On the Early Stages of Electric Sparks

Abstract

With the Kerr-cell electro-optical shutter of Abraham-Lemoine and Beams, phenomena in the early stages of sparks between electrodes of Zn, Cd and Mg have been studied. It was found that, during 50(${10}^{\ensuremath{-}8}$) sec after beginning of the sparks, the spark doublet lines $3{d}_{1,2}\ensuremath{-}4{f}_{1,2}$ of Zn have widths of 45A, while the corresponding lines of Cd and Mg are about 30A in width. The luminosity of the metallic vapors of Zn, Cd and Mg was observed to spread from the electrodes with speeds of 2.1(${10}^{5}$) cms/sec, 1.5(${10}^{5}$) cms/sec, and 1.2(${10}^{5}$) cms/sec, respectively. Photographs of the early stages of single sparks with exposure times as short as 4(${10}^{\ensuremath{-}8}$) sec were obtained. The snapshots showed that during these short intervals of time after beginning of a spark the discharge is confined to a filament having a cross-section at the anode of 5(${10}^{\ensuremath{-}4}$) ${\mathrm{cm}}^{2}$ which broadens out to four times this size at the cathode. From the circuit constants and these dimensions of the discharge it was accordingly estimated that the discharge current density attained the magnitude of 1.7(${10}^{6}$) ${\mathrm{a}\mathrm{m}\mathrm{p}\mathrm{s}/\mathrm{c}\mathrm{m}}^{2}$. The asymmetry of the photographed images of the sparks disappeared when the exposure times were extended to include a complete cycle of the discharge, thereby proving the satisfactory operation of the shutter.Assuming that the broadening of the lines to be due to the Stark effect of interionic fields and with the aid of existing data on the Stark effect behavior of the lines, it was calculated that the average inter-ionic field was ${10}^{6}$ volts/cm. This result in turn implies that the average separation of the ions was 3.8(${10}^{\ensuremath{-}7}$) cm and therefore that 1/3 of the molecules in the path of the discharge were ionized. Approximately the same degree of ionization follows from the assumption that 1/2 of the discharge current is carried by the positive ions moving towards the cathode with the observed velocity of 2.1(${10}^{5}$) cms/sec.Four more or less independent methods of estimating the temperature of the early periods of the spark from the present observations lead to values of the order of magnitude of 10,000\ifmmode^\circ\else\textdegree\fi{}K which point to thermal ionization as a prominent feature of such discharges.