An electrostatic charge of −600 volts is uniformly distributed over the surface of the drum by a corona discharge from a corona unit (Corotron), with output limited by a control grid or screen. This effect can also be achieved with the use of a contact roller with a charge applied to it. Essentially, a corona discharge is generated by a narrow wire 1⁄4 to 1⁄2 inch (6.35 to 12.7 mm) apart from the photoconductor.