A fascinating feature at the Boston Museum of Science gives audiences an up-close look at energy in action. Two pillars topped with enormous, hollow aluminium orbs flash an almost sinister glow, making them seem better suited to a mad scientist’s lair. Lightning sizzles as it streaks from the massive machine.

The colossal contraption responsible for the indoor storms is the world’s largest air-insulated Van de Graaff generator. An average of two million volts surge through the 40-foot-high machine. With the help of several Tesla coils, the generator puts on a cracking display of static electricity.

Van de Graaff generators use an internal, moving belt to produce an electric charge. The belt employs friction to generate electricity the same way a prankster would when rubbing a balloon on top of someone’s head to make their hair stand up—except on a much larger scale. A large Van de Graaff generator can make an electric charge strong enough to shoot bolts of lightning and even power particle accelerators.

The machine on display at the Boston Museum of Science is the star of the museum’s Theater of Electricity. Dr. Robert J. Van de Graaff, a professor at the Massachusetts Institute of Technology, built it in the early 1930s to power the university’s particle accelerator. Since it retired from its duties of helping the bright minds of the 20th century research high-energy X-rays and atom smashing, the generator spends its time sparking curiosity and wonder among the audiences who attend its daily shows.