Sparkon

Meet Sparkon, a shiny metal dome who can hold a lot of charge, until the air beside him gives up.

Air is normally a good insulator. It has very few free charges, so it blocks current. But if the electric field becomes strong enough, the few stray electrons in the air are pulled so hard that they knock more electrons free from the molecules they hit.

This runs away in an avalanche, the air turns into a conducting plasma, and a spark jumps across. For dry air at normal pressure this happens at a field of about 3 × 10⁶ V/m, or 3 kV for every millimetre of gap.

A Van de Graaff dome with a radius of 15 cm reaches that field at its surface when its voltage is about 450 kV, so it cannot be charged any higher. Bigger domes can hold more voltage before sparking.

Lightning is the same breakdown on a giant scale. High-voltage equipment uses smooth rounded shapes, because sharp points concentrate the field and spark first.