The onset of natural convection has been investigated in a CuSO₄-H₂SO₄-H₂O solution placed between parallel and horizontal plane electrodes. The onset time was obtained experimentally by directly comparing the current of diffusion control with that of natural convection control. The critical Rayleigh number based on the Nernst diffusion layer thickness was found to be 1250±100. This critical time marking the detection of convection was four times larger than that predicted by the propagation theory.