> , by means of the zinc two-line intensity ratio method, and the rotational temperature, <i>T</i><sub>r</sub> , by the C<sub>2</sub> band method. Relative electron densities were estimated by comparing Hα line widths in each of the four arc atmospheres. These measurements were performed to gain some understanding of the influence of small amounts of CO on the excitation characteristics of low amperage arcs in argon and helium. The resulting temperatures and electron density estimates indicate that only the arc in argon with CO present approaches local thermodynamic equilibrium. Tables of the measured temperatures and plots of the Hα line widths for the estimation of electron densities are given. Implications with regard to the excitation of metal impurities vaporized into these arcs are discussed.

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