Benjamin Franklin’s discoveries resulted from his investigations of electricity.
Franklin proposed that “vitreous” and “resinous” electricity were not different types of “electrical fluid” (as electricity was called then), but the same electrical fluid under different pressures.
Ben was the first to label them as positive and negative respectively, and he was the first to discover the principle of conservation of charge.
In 1750, Ben Franklin published a proposal for an experiment to prove that lightning is electricity by flying a kite in a storm that appeared capable of becoming a lightning storm.
On May 10, 1752 Thomas-François Dalibard of France conducted Franklin’s experiment using a 40-foot tall iron rod instead of a kite, and he extracted electrical sparks from a cloud.
Then on June 15, 1752, Benjamin Franklin possibly conducted his famous kite experiment in Philadelphia, successfully extracting sparks from a cloud (traditional date, the exact date is unknown).
Franklin’s experiment was not written up with credit until Joseph Priestley’s 1767 History and Present Status of Electricity; the evidence shows that Franklin was insulated (thus not in a conducting path, where he would have been in danger of electrocution).
Others, such as Prof. Georg Wilhelm Richmann were indeed electrocuted during the months following Franklin’s experiment.
In his own writings, Franklin indicates that he was aware of the dangers and offered alternative ways to demonstrate that lightning was electrical, as shown by his use of the concept of electrical ground. When Franklin performed this experiment, he may not have done it in the way that is often described, flying the kite and waiting to be struck by lightning, as that could have been dangerous.
Instead he would have used the kite to collect some electric charge from a storm cloud, which implied that lightning was electrical.
On October 19, in a letter to England explaining directions for repeating the experiment, Franklin wrote:
“When rain has wet the kite twine so that it can conduct the electric fire freely, you will find it streams out plentifully from the key at the approach of your knuckle, and with this key a phial, or Leiden jar, may be charged: and from electric fire thus obtained spirits may be kindled, and all other electric experiments [may be] performed which are usually done by the help of a rubber glass globe or tube; and therefore the sameness of the electrical matter with that of lightening completely demonstrated.”
Franklin’s electrical experiments led to his invention of the lightning rod.
He noted that conductors with a sharp rather than a smooth point were capable of discharging silently, and at a far greater distance. He surmised that this knowledge could be of use in protecting buildings from lightning by attaching “upright Rods of Iron, made sharp as a Needle and gilt to prevent Rusting, and from the Foot of those Rods a Wire down the outside of the Building into the Ground;…Would not these pointed Rods probably draw the Electrical Fire silently out of a Cloud before it came nigh enough to strike, and thereby secure us from that most sudden and terrible Mischief!”
Following a series of experiments on Franklin’s own house, lightning rods were installed on the Academy of Philadelphia (later the University of Pennsylvania) and the Pennsylvania State House (later Independence Hall) in 1752.
In recognition of his work with electricity, Franklin received the Royal Society’s Copley Medal in 1753 and in 1756 he became one of the few 18th century Americans to be elected as a Fellow of the Society. The cgs unit of electric charge has been named after him: one franklin (Fr) is equal to one statcoulomb.
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