Location of impact points in the impulse flashover of air from a pointed electrode

The position of the flashover impact point on the plane electrode in a point-plane system has been investigated using both paper and aluminium foil to record the impact points. The impact points are close to the centre at low voltages where the probability of flashover is very low. The spread increases as the voltage is raised through the 100% flashover level, then closes in again towards the centre as higher overvoltages are applied. For a long gap the widest deviation of the impact points from the centre is produced by a negative voltage, but with a small gap the widest deviation is produced by a positive voltage. Although the two methods of recording the impact points yield substantially the same distribution when flashover occurs, when the paper is used it considerably reduces the negative flashover voltage so that the usual polarity effect disappears. These results are discussed in terms of space-charge fields, which first modify the trajectory of streamers from the point electrode, and secondly initiate a triggering discharge between the plane electrode and the recording paper