## Abstract

Experiments on the onset of thermal convection in a rotating layer of mercury heated from below and subject to an impressed magnetic field are described. Experiments have been performed for values of the parameter Q$_{1}$ (=$\sigma $H$^{2}$d$^{2}$/$\pi ^{2}\rho \nu $, where H denotes the strength of the magnetic field, $\rho $ the density of fluid and $\sigma $ the coefficient of electrical conductivity) varying between 9$\cdot $85 and 2$\cdot $01 $\times $ 10$^{4}$ and a value approximately 10$^{6}$ for the parameter T$_{1}$ (= 4$\Omega ^{2}$d$^{4}$/$\pi ^{4}\nu ^{2}$, where $\Omega $ is the angular velocity of rotation, d the depth of the layer and $\nu $ the coefficient of kinematic viscosity). The critical Rayleigh number R$_{c}$ for the onset of instability as well as the manner of its occurrence-overstability or cellular convection-have been determined for fourteen different values of Q$_{1}$. In agreement with Chandrasekhar's theoretical predictions, the experiments confirm that the transition between overstability and convection occurs discontinuously at a critical field strength (when the angular velocity is maintained constant). The nature of the dependence of R$_{c}$ and Q$_{1}$ (for given T$_{1}$) is also in agreement with theory.