The aiming of the bolt: how a flashover finds the weak spot

Purpose: To find how a spark from a positive or negative rod locates an object to connect to, or finds a weak spot in a cable insulation. Experimental: 1-2-m gap in ambient air between a rod and a horizontal wire suspended 0.25-0.5 m above ground, subjected to fast-rise (1.2 /spl mu/s) and slow-rise (150 /spl mu/s) positive and negative impulse voltages, and observed with photomultipliers, streak/framing camera, and open-shutter cameras. Main result : A leader channel from the rod feels its way by spraying a conical volume in front with streamers. When these connect to object(s) the leader follows either the direction of the highest streamer density or the most conductive streamer channel (our cameras cannot decide). If the streamers do not connect the influence of the missed object(s) is small. Additional : Evidence has been found for the slow increase with time of the field-to gas density ratio E/n in streamer channels due to channel heating and subsequent gas density reduction.