If no light is exposed to the drum, then the surface is not conductive and the resistance is high. This causes a corona discharge to occur from the wire. The discharge exposes the drum surface with positive and negative charges. Primary Corona Charging During the Primary Charging phase a corona unit exposes the drum surface with a uniform negative potential. This causes the surface of the drum to have a negative potential.
Positive charges occur in the base of the drum to balance the negative charges on the surface.
The light from a lamp exposes an original and the light reflected strikes the drum surface via a lens and mirrors. When light strikes a black color on the original the light is absorbed and not reflected back to the drum. When the light strikes a white area the light is reflected back to the drum. When light strikes other colors or grays the intensity of the light reflected back to the drum varies. When light strikes the photosensitive drum the resistance of the drum surface is lowered and becomes conductive which allows the positive and negative charges in those lighted areas to cancel out.