Grid-controlled x-ray tube,
an X-ray tube which is equipped with a grid, i.e. an extra electrode
between the cathode and anode to control the flow of electrons. The
third electrode is actually the focusing cup that surrounds the
filament. Normally, the focusing cup is kept at the same negative
potential as the filament. In a grid-controlled tube, the focusing
cup may be negatively charged (as compared to the filament) to such
an exist that the flow of electrons from the filament is completely
stopped. The voltage applied between the focusing cup and filament
may thus act like a switch to turn the tube current on and off. This
is particularly useful when very short exposures are needed, e.g. in
cinefluorography. Grid-controlled X-ray tubes provide secondary
switching as opposed to the primary switching provided by e.g.the
silicon controlled rectifier SCR . See also exposure switching.
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Useful Link :http://www.amershamhealth.com/medcyclopaedia/Volume%20I/grid%20controlled%20X%20ray%20tube.asp
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