In school, I learned that Benjamin Franklin's lighting rods were very popular in France because they stopped the plague of churches catching fire from lighting strikes. At the time churches were the tallest things around, with a metal cross on top to boot, so were hotbed for lightning strikes. The French people were very upset because God kept smiting their churches. So Benjamin Franklin, while serving as ambassador, gave them lighting rods which solved this problem, and French opinion of the Colonies rose dramatically enough to support them in the Independence War.
Or so the story went.
I find it hard to believe that churches burning down was that much of a problem, or that solving it with a simple invention would have factored into the decision to support a war on the other side of the Atlantic. How big of a deal were the lightning rods really?
It's a cute theory, but like many cute theories it comes with major problems. That churches were always being hit by lightning and burning down , that it was interpreted as God disliking them, that lightning rods were immediately mounted on churches, and that gratitude for this was enough for France to send aid to the Colonies...there's a lot of dubious stuff, there. Plenty of churches survive from before 1700 in France: why wouldn't mounting lightning rods, instead of reforming sinners , have been seen by God ( and the Church) as making France deserving of other plagues: gratitude can motivate governments to award medals, but not enter alliances.
At the heart of this is Franklin's general diplomatic effort to make the American cause seem worthwhile for France, which was part public relations and part negotiation: partially a a result of his scientific inquires and interests he did become extremely popular there, and that was very important. But even with Franklin's efforts, the decisive event was the battle of Saratoga, one of the first ( and only) Colonial victories, which convinced the French that Americans actually had a chance of winning.