On February 12, 1935, U.S. Patent No. 1,991,236, issued to Robert Jemison Van de Graaff on an Electrostatic Generator:

Van de Graaff’s generator could generate direct-current voltages much higher than the 700,000-V which was the state of the art at the time using other methods, on the order of 1,500,000-V. between them.

Van de Graaff’s generator was originally developed as a particle accelerator for physics research, because its high potential can be used to accelerate subatomic particles to great speeds. It was the most powerful type of accelerator until the cyclotron was developed. Van de Graaff generators are still used as accelerators to generate energetic particle and X-ray beams for nuclear research and nuclear medicine, and really cool exhibits in science museums.