The experiments and the letters to Collinson describing them continued, and about this time we find Franklin writing a long and apparently the first intelligent explanation of the action of the Leyden jar. Then followed attempts to explain thunder and lightning as phenomena of electricity, and on July 29, 1750, Franklin sent to Collinson a paper announcing the invention of the lightning-rod, together with an explanation of its action.
In these papers he also suggested an experiment which would prove positively that lightning was a form of electricity. The two phenomena were alike as regarded light, color, crooked direction, noise, swift motion, being conducted by metals, subsisting in water or ice, rending bodies, killing animals, melting metals, and setting fire to various substances. It remained to demonstrate with absolute certainty that lightning resembled electricity in being attracted by points; and for this purpose Franklin proposed that a man stand in a sort of sentry-box on the top of some high tower or steeple and with a pointed rod draw electricity from passing thunder-clouds.
This suggestion was successfully carried out in France, in the presence of the king, at the county-seat of the Duke D'Ayen; and afterwards Buffon, D'Alibard, and Du Lor confirmed it by experiments of their own. But they did not use steeples; they erected lofty iron rods, in one instance ninety-nine feet high. Nevertheless, it was in effect the same method that Franklin had suggested. The experiment was repeated in various forms in England, and the Philadelphia philosopher, postmaster, and author of "Poor Richard" became instantly famous as the discoverer of the identity of lightning with electricity.
Two years before these experiments were inaugurated he had retired from business for various reasons, chief among which was his strong desire to devote more time to science. His letters continue to be filled with closely reasoned details of all sorts of experiments. So earnest were these Philadelphia investigators, that when Kinnersley wrote complaining that in travelling to Boston he found difficulty in keeping up his experiments, Franklin, in reply, suggested a portable electrical apparatus which would not break on a journey.