The diode was the first thermionic device. Sir John Ambrose Fleming produced the first diode for radio detection, his Oscillation Valve, in 1904. The term Oscillation Valve was used to describe the fact that his device would pass current in one direction only and therefore make audiable the high frequency radio frequency oscillations. Today the diode is used as a term for a signal detector.
The filament in the early devices had to be strongly heated to emit electrons. Practical valves used tungsten wire heated to 2,800 °C. At this temperature the filament was as bright as a light bulb.
The power rectifier is also a diode but with a heavy duty cathode. The diode conducts when the anode is positive of the cathode. Normally a series resistance is included in the circuit to limit the current.
An indirectly heated oxide cathode would be suitable for anode voltages up to 1000 Volts. Above this figure bombardment of the cathode would 'poison' the emissive surface and so reduce the life of the device. Thoriated tungsten filaments would be used for high voltage service. For very high power pure tungsten would also be used as a cathode, this was popular until the cost of the heating power became a significant factor in later years.