Van de Graaff Generator

In 1929, the American physicist Robert Van de Graaff designed a machine, now called the Van de Graaff generator, capable of building up an extremely large electrostatic potential difference, of the order of several million volts (up to about 10^7 V). It works by combining electrostatic induction and action at points (corona discharge), two principles developed earlier in this chapter. A large hollow spherical conductor is mounted on top of an insulating stand. A continuous insulating belt, driven by a motor over two pulleys, runs from near the ground up into the interior of the hollow sphere. Near the bottom, a metal comb with sharp points, connected to a high-voltage source, sprays charge onto the moving belt by corona discharge (action at points). The belt physically carries this charge upward and into the interior of the hollow sphere, where a second sharp-pointed comb, connected to the sphere's inner surface, collects the charge off the belt (again by action at points, now acting in reverse to strip charge from the belt onto the comb). Because the field inside a hollow conductor is always zero (electrostatic shielding, section 1.7.2) regardless of how much charge the outer sphere already carries, the incoming charge is always able to flow onto the sphere's inner surface, where it immediately redistributes to the outer surface, and the whole process repeats continuously, allowing charge -- and hence the sphere's potential -- to build up to extremely high values, limited in practice mainly by dielectric …