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The results of an experimental study of the critical thermal flux under transient conditions when a liquid bolls in a large volume are presented.The appearance of various new engineering systems with high thermal stresses at the hot surfaces has stimulated interest in the critical boiling mode under transient conditions. Dynamic effects of changes in the mode of heat transfer should be felt especially during an instantaneous increase in the heat load.Critical boiling during a sharp increase of heat load has been studied by several authors [1, 2]. Experimental results with organic liquids have been reported in [1], but no consideration was given there to the thickness of the heating element. Subsequent reports were published in [2] on similar experiments with water, but the authors there conclude that the critical heat flux in a large volume is the same under steady and transient conditions of heat transfer. Our studies have shown that such a conclusion can be valid only for a heating element the wall thickness of which exceeds a certain minimum.Our experiments were performed on an apparatus the electrical part of which is shown schematically in Fig. 1. The test element was a shaped plate of KhlSN9T steel with three lugs, for measuring the voltage drop across the active segment and for switching on the automatic control device. The active segment of the test element had an area of 30 x 10 mm with the active surface turned up. Its underside was thermally insulated by means of the Textolite segment 200 mm in radius over which the test element had been spread. Both were separated by a thin (0.2 mm thick) Teflon-4 sealing pad; owing to the water repellant qualities of the latter, the possibility of water oozing under the active surface was thus eliminated. The test element, together with the entire assembly, was placed in a vessel with a double wall containing doubly , Fig. 1. Basic electric circuit of the test apparatus: 1) active segment; 2) oscillograph; 3) vibrator; 4) amplifier.Chelyabinsk Polytechnical Institute.