Post-Arc Conductivity and Metastable Helium

Abstract

Conductivity of helium immediately after arcing through it.—A study has been made of the abnormal conductivity which Kannenstine found persisted for.007 sec. after the arc was extinguished. The results prove that the residual conductivity is due to positive ions and not to metastable excited helium atoms, as suggested by Kannenstine. By means of adjustable commutators, the arc tube was connected first to a battery which caused an intense arc (period A), then after a brief interval (period B) to a source of voltage $V_c$ through a micro-ammeter (period C). As $V_c$ was increased the micro-ammeter showed a small negative current (positive ion) to 1.2 v., then a regularly increasing positive current reaching saturation at 8 v. The logarithm of the saturation current was a linear function of the length of period B for $V_c=-15\mathrm{}$ V. and for $V_c=-26\mathrm{}$ V., the current decreasing to one tenth in from.004 to.007 sec. (Similar observations in mercury gave a constant of.01 to.02 sec.). Observations with a Braun tube oscillograph gave direct additional evidence that after the arc ceased, the current decreased exponentially with time, independent of the length of the interval B. An attempt to sweep the positive ions away by applying a high negative voltage during B failed. Spectrographic observations made with a rotating slit attached to the commutator showed that no light was emitted during period C, even when the helium had been so carefully purified that the bands were distinctly visible in the spectrum of the arc. Life of metastable helium (states 2S and 2S).—While there is good evidence that metastable helium atoms persist for a longer time than ordinary excited atoms, the only evidence for a life of the order of 1/140 sec. rests on Kannenstine's interpretation of the persistent post-arc conductivity, and the experiments in this paper prove his interpretation to be unjustified.