Joint Instability of Hydromagnetic Fields which are Separately Stable

It is found that certain parallel magnetic fields can destabilize the plane Couette flow of a perfectly conducting fluid, though this flow is known to be stable when there is no field. The destabilizing role of the rigidity of the field is attributed to its ability to prevent the stretching of curves of constant perturbation vorticity by the shear. Unstable long waves transfer energy to the mean magnetic field from the kinetic field by means of the Reynolds stresses. This suggests the possibility of maintaining an electrical current which is initially introduced between two differentially rotating cylinders.