Variations in the distribution of steam bubble, the "void" distribution, in a boiling channel as a function of changes in heating power were studied. A rectangular test tube, of 1.11 x 4.44-cm cross section and 127-cm height, was inserted in a forced-circulation pressure loop. The tube was heated by passing an a-c current through the tube walls. A power oscillator was built which could give a 10% peak-topeak sinusoidal power modulation at any frequency in the interval from 0.01 to 10 cps. Variations in the volume fraction of steam were observed by means of a gamma densitometer built for the purpose. Accurate void profiles could be taken by traversing the test channel vertically and horizontally. With the void detector stationary at a given height, the amplitude and phase delay of the steam void variations were measured in the frequency range mentioned. The signal from the gamma detector was passed to a harmonic analyzer built for the experiment. This instrument could pick out the void variations coherent with the power variation in the presence of much greater random signal variations caused by the boiling process. The frequency response of steam void was measured at 4 different pressures ranging from 27.2 tomore » 68 atms, at conditions comparable to those in pressurized boiling water reactors. Void phase and void amplitude are plotted as functions of frequency, and the data are also presented in tables. The most important result of the experiments is to show that the void response falls off at a frequency that is much lower than that predicted by theoretically derived power-to-void transfer functions used previously in reactor calculations. Also, the void amplitude in the lower part of the channel was larger than expected. By taking into account the pressure changes in the channel caused by the power variations, an expression was derived for the power-to-void transfer function that could be fitted very well to the data. A constant, associated with the completeness of the mixing in the direction perpendicular to flow, had to be chosen in order to fit properly the break frequency in the amplitude curve. (auth)« less