THYRATRON

gas-filled, three-element, hot-cathode tube in which the grid controls only the starting of a current and thus provides a trigger effect. The normal grid potential is negative with respect to the cathode, which prevents electrons from flowing to the plate and exciting a discharge. At a chosen instant the grid potential is raised enough to start a discharge (i.e., the grid signal triggers the discharge), initiating a large flow of current through the tube. The discharge stops when the anode voltage is sufficiently lowered. Thyratron tubes are useful in devices, such as rectifiers and inverters, that convert ac (alternating current) to dc (direct current) or vice versa.