NO2 during decomposition by the silent electric discharge at 5 kV (r.m.s.) gives the negativeJoshi-Effect, i.e., an instantaneous and reversible diminution, − Δi of 4 to 5% in the discharge current on irradiating the ozonizer with visible light. The effect is given at all stages of the cecomposition. A positiveJoshi-Effect, +Δi, is also obtained when the gas is exposed to potentials near the threshold value. Joshi’s oscillographic studies of the phenomenon and subsequent work in these laboratories thereon have shown that the effect is chiefly due to the suppression, on irradiation, of the high frequencies present in the discharge current.