In June 1752 Ben Franklin conducted his famous “kite experiment.” He flew a kite amid lightning and thunder to prove that a storm’s bright flashes were electrical in nature. He did this by lofting a kite in a thunderstorm that had a metal key tied to the kite line. The idea sounds a bit dangerous and Ben, indeed, is usually shown out cavorting in a field as lightning cracks all around him. In reality, the wily Franklin wasn’t quite so willing to sacrifice himself for science. Rope was an important element in the experiment since the hemp control line not only held the kite aloft but also was part of Ben’s electrical sensing system.

Rather than standing out in the open where he could have been struck, Ben actually picked a field that had a shed or lean-to on it. He stood in the shed, which protected him from a direct strike. The control line extended out the open end of the shed and up to the bobbing kite.

The control line was made of hemp cordage, which can soak up water and become an electrical conductor. To protect himself from getting a shock from the hemp line, he tied a length of silk cord to the end of the hemp line and held onto that. While not strictly an insulator, silk is less conductive than hemp, so this provided Ben with some protection from a surge of current coming down the line. Where the hemp and silk were joined, Ben tied in the metal key.

But how did the hemp cord assist Franklin in his experiment?

Ben, ever the keen observer, took note that when the storm was overhead and lightning was present, the thin fibers along the outside of the hemp cord became electrically charged and stood straight out, like your hair can stand up when charged after rubbing it with a plastic balloon. When Ben saw the fibers stand up he knew that cord was charged with electricity. It was at that point that he brought the knuckle of his index finger close to the key and a thin spark jumped from the key to his hand.

A little hemp and Franklin was flying! He had proven that lightning was an electrical discharge.