Around 1750, Benjamin Franklin invented the lightning rod, a sort of pointed metal decoy designed to be struck by a lightning bolt that might otherwise strike a more fragile part of a building. Nikola Tesla thought this was a terrible idea. The pointed end of Franklin's lightning rod, he argued, actually attracts bolts of lightning.
[T]he pointed lightning-rod is quite ineffective in the one respect noted, it has the property of attracting lightning to a high degree . . . . and in this feature lies the chief disadvantage of the Franklin type of apparatus.
Tesla set out to replace the pointed lightning rod, and in 1918, he received a patent (1,266,175) on his new design that was rounded rather than pointed: a lightning umbrella.
"the probability of being struck is decreased by the presence of my protector,
whereas it would be increased by the presence of the Franklin rod"
Tesla believed that his new design was "equivalent to a repellent force" against lightning.
The improved protector . . . behaves in a manner just opposite to the Franklin type and is incomparably safer for this reason.
Later studies have shown that lightning strikes are essentially unpreventable, vindicating Franklin's design. If lightning is going to strike anyway, then you want a rod that attracts lightning, to keep the lightning from striking someplace where it might do significant damage.
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