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To prevent any remaining gases from remaining in a free state in a vacuum tube, modern tubes are constructed with "getters", which are usually small, circular troughs filled with metals that oxidize quickly, barium being the most common. Once the tube envelope is evacuated and sealed, the getter is heated to a high temperature (usually by means of RF induction heating) causing the material to evaporate, absorbing/reacting with any residual gases and usually leaving a silver-colored metallic deposit on the inside of the envelope of the tube. If a tube develops a crack in the envelope, this deposit turns a white color when it reacts with atmospheric oxygen. Large transmitting and specialized tubes often use more exotic getters.
Other Getter Types
In addition to barium, aluminum, magnesium, calcium, sodium, strontium, and phosphorus may be used as getters. These getters are also known as "flashed getters" since the element is quickly vaporized and deposited on the tube envelope. The metallic getters, aluminum, magnesium, calcium, sodium, and strontium, look much like the common barium getter. The phosporus getter looks very different. The phosphorus getter produces a yellowish white deposit on the tube envelope. This deposit can give the appearance of a spoiled vacuum when the vacuum is actually intact. One unique clue to discern a phosphorus getter is that this deposit produces a rainbow effect when light shines on it.
High power transmitting and industrial tubes may not employ a flash type getter. In these tubes, the getter material is either the actual tube plate, or the getter material coats the plate structure of the tube. The plate operates at a red heat which serves to activate the getter material to absorb the oxygen and trace gases in the tube. Two common materials are carbon (usually in the form of graphite) and zirconium.
References
Stokes, John W. 70 Years of Radio Tubes and Valves: A Guide for Engineers, Historians, and Collectors. Vestal Press, 1982.
Reich, Herbert J. Principles of Electron Tubes. Understanding and Designing Simple Circuits. Audio Amateur Radio Publication, May 1995. (Reprint of 1941 original).
"Getter"