Spark Breakdown Potentials as a Function of the Product of the Pressure by the Plate Separation in A,N2andH2for Pt and Na Cathodes

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Abstract
Measurements were made on the sparking potentials between fixed plane parallel electrodes in purified, mercury-free A, N2 and H2 for Pt and Na cathodes for extended pressure ranges. The effect on the sparking potentials in H2 produced by heated Pt and Na cathodes was investigated. In all of the gases studied, the values of the sparking potential are lower for a Na-coated Pt cathode than for a clean Pt cathode throughout the pressure ranges investigated. The percentage lowering, which is especially pronounced near the minimum sparking potential, decreases with increasing pδ. In A, the sparking potentials for a Pt cathode agree very closely with the Fe cathode values of Penning except in the vicinity of the minimum sparking potential; the discrepancy at low values of pδ is caused by the difference in cathode surface. For a Na-coated surface, the sparking potentials are 50 percent lower in the vicinity of the minimum and 7.5 percent lower at pδ equal to 240 mm×cm. The predischarge currents within a few volts of breakdown were less than 10−9 amp. for both Na and Pt surfaces. The effect of a Na cathode on the breakdown potentials in A is in satisfactory agreement with the quantitative predictions of classical Townsend theory given by Loeb. In N2, the lowering produced by Na is 28 percent near the minimum and 20 percent at pδ equal to 340 mm×cm. The N2 results are not applicable to the theory discussed by Loeb because of the heavy predischarge currents, 360 microamperes at pδ equal to 356, which occur in the presence of Na. The lowered sparking potentials and the heavy predischarge currents, as well as the increase in the values of the Townsend coefficient α in the presence of a Na-covered cathode which was observed by Bowls, indicate that there occurs in the presence of Na a volatile substance, Na or something else, of vapor pressure greater than 10−4 mm and ionization potential less than that of N2 which acts independently of cathode phenomena. H2 is intermediate between N2 and A both with respect to the approach of the Na and Pt curves at high pδ and the magnitudes of the predischarge currents. In the H2—Na case, the marked predischarge currents and the lowered sparking potentials are probably caused by a volatile impurity, Na or something else. The sparking potentials in H2 with heated Pt and Na cathodes are identical within the limits of experimental error. The coincidence at both low and high values of pδ may be explained by the formation of NaH and the distilling off from the heated surface of the Na and/or NaH. At low values of pδ, the sparking potentials are governed considerably more by cathode phenomena than by the primary process of ionization by collision. The heated Na cathode has no Na nor NaH deposited on it; the cathode emission properties are the same therefore as those of the heated Pt surface. Hence, the sparking potentials are the same. At high values of pδ, α is predominant in determining the sparking potentials, and

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