Is air a PERFECT electrical insulator?
There is no such thing as a PERFECT electrical insulator, not even a perfect vacuum, since an electrical current will flow in anything if the electrical field is strong enough. Dry air is a good insulator but doesn’t even come close to being perfect. Better than dry air is the gas sulfur hexafluoride, used to insulate very high voltages, but even SF6 will break down and conduct electricity if the electrical field is strong enough. A perfect insulator would not allow electrons to move at all. In solids, Teflon (PTFE) is close to perfect, but it too will break down and conduct electricity if enough potential difference is applied. In vacuum, electrons must first be liberated from a surface, by field emission or thermionic emission in the presence of an electrical field, but once in the vacuum the electrons are free to follow the electrical field lines unimpeded.
