The Persistence of the Radiation Excited in Mercury Vapor

Abstract

A photographic method was used. Light from an intermittent mercury arc passed through a quartz cell containing Hg vapor at 40° to 100°C, and then together with the excited radiation through a hole in a rotating sector into a spectrograph. The sector was mounted on the shaft of the commutator which operated the arc. Thus for $\lambda 2537$ a trace was obtained on the plate, whose length depended on the time of persistence of the radiation, the time scale being determined by the speed of rotation. Measurements of the blackening showed that the decay was exponential. The time to decrease to intensity $\frac{1}{e}$ came out 4.0×${10}^{-5\mathrm{}}$ and 6.8×${10}^{-5\mathrm{}}$ sec. for cells of length 1.6 and 3.2 cm respectively. For the arc alone in a tube of 0.9 cm radius the constant was 0.6×${10}^{-5\mathrm{}}$ sec, while for the arc at the center of a bulb of 3 cm radius the constant was 5.5×${10}^{-5\mathrm{}}$ sec, The decay was found to be independent of the vapor pressure from.006 to.27 mm for the cell and from.05 to 1.7 mm for the bulb. No persistence was observed when the cell did not contain distilling mercury or when it was sealed off from the pump. Fresh mercury seems to be necessary and also the absence of impurity. It is interesting to note that the time constants are of the order of magnitude of the times required for atoms at the temperatures involved to go without impact from the center to the wall of the cell.