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This Recommendation refers to hard (i.e. vacuum) tubes and valves for pulse modulator applications. Measurements on such tubes and valves are commonly made using nominally rectangular pulses and this will assumed to be the general case. The definitions and methods may, of course, be extended to include other forms of pulse. Vacuum switch tubes which operate at voltages in excess of 16 kV can be expected to procedure X-rays during the "turn-on" and "turn-off" periods of their "on" periods, and some very high voltage modulators may produce considerable quantities of soft X-rays during the "on" periods. Operators should ascertain that unsafe levels of radiation do not excist. The manufacturer should be consulted to determine the necessity for and the general design of suitable radiation shields.