A second smaller grounded bar stands nearby.
As the large ball charges, the electric field eventually becomes large enough to cause dielectric breakdown of the air. The nuclei of the atoms in the air are pulled in the direction of the electric field, while the electrons are pulled in the opposite direction. When the electric field is large enough, some of the electrons are pulled completely of their atoms.
Once this happens, the electrons and the charged ion left behind each begin to rapidly accelerate. The collide with other atoms, hastening the ionization process of those ions. The resulting avalanche, an "arc," is what happens with a lightning strike.
The arc discharges the large ball, after which it must charge again before another arc can occur.
| A grounded ball is brought near the Van de Graaff. The pops are the sounds of arcs. | Video. | Still. |
| When a grounded pin is brought near the Van de Graaff, no arcs occur. The field is great enough at the tip of the pin to cause ionization and discharge the ball before a large charge can build up. This is the principle behind lightning rods. | Video. | Still. |
