Cathode heater
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A cathode heater is a coil or filament used to heat the cathode in a vacuum tube or cathode ray tube. Before transistors and integrated circuits came into widespread use, electronic devices used vacuum tubes as switching elements. The cathode element had to achieve the required temperature in order for these tubes to function properly. This is why older electronics often needed some time to "warm up" after being powered on; this phenomenon can be observed in the cathode ray tubes of modern televisions and computer monitors.
The simplest type of vacuum tube operates as a diode: that is, it allows current to flow in only one direction. The cathode heater is used to raise the temperature of the cathode filament, permitting thermionic emission of electrons into the evacuated tube. The other element inside the tube is called the "plate", or anode. If the anode is positively charged relative to the cathode, the emitted electrons will be attracted to it, and current will flow. This exhibits the characteristics of a diode as current flow in the reverse direction is not possible (the anode is not heated, prevention thermionic emission.) More complex vacuum tubes operated as triodes (the predecessor to the modern transistor) or other circuit elements, but all tubes required some type of cathode heater in order to trigger electron emissions.
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