The onset of convection in a horizontal layer of fluid subject to time-dependent heating from below is studied both experimentally and analytically. The fluid layer is confined by a rigid boundary at the bottom and a shear-free surface at the top. The fluid, which is initially quiescent, is heated by increasing the temperature of the lower surface at a constant time-rate. The experimental observation of the time of the onset of convection is made through the sudden change in variation of the temperature of the lower surface with respect to time. The onset is also observed through the sudden change in the fringe pattern created by holographic interferometry. Various layer depths are considered in order to observe the influence of this variable on the onset time. The analytical prediction is made by application of the linear amplification theory subject to boundary conditions similar to those of the experiment. Good agreement is found between the experimental and analytical results. Comparisons are also made with the experimental and analytical results available in the literature.

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