An apparatus (Machlett's Dynalyzer II system) is described that makes real-time dynamic tests on the output and performance of the X-ray tube and generator so that a single exposure gives information on mA and mAs; exposure time (ms); kVp anode to earth; kVp cathode to earth; kVp cathode to anode; tube filament current; line voltage and radiation output of the tube (mR). The method of use is described together with the results of comparisons made with other test equipment. Some novel design features are of particular interest. The results show that by using this apparatus present design characteristics of X-ray generator-control-tube systems are such that the accuracy of calibration can be improved by at least an order of magnitude.