 Since electrically charged conductors do not have any electric field inside, charges can be added continuously from the inside without increasing them to the full potential of the outer shell.Since a Van de Graaff generator can supply the same small current at almost any level of electrical potential, it is an example of a nearly ideal current source.The maximal achievable potential is roughly equal to the sphere radius "R" multiplied by the electric field "E"max at which corona discharges begin to form within the surrounding gas.