Sparks leaping between two brass spheres connected to a Wimshurst machine, an early type of electrostatic generator. The potential difference between the two spheres is approximately 4000 volts. Notice the mica vanes of the charging discs visible in the background. The Wimshurst machine was superseded by the Van de Graaff generator, invented in 1931 by Robert Van De Graaff at the Massachusetts Institute of Technology (MIT). By storing the electric charge on a hollow metal sphere the Van de Graaff generator was able to achieve potentials many times larger than those produced by the Wimshurst machine.