(oil on aluminum). Representing the “sentry-box experiment” organized by naturalist Thomas-François Dalibard and performed during a storm at Marly-la-Ville, France, in 1752. It was based on ideas outlined in in Franklin’s 1751 publication "Experiments and Observations on Electricity." When sparks were visible on the insulated iron rod, it was clear that electricity was present in thunderclouds and lighting. Franklin performed his famous key and kite experiment a month later. My interest was piqued when I saw a rare bolt of positive lightning.