engineering question #2350

Ernie Epp, a 75 year old male from Kenora, ON Canada asks on November 4, 2004,


Regarding the behaviour of a high-voltage spark. I have a device which generates a pulse of high-voltage of about 20,000 volts at about two times per second rate. Its called an electric fence energizer and is normally used to control cattle inside a pasture fence. The fence wire is insulated and the cattle will avoid attempting to push the wire out of the way due to the jolt that they will receive if they touch the wire. The inside of the container which is made out of sheet metal contains a so-called spark gap of about 2 cm which I believe is placed there in order to protect the pulse transformer from lightning induced voltages or excessive battery voltage. The permanent gap placed there wil limit the maximum voltage that will appear at the transfomer secondary winding. My question is, when this energizer is tested on the workbench with the lid rmoved, the spark can be observed jumping across the gap of the fixed spark gap. The spark gap actually consists of eight possible spark directing points and are all about the same distance from the metal container surface. When the sparking occurs, the location of the spark changes from place to place among these eight points. Why does the place that the spark occurs move to different point locations? My personal logic tells me that the spark should occur at the point of shortest gap continually. It doesn't . It moves between the various eight points here and there. I doubt that all of these points are all of exactly the same distance from the metal case so why doesn't the spark over to the case not pick the easiest route continually? Is the air being ionized randomly by stray radioactivity or cosmic rays or by neutrinos?

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the answer

John Jones answered on November 22, 2004, A:

As the questioner suggests, it is the ionization of the air that makes the path of the spark change. Stray radioactivity or cosmic rays will ionise air (neutrinos won't), but I suspect the most important source of ionisation here is the sparking itself: the first spark to discharge will produce ions all along its path, and these ions will be mixed in with the surrounding molecules, turbulently and unpredictably, by the currents generated by the spark's heating of the air. Of the ions produced by a spark, about half will recombine within 0.7 seconds, and about 90% after 6 seconds. So if the second spark occurs within a few seconds of the first, the spatial distribution of the ions produced by the first spark will be the chief factor determining the path of the second spark.

It would be interesting to see what happened if the device were allowed to rest for a few minutes in between each spark, so that the ions generated by the first spark would all have recombined before the second spark appeared.

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