The properties of the cathode-directed positive streamer discharge, propagating in a uniform electric field, have been investigated. Precautions have been taken to keep the effects of space charge, due to branching, to a minimum, and conditions under which streamers possess minimum initial energy have been obtained. In this way, a threshold field of 440 kV m-1 for the propagation of a single streamer has been determined. The velocity of streamers in the uniform field has been studied. This rises linearly with the electric field and the results confirm that space-charge effects are negligible. Comparison is made with previously published work.

Measurements are reported of the variation of the electric field required for propagation of positive streamers in air as a function of ambient temperature. Propagation velocities have also been measured as function of field and temperature. The results are discussed in terms of changes of air density consequent upon the temperature changes and comparisons are made with other work in which density has been changed by variation of pressure at constant temperature. There is some evidence that specific temperature effects exist. The variation of streamer velocity with air density indicates that a constant reduced total field E/N is maintained in the avalanches, replicating the streamer tip as the density is changed.

The positive streamer in air has been shown to exhibit reproducible properties which can be measured precisely in a uniform electric field; examples are the field are the field required for its propagation and its velocity as a function of field. It is thus an excellent tool for assessing the influence of the material upon discharge propagation over insulating surfaces. The paper compares streamer propenies in air alone with those when propagated over a surface in a uniform field. In the latter case, streamers are usually found to propagate in two modes, namely, a "fast" component on the surface and a slower one in the ambient air. Materials studied include ceramic, PTFE and silicone rubber; each material is shown to have its own "signature" in terms of light output, minimum field for propagation and velocity over the surface.

Arabia. He is currently working towards the Ph.D. degree in the School of Mechanical Engineering,