3 megavolt <a href="https://en.wikipedia.org/wiki/Cockcroft-Walton_generator" class="extiw" title="w:Cockcroft-Walton generator">Cockcroft-Walton particle accelerator</a> at <a href="https://en.wikipedia.org/wiki/Kaiser_Wilhelm_Institute_of_Physics" class="extiw" title="w:Kaiser Wilhelm Institute of Physics">Kaiser Wilhelm Institute of Physics</a> 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 <i>(not visible)</i>.  Subatomic particles are accelerated to high speeds in the tube by the high potential.  The black vertical segments on each stack are <a href="https://en.wikipedia.org/wiki/capacitor" class="extiw" title="w:capacitor">capacitors</a> which store the charge, while the diagonal "rungs" between the columns are <a href="https://en.wikipedia.org/wiki/vacuum_tube" class="extiw" title="w:vacuum tube">vacuum tube</a> rectifiers called <a href="https://en.wikipedia.org/wiki/kenotron" class="extiw" title="w:kenotron">kenotrons</a>, 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 <a href="https://en.wikipedia.org/wiki/corona_discharge" class="extiw" title="w:corona discharge">corona discharge</a> which causes leakage of current into the air.   The output voltage of this machine was close to the limit for open-air electrostatic generators; even modern Cockcroft-Walton machines cannot produce more than about 5 megavolts.<br><br>
Alterations to image: cloned in a small amount of wall behind lefthand column to replace overlap of adjacent picture on page.
Unknown author<span style="display: none;">Unknown author</span>
Retrieved April 12, 2015 from <a rel="nofollow" class="external text" href="https://books.google.com/books?id=bSgDAAAAMBAJ&amp;pg=PA53">"World's biggest atom smasher uses 3,000,000 volts" in <i>Popular Science Monthly</i>, Popular Science Publishing Co., New York, Vol. 131, No. 4, October 1937, p. 53</a> on Google Books