Studies of the first time dependence in Rayleigh-Bénard convection in ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ are reported. The aspect ratio $\Gamma$ could be varied continuously. Data at 31 $\Gamma$ values with $4<\mathrm{}\Gamma <13\mathrm{}$ show that near onset the first time dependence is always periodic and associated with changes in the heat transport. The dominant periodicity has a nonzero amplitude, $A$, and vanishing frequency at onset. Changes in $A$ with $\Gamma$ show wave-number effects. Except for the time dependence, the transition is consistent with predictions for the skew-varicose instability.

• Received 11 February 1983

DOI:https://doi.org/10.1103/PhysRevLett.50.1199