Karl,
that spark plug tester seems nice, it's a shame you do not have one anymore It would answer our questions. I always thought that the less pressure, the higher the resistance through the spark gap. And the more pressure, the less resistance. I assumed this because what is a spark? A spark is a plasma, that conducts electricity. But this plasma appears only when the atoms between the high voltage source and the ground (spark gap) are ionized so far, that they start to conduct electricity and a spark appears. So, if we have a vacuum in the spark gap, there are no atoms to ionize in order to get the electricity to flow. But if we have a high pressure gas between the electrodes, there is much more atoms there to ionize. I may be wrong, so please someone correct me
Thanks,
Jetijs