English: High voltage laboratory at the U.S. National Bureau of Standards, Washington DC, USA, in 1948. The caption of the image in the source is: "A special 6-story laboratory at the Bureau houses a 1,400,000 volt x-ray generator (rear) and a 1,050,000 volt cascade transformer (foreground). The latter is used for testing and calibrating commercial power equipment as well as for producing simulated lightning, used in investigating protective measures in installations and aircraft." The cascade transformer is shown in operation, producing a long electric spark. The luminous glow visible on the wires carrying the high voltage is called corona discharge, and is caused by electricity leaking into the air. The cascade transformer in the black column (right) that generates the high voltage consists of several transformers connected in cascade, with each successive transformer at the secondary potential of the previous one, so no single transformer winding will have to withstand the full output voltage.