Experiments on the instability of a layer of mercury heated from below and subject to the simultaneous action of a magnetic field and rotation

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 Q1 ( = σH2d2/π2pv, where H denotes the strength of the magnetic field, ρ the density of fluid and σ the coefficient of electrical conductivity) varying between 9·85 and 2·01 x 104 and a value approximately 106 for the parameter T1 ( = 4Ω2d4/π4v2, where Ω is the angular velocity of rotation, d the depth of the layer and v the coefficient of kinematic viscosity). The critical Rayleigh number Rc 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 Q1. 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 Rc and Q1(for given T1) is also in agreement with theory.