A thyratron is provided in which an anode 2 and at least a control grid 3 are included in a screening box 10 provided to operate at cathode potential, and a screening electrode 11 is interposed between the anode and control grid, is carried by the screening box and is electrically common therewith. As shown the screening electrode is of double layer construction -11', 11''- and a preionising grid is also included within the screening box 10. IMAGE
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