English: Control board and a voltage multiplier stack in the 3 megavolt Cockcroft-Walton particle accelerator at Kaiser Wilhelm Institute of Physics in Berlin in 1937. The source claims it was the world's most powerful accelerator at the time. It consisted of two 4 stage Cockcroft-Walton voltage multiplier stacks of opposite polarity, with the high potential appearing at the top of the stacks applied to electrodes at opposite ends of an evacuated accelerator tube. Subatomic particles are accelerated to high speeds in the tube by the high potential. A multiplier stack is visible behind the control desk. The black vertical segments are capacitors which store the charge, while the diagonal "rungs" between the columns are vacuum tube rectifiers called kenotrons, which only allow charge to pass in one direction. An alternating voltage of several hundred kilovolts is applied between the bottom of the columns, which act as a "charge pump" forcing charge into the top electrode. All exposed parts at high potential must have smooth gently curving surfaces to prevent corona discharge which causes leakage of current into the air. For safety, after setting the controls of the machine at the console shown, the operator left the room and operated the machine from another room, wearing dark glasses to protect their eyes from the brilliant arcs.