The general features of the interaction of hydrodynamic and electromagnetic effects, when an electrically conducting fluid medium is in motion in the presence of a magnetic field, are now well established. A discussion of these effects in which the equations of the system are set out and an analogy is drawn between the behaviour of the magnetic field and that of vorticity in a viscous fluid, is given by Batchelor [1]. The development of the subject may be divided broadly into two sections. First, there are the problems of engineering interest, such as flow of conducting fluid in pipes, with associated problems of stability, and secondly there are the problems of astrophysical and geophysical interest.