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A multiple exposure picture (one of 68 images created by Century Magazine photographer Dickenson Alley) of Tesla sitting in his Colorado Springs laboratory with his “magnifying transmitter” [= Tesla Coil] generating millions of volts. The 7-metre (23 ft) long arcs were not part of the normal operation, but only produced for effect by rapidly cycling the power switch.[2]
What is happening in that photo?
Here’s a similar photo on Wikipedia. Image description: