Brilliant_ The Evolution of Artificial Light - Jane Brox [40]
Franklin advanced the understanding of electricity with countless experiments and considerable writings on the subject, and he clarified some of its mystery. Philip Dray notes that Franklin "was the first to discover that the [Leyden] jar's stored charge was not in the water, as others had believed, but in the glass. The glass was a dielectric, meaning it stored and allowed the passage of electricity but did not conduct it." Perhaps most significantly, Franklin—like Stephen Gray and Abbé Nollet—suspected that lightning and the electrical charges they'd created in their experiments were one and the same substance. The common belief at the time, however, held that lightning—heavenly fire—was its own distinct phenomenon and a manifestation of the will of God, a belief that may have been reinforced by the fact that churches and monasteries, with their high steeples and bell towers, were often struck during storms. "There was scarce a great abbey in England which was not burnt down with lightning from heaven," notes a church history of Britain. Many thought such destruction could be warded off by the sounding of church bells during electrical storms, though the practice only served to hasten the deaths of countless bell ringers.
Franklin suggested a new way to ward off such destruction. "There is something ... in the experiments of points, sending off or drawing on the electrical fire," he wrote. "For the doctrine of points is very curious, and the effects of them truly wonderful.... I am of the opinion that houses, ships, and even towers and churches may be effectually secured from the strokes of lightning by their means." When he began to promote the use of lightning rods on buildings, he encountered considerable resistance from church leaders, who claimed the rods were blasphemous and warned that drawing lightning from the sky would cause earthquakes. He was undeterred, however, and his observations of the workings of lightning rods led to his most renowned experiment, which proved that the charges in the heavens and those in Leyden jars were one and the same.
In July 1750, Franklin proposed that a sentry box, large enough to house a man and with a pointed rod rising from it, be built. It would contain an electrical stand which, if it
be kept clean and dry, a man standing on it when such clouds are passing low might be electrified and afford sparks, the rod drawing fire to him from a cloud. If any danger to the man should be apprehended (though I think there would be none), let him stand on the floor of his box, and now and then bring near to the rod the loop of a wire that has one end fastened to the leads, he holding it by a wax handle; so the sparks, if the rod is electrified, will strike from the rod to the wire and not affect him.
In May 1752, before he could conduct his experiment, a French physicist successfully followed his suggestion. The following month, Franklin, knowing nothing of the events in France, carried out a similar experiment with a silk kite, a hemp rope, and a key, which he later detailed:
As soon as any of the thunder-clouds come over the kite, the pointed wire will draw the electric fire from them, and the kite, with all the twine, will be electrified, and the loose filaments of the twine will stand out every way, and be attracted by an approaching finger. And when the rain has wetted the kite and twine, so that it can conduct the electric fire freely, you will find it stream out plentifully from the key on the approach of your knuckle. At this key the phial may be charged; and from electric fire thus obtained spirits may be kindled, and all the other electric experiments be performed which are usually done by the help of a rubbed glass globe or tube, and thereby the sameness of the electric matter with that of lightning completely demonstrated.
To connect heavenly forces to the "virtue" that humans had puzzled over since the first sparks were rubbed from amber elevated