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A diode has been designed and used for testing various samples of cathode material in several plants and laboratories during recent years. Several criteria have been used for evaluating the emissive power of the various materials tested. To simulate the usual space-charge-limited emission test commonly used on receiving tubes, a cathode-temperature versus emission characteristic has been taken on each test lot. Temperature-limited emission has been examined under both low-field, low-temperature conditions and normal-temperature, high-field conditions. Results indicate that this method has several important advantages over the present approved method.