Using ordinary diodes and also diodes having a getter chamber, the initial decay of thermionic emission from oxide-coated cathodes over about 0.1 second was observed on the conditions of various cathode temperatures and also various anode voltages, and the change of initial decay was examined by keeping a cathode at high temperature and also by heating or cooling a getter deposit. As a result, it was found that by heat treatment of cathode at high temperature in the ordinary diode the initial decay of saturation current at observation temperature became slower than that before the treatment, that the decay characteristic could be restored to that before the treatment by applying high anode voltage and drawing out emission current, and that these phenomena similarly occurred also in the case of heating a getter deposit in both tubes. Also an initial decay of emission current more rapid than the initial decay of saturation current was found to occur at low anode voltages. In order to explain the above-mentioned phenomena, the poisoning of cathode due to residual gases was considered as the main cause of initial decay of saturation current, while the drop of anode potential caused by charging up of contamination on the anode was considered as the main cause of initial decay of emission current at low anode voltage
