Measurements were made on the sparking potentials between fixed plane parallel electrodes in purified, mercury-free A, ${\mathrm{N}}_{2}$ and ${\mathrm{H}}_{2}$ for Pt and Na cathodes for extended pressure ranges. The effect on the sparking potentials in ${\mathrm{H}}_{2}$ 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 $p\ensuremath{\delta}$. 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 $p\ensuremath{\delta}$ 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 $p\ensuremath{\delta}$ equal to 240 mm\ifmmode\times\else\texttimes\fi{}cm. The predischarge currents within a few volts of breakdown were less than ${10}^{\ensuremath{-}9}$ 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 ${\mathrm{N}}_{2}$, the lowering produced by Na is 28 percent near the minimum and 20 percent at $p\ensuremath{\delta}$ equal to 340 mm\ifmmode\times\else\texttimes\fi{}cm. The ${\mathrm{N}}_{2}$ results are not applicable to the theory discussed by Loeb because of the heavy predischarge currents, 360 microamperes at $p\ensuremath{\delta}$ 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 $\ensuremath{\alpha}$ 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 ${10}^{\ensuremath{-}4}$ mm and ionization potential less than that of ${\mathrm{N}}_{2}$ which acts independently of cathode phenomena. ${\mathrm{H}}_{2}$ is intermediate between ${\mathrm{N}}_{2}$ and A both with respect to the approach of the Na and Pt curves at high $p\ensuremath{\delta}$ and the magnitudes of the predischarge currents. In the ${\mathrm{H}}_{2}$---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 ${\mathrm{H}}_{2}$ with heated Pt and Na cathodes are identical within the limits of experimental error. The coincidence at both low and high values of $p\ensuremath{\delta}$ 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 $p\ensuremath{\delta}$, 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 $p\ensuremath{\delta}$, $\ensuremath{\alpha}$ is predominant in determining the sparking potentials, and $\ensuremath{\alpha}$ is very sensitive to the presence of Na. The formation of NaH, which occurs because of the presence of the heated cathode, probably removes most of the Na vapor and destroys all of the volatile substance which causes the lowering in the case of the cold Na cathodes.
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Jan 20, 2023
Jian-Quan Li + 2
Open Access