[Liberal Classic]

Greatest single invention was the lightning rod. To the topic of this thread, Franklin purposely refused to patent his invention.

[bvw]

I'd argue that the lightning rod and the piped iron stove are both a paired set that are the greatest -- for they saved many thousands of lives from fire.

The fire department and fire insurance is another, but those might best be viewed originally as a gang-control device as I understand.

[Wonder Warthog]

"Greatest single invention was the lightning rod."

Not. It might have been FRANKLIN's greatest invention (althoug h I would argue that bifocals have had more long-term impact). The greatest SINGLE invention was the generation and tranmission of AC current (Tesla). That is the thing that makes our current (no pun intended) civilization possible.