Lightning was a supernatural scourge to the wooden cities of the 18th century. Churches were particularly susceptible, since they were often the tallest structures around, and a single electrical storm was known to lay waste to buildings across entire regions. In Franklin's lifetime, a bolt of lightning even killed 3,000 people in Italy after it struck a church basement packed with gunpowder. Aside from fervent praying, no one knew how to protect buildings from this "electrical fire."
Franklin retired from the publishing business at 42 to work full time on electrical experiments. After countless hours spent tinkering with static electricity, Franklin figured that if a metal rod could be fixed to the top of a building and wired to the ground with a cable, it could gently extract the "fire" from a cloud before it had a chance to do any damage.
Franklin sent news of his protective rod across the Atlantic, where it was first adopted in the churches and cathedrals of the French countryside.