We report experiments measuring the threshold electric field for electrohydrodynamic instabilities in nematic liquid crystals as a function of magnetic field applied parallel to the electric field. At low magnetic fields, the threshold voltage decreases in quantitative agreement with theoretical predictions. However, at higher magnetic fields, a crossover is seen and the threshold voltage increases linearly with the magnetic field. In this region, the bifurcation from conduction to convection is subcritical, with the discontinuity increasing linearly with magnetic field. The wavelength exhibited by the roll structure at onset also increases dramatically with the applied magnetic field.
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