Van de Graaff accelerator It is based on the principle that the net charge on a conductor resides on its surface. Fig. v1 shows the elements of such an accelerator. Charge is transferred at a high potential (~ 10⁴ V) on to a closed moving belt made of an insulating material via a metal comb (A). The charge on the belt is transferred to a large metallic spherical dome through the comb (B). The charge will accumulate on the surface of the sphere until its potential rise to the breakdown value. The ions produced by a ion source placed inside the sphere would accelerate between the high potential difference ~ 10⁶ V of the sphere and ground. Such machines are generally used for the study of nuclear collisions and are called pelletron.