Photosensitivity and Joshi Effect in Counters†

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Abstract
Joshi effect (±δi) an almost instantaneous and reversible photo-variation of the discharge current (i), is studied in various types of discharge vessels, charged with air, H2 and I2 vapour, and excited by fields due to continuous and alternating potentials (V). t was observed by a counter-sealer, and/or a galvanometer after rectification. +δi was observed under β-rays from a 50 millicurie Ra-Be source, and various frequency bands of the visible. The intensity (I) was varied over 30 times using the former, and 500 times with the latter. A 100% negative effect is observed near and above the threshold Vm; it decreases with increase of V. ± δi (disfavoured by a prolonged discharge) occurs within a restricted range of conditions, viz. low J and V. As these are increased, a sharp inversion equation follows which is fully reversible in respect of I and V. The minimum inversion intensity and the inversion potential vary in the order: violet < blue < green < yellow < red and, I2 H2 < air. ±δi is independent of, but possibly favoured by, selective light absorption, other conditions being equal. Compared with the galvanometer, the use of a counter-scaler is better adapted as a detector over a wider range of operative conditions. A second inversion of −δi into + δi at large V, observed earlier, is now confirmed using a counter-sealer. The results agree with Joshi's theory that photoemission from an adsorption-like wall layer with a low work function formed under the discharge gives + δi per se. −δi is ascribed to a negative space charge produced near the cathode at large I and V.