What I meant was a methode also to check the porosity of ignition cable, not only to detect a totally damaged cable, where the spark never more reaches the sparking plug.
Many of us have experienced, that the ignition system sometimes tends to behave strange, when the motor becomes wet, because it's cold and rainy. Then, the question arises, what is it? A break in the ignition coil, porous ignition cables, which become less insulating after some time, a damaged distributor, a break in one of the ignition plugs?
Because today all these parts are rather expensive, an exchange of all of them isn't useful. So, observing this ionization of air arround the ignition ables can help to find out, whether the source of error is an porous ignition cable.
This methode allows to find a damaged cable, which will still work perfectly at dry weather, but only becomes problematic when it's getting wet: The cable gets a blue aura at those places then, where "parts" of the high voltage leave the cable and run to grounded motor parts. It's very nice looking...

Yes, to see this glimming it should be rather dark. The more the less the damage is. But bigger damages can be seen very clearly, even if it's not so dark, as Terry mentioned.

Kai