Paul,

     The experiment with a 12v battery that you describe is not measuring
     sparking potential. Even a 9v battery will make a spark if you first
     short the terminals then separate them by a small gap.

     This is because the air around the electrodes is already ionized,
     due to the current that was flowing while you were shorting them.
     The ions and the metal vapor from the contacts form a plasma that
     conducts current through the gap.

     But this does not happen unless you short the two contacts first.
     What we are concerned with is the dielectric breakdown voltage of
     air, which is the minimum voltage at which an insulator will
     breakdown and conduct.

     There is a minimum voltage that needs to be present in order for a
     spark to occur in air. This voltage varies among idfferent gases.
     See this Wikipedia article:

     http://en.wikipedia.org/wiki/High_voltage

     Go down to the section, "Sparks in Air." It states that the minimum
     sparkover voltage for air at standard temperature and pressure is
     380 volts. This is actually a little more than the 327 volts I
     quoted, which I got from this source:

     http://home.earthlink.net/~jimlux/hv/paschen.htm


     Regards,

     Gordon.

Cool you answer my question before I ask it. So it is sort of like
static and kinetic friction. Once it starts it gets easier to
continue.