Paschen's Law

Paschen's Law describes how the breakdown voltage of a gas—essentially the voltage needed to make an insulator conductive—depends on the pressure of the gas and the distance between two electrodes. Specifically, it shows that, for certain pressures and distances, there is a minimum voltage required to create an electrical discharge, like a spark. At very low or very high pressures, this required voltage increases, but at a certain pressure range, it decreases to a minimum before rising again. This law is important in designing electrical equipment and understanding phenomena like lightning and sparks.