Run the simulation to activate the motor and power the lower of two rollers. The rotating roller causes a belt to circulate between it and a second roller positioned above it. The lower roller and the motor are in a metal box, as is a metal brush located close to the lower roller and connected electrically to the box. A second brush positioned with its teeth facing the top of the upper roller, and enclosed with that roller in a hollow metal sphere, is similarly linked to its encasement.
You can pause the simulation at any point in the tutorial to turn off the generator's motor and the corresponding action of the rollers and belt. Notice that the charge on the metal sphere of the generator will remain constant when the motor is off, but if it is turned back on will continue building up electrons until the discharge point is reached (or the motor is halted again).