De Buffon erected his rod on the tower of his château at Montbar; de Lor, over his house in Paris, and d'Alibard, at his country seat at Marly, a little town eighteen miles from Paris. D'Alibard was not at home on the eventful afternoon of May 10th; but before leaving Marly, he had drilled a certain Coiffier in what he should do in case an electric storm came on during his absence. Though a hardy and resolute old soldier and proud of the confidence placed in him, Coiffier grew alarmed at the long and noisy discharges which he drew from the insulated rod on the afternoon of May 10th. While the storm was still at its height he sent for the Prior of the place, Raulet by name, who hastened to the spot, followed by many of his parishioners. After witnessing a number of brilliant and stunning discharges, the priest drew up an account of the incident and sent it, at once, by Coiffier himself to d'Alibard, who was then in Paris. Without delay d'Alibard prepared a memoir on the subject which he communicated to the Académie des Sciences three days later, viz.: on May 13th. In the concluding paragraph, the polished academician pays a graceful tribute to the philosopher of the Western World:
"It follows from all the experiments and observations contained in the present paper, and more especially from the recent experiment at Marly-la-ville, that the matter of lightning is, beyond doubt, the same as that of electricity; it has become a reality, and I believe that the more we realize what he (Franklin) has published on electricity, the more will we acknowledge the great debt which physical science owes him."
We may, in passing, correct the error of those who credit French physicists with having originated the idea of the pointed conductor. Such writers should read the words of d'Alibard in the beginning of his memoir, where he says: "En suivant la route que M. Franklin nous a tracée, j'ai obtenu une satisfaction complète"; that is, "In following the way traced out by Franklin, I have met with complete success." To Franklin, therefore, belongs the idea of the pointed rod of 1750, which became the lightning conductor of subsequent years; to the Parisian savants belongs the great distinction of having been the first to make the experiment and verify the Franklinian view of the identity of the lightning of our skies with the electricity of our laboratories.
Franklin had precise ideas on the action of his pointed conductors, clearly recognizing their twofold function: (1) that of preventing a dangerous rise of potential by disarming the cloud; and (2) that of conveying the discharge to earth, if struck. In some of his letters, he complains of people who concentrate their attention on the preventive function, forgetting the other entirely. "Wherever my opinion is examined in Europe," he wrote in 1755, "nothing is considered but the probability of these rods preventing a stroke, which is only a part of the use I proposed for them; and the other part, their conducting a stroke which they may happen not to prevent, seems to be totally forgotten, though of equal importance and advantage."
A favorite illustration of Franklin's showing the discharging power of points, consisted in insulating a cannon ball against which rested a pellet of cork, hung by a silk thread. On electrifying the ball, the cork flies off and remains suspended at a distance, falling back at once, as soon as a needle is brought near the ball. (1747.)
He also used tassels consisting of fifteen or twenty long threads (Fig. 17), and even cotton-fleece, the filaments of which stand out when electrified, but come together when a pointed rod is held underneath. He also noticed that the filaments do not collapse when the point of the rod is covered with a small ball. (1762.)
Fig. 17
Tassel of Long Threads or Light Strips of Paper
Franklin's views on lightning-rods met with some opposition in France from the brilliant Abbé Nollet, and in England from Dr. Benjamin Wilson. The latter was mainly instrumental in bringing about the famous controversy of "Points vs. Knobs." In 1772, a committee was appointed by the Royal Society to consider the best means of protecting the powder-magazines at Purfleet from lightning. On the committee with Dr. Wilson were Henry Cavendish, the distinguished chemist and physicist, and Sir John Pringle, President of the Royal Society. The report favored sharp conductors against blunt ones advocated by Dr. Wilson. Five years later, in 1777, the question was again brought up, and again the new committee decided in favor of pointed terminals, convinced "that the experiments and reasons made and alleged to the contrary by Mr. Wilson were inconclusive."