National Geographic : 1993 Jul
SAFE BEHIND BARS n oversize birdcage keeps operator Don Salvatore from being jolted by a 2.5 million-volt Van de Graaff static electric ity generator at Bos ton's Museum of Science. The artificial lightning passes through the metal frame -a principle that protects motorists in thunderstorms. Grounded wires surrounding the exhibit serve as lightning rods to protect visitors. Physicist Robert J. Van de Graaff built this model in 1931. Reaching toward thunder clouds, skyscrapers can initiate lightning. Branches streaking upward from an antenna atop New York's World Trade Center (below) typify ground-to-cloud lightning, common from tall buildings and mountaintops.