Flew kites to demonstrate that
lightning is a form of static electricity (ESD).
He would run a wire to the kite and produce sparks at the
ground, or charge a Leyden jar. This led Franklin to invent
the lightning rod.
Franklin also made
several electrostatic generators with rotating glass balls
to experiment with.
These experiments led him to formulate
the single fluid (imponderable fluid) theory of electricity.
Previous theories had held there were two electrical fluids
and two magnetic fluids. Franklin theorized just one
imponderable electrical fluid (a fluid under conservation)
in the universe.
The difference in electrical charges was
explained by an excess (+) or defect (-) of the single
electrical fluid. This is where the positive and
negative symbols come from in electrical science.