The temperature dependence of the photosensitivity of an electric discharge with 50-cycle potentials in halogen-filled Siemens' tubes has been studied at constant gas pressure p as well as at constant mass conditions. The potential width of the photosensitive non-self-maintained region and the onset potentials of the self-maintained region of the discharge increased at constant p with the temperature T > 100 °C. In the self-maintained region of the discharge, irradiation by external light produced the familiar photoreduction of the discharge current i, viz. the negative Joshi effect, −Δi, which increased very little with T at constant p and applied potential V and reversed sign, depending on V, at 100–120 °C to the positive Joshi effect, +Δi, i.e., the photoincrease of i.Oscillographic studies of the current structure at [Formula: see text], the breakdown voltage in the dark, confirmed the co-occurrence of both +Δi and −Δi at T < 100 °C. They confirmed also that −Δi was absent at T > 100 °C. An integrating current detector which measured the resultant Δi showed that for a given p and T, +Δi increased with [Formula: see text] to a maximum; above Vm, +Δi decreased and reversed sign to −Δi as V was increased. At constant mass conditions, the Δi−V curve shifted towards the increasing potential axis by an increase in T. The potential at which +Δi was maximum and the potential at which the sign reversal of Δi occurred increased with T; the maximum magnitude of +Δi also increased with T > 100 °C.
Threshold Potential for the Low-Frequency Electric Discharge in Air