A Study of Arc Temperatures by an Optical Method

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Abstract
Sound waves passing through arcs in air are photographed with a rotating mirror camera, whereby the sound velocity in the arc core is determined. From the observed sound velocity the arc temperature may be calculated by a method previously described. The arcs measured by this method, together with the sound velocities and corresponding temperatures, are: Cored carbon1.85×105 cm sec.−15500∘KNaCl cored C1.574740Al cored C2.146160W cored C2.176220W 6 mm electrodes2.045950W 9.5 mm electrodes2.276440W–Fe2.136150Fe–Fe welding arc2.086020The experimental error corresponds to ±100° at 5500°K. The experimental requirements of the sound source are discussed in detail. The gas temperature across the arc core is found to be constant within the error of observation. For most of the arcs studied the current density and electric gradients have also been measured, so that the data may be used as a test of the thermal ionization theory. The ``effective ionization potential (I.P.)'' of the arc gas is in all cases above the I.P. of the metallic vapor and below that of nitrogen. For the W arcs the effective I.P. is 15 volts, midway between the first I.P. of O2 and N2. For the arcs other than tungsten, this quantity is lowered appreciably by electrode vapor. A small partial pressure of Na lowers the arc temperature by approximately 1500°. The temperature of a welding arc between a coated iron welding rod and a steel plate is measured to be 6000°, from which the dissociation of the arc atmosphere is found to be 81 percent. This high degree of dissociation points to the importance of the dissociation-diffusion-recombination process for heat transfer in welding arcs and explains the recently reported negative results of attempts to arc weld in pure argon.