Abstract
The thermal convective instability of a nematic layer aligned perpendicular to horizontal plates displays original characteristics, due to the coupling of the nematic distortion with the temperature gradient; in particular the adverse temperature gradient threshold ΔTc can be modified by the application of a vertical (stabilizing) or horizontal (destabilizing) magnetic field. In addition, the application of a magnetic field H controls both the threshold of this instability and the geometric form of the instabilities above ΔTc.