Here you can see a pendulum that draws an electrical arc from a high voltage Transformer. More details: As the pendulum gets near the static ball the current jumps over because of the high voltage. Due to the high power, the air turns into plasma (free moving electrons which are no longer bound to the atomic nucleus). Because of the free moving electrons, a plasma is very conductive. That's why the current keeps flowing even though the pendulum travels away from the static metal ball. This increases the length of the arc. It also travels upwards because of the heat.