Discharge tubes contain electrodes in a gas chamber, generating voltage and current from chemical reactions. They emit light and act as electrical switches, with neon and xenon commonly used for illumination. Hot cathodes emit electrons through heat, while cold cathodes emit light without heating.
A discharge tube is an object that contains a certain arrangement of electrodes within a gas chamber. Usually, these objects are designed within glass shields; however, some use ceramic or enameled metal. They work by applying voltage to start the movement of particles through electrical conductors. This causes ionization of the gas inside the exhaust pipe. The end result is gas discharge tube illumination.
Discharge tubes are electrochemical cells, which means they generate electromotive force, or voltage, as well as current from the chemical reaction in the gas. Electrons are released and accepted at each end of the tube, just like a battery. A discharge differs from a battery, however, in that it generally doesn’t act as a power source. When the electrons leave the gas cell, this is called oxidation. However, when electrons enter the cell, it is known as reduction.
Gas discharge tubes are considered cathodes as opposed to anodes, in terms of electrochemical cells. This means that electric current flows out of the electrode. Polarity can be either positive or negative depending on design or function, however, most are usually negative. Cations, ions with more protons than electrons and therefore positively charged, are attracted to the cathode. Conversely, anions, ions with more electrons than protons and negatively charged, drift apart. Essentially, any discharge tube that produces energy is positive, and any tube that consumes energy is negative.
A vacuum waste tube is known as a hot cathode. This emits electrons caused by thermionic emission, the flow of electrons or ions caused by heat. Basically, a hot cathode uses an electrical filament to emit electrons. Examples include electron guns, an electron microscope, and most fluorescent lamps. Cold cathodes, on the other hand, are discharge tubes that aren’t independently heated, but are still capable of emitting light, like the liquid crystal displays in televisions and computer screens.
One of the most popular uses of a gas discharge tube is to act as an electrical switch. Ignitron, krytron and thyratron are the most common examples of gases used in this technology. These are not generally used to amplify signals, as a normal vacuum tube would, but are designed to interrupt and divert a current to one conductor or another.
Due to the ionizing effects of discharge tubes, one of the most common uses is for neon lights. Other similar devices use xenon in the form of flash lamps seen in cameras and strobe lights. Another common use for the waste tube is in the Geiger counter.
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