Van de Graaff generator
A moving belt carries charge from a lower comb to a hollow metal sphere, building up enormous static potential. When the electric field at the surface exceeds the dielectric breakdown threshold of air (~3 MV/m), corona discharge begins and sparks leap to a nearby grounding sphere. Hair strands on the dome stand on end as like charges repel.
The Van de Graaff generator was invented by Robert J. Van de Graaff in 1929 at Princeton University. A motor-driven insulating belt transports charge from a lower corona point (or comb) to the interior of a hollow metal sphere. Because excess charge on a conductor resides entirely on the outer surface (a consequence of Gauss's law), the belt can continuously deliver charge to the interior, and the sphere's potential rises without limit — in principle.
In practice, the potential is limited by dielectric breakdown of the surrounding air. When the electric field at the sphere's surface exceeds approximately 3 MV/m (for dry air at sea level), air molecules ionize and a corona discharge begins, leaking charge. At higher fields or when a grounded conductor is nearby, a full spark discharge occurs: a branching plasma channel that neutralizes the accumulated charge in microseconds.
Humidity dramatically affects the breakdown threshold. Water molecules are polar and provide free charge carriers; humid air breaks down at lower field strengths. This is why Van de Graaff demonstrations work best on dry winter days.
The "hair-raising" effect is a direct consequence of Coulomb repulsion. Hair strands touching the charged sphere acquire like charges and repel each other, standing on end. The force on each strand is proportional to Q²/r².