The propagation characteristics of a 70 GHz slow surface wave in a transversely magnetised, partially filled, parallel-plate, solid-plasma waveguide have been investigated experimentally using n-InSb as the plasma material to which a pulsed current was applied. The surface-wave resonant magnetic field increased substantially with an increase in the applied current. Subsequently, a variation of the electron density of the plasma material with the current was determined by measuring the reflection from the plasma slab. The results indicate that the variation of the propagation characteristics is primarily explained by the variation of the electron density in the plasma material, and that the propagation characteristics of the slow surface wave in the above mentioned plasma waveguide can be controlled electronically by applying a current to the solid-plasma material.