The only [method] which we shall use in this thesis is to treat the field quantities as continuous functions, so that we can use the usual field equations of hydrodynamics and electromagnetism. Conditions for this assumption to be valid have been given by Bernstein et al. They are: a) Quadratic terms in the velocity and current density are negligible. b) The plasma is locally quasi-neutral. c) The ratio of the electron mass to the ion mass is negligible compared with unity. d) Unless otherwise stated, (i.e. in Chapter 2), all heating effects are to be neglected. We shall also assume a scalar pressure, the condition for this being that there should be many particle collisions during a characteristic time, and shall neglect the displacement current [?] in Maxwell’s equations, this being justifiable if the magnetohydro- dynamic velocity [?] is small compared with the velocity of light.
