The work of Benjamin Franklin on thunderstorms and the development of the lightning rod

Abstract An account is given of the experiments and hypotheses which led Franklin to propose ( a ) the “Philadelphia Experiment” (10th May, 1752) on the identity of laboratory and thunderstorm electrification and ( b ) the lightning rod as a protective device. These and Franklin's other experiments and ideas on lightning and thunderstorms are surveyed in the light of modern knowledge. His conception that one function of the lightning rod was to neutralise all or part of the charge on the cloud-base by point-discharge is shown to be correct in the exceptional case of an extremely high rod, which was his original view. Rods of moderate height are not able to do this but perform the second function suggested by him, drawing the discharge to them and leading it safely to ground.