In his scientific career Franklin was not less illustrious than in his political life. The founding of the American Philosophical Society, which was in constant communication with the Royal Societies of London and Dublin, and of which he was the first secretary, led to a familiarity with the progress of science in Europe, and throughout his long and busy life he never failed to keep up his interest in the scientific pursuits which at that time received such an impetus.

The discoveries of Gray and Du Fay in electricity produced a great impression on him, and his studies in the same department were followed with an absorbing interest. What this mysterious power called electricity was, became the question of the day, and scientific minds set resolutely to work to solve the question.

Franklin's experiments in electricity were confined to the problems of finding out, if possible, what electricity was, and its distribution throughout nature. Gray and Du Fay had shown that there were two kinds of electricity, which repelled and attracted each other mutually, and that the electric current could pass from one body to another. Franklin attempted to find out the reason for this attraction and repulsion, to discover why there existed conductors and non-conductors, or why some bodies would allow the electricity to pass through them and others would not.

After many careful and interesting experiments, he was led to the belief that electricity was not created  or produced, either by friction or any other process whatsoever, but that it was present everywhere, and that every body contained some quantity of this mysterious force, though what its nature was and how great its power might be, no one cold decide. Franklin reasoned that as all bodies were equally supplied with electricity, there would be a state of equilibrium which would show no signs of its existence unless it were in someway disturbed, and that the electricity manifested itself only when something occurred to disturb the normal condition of the body, either by giving it more electricity or taking some away from it.

Instead of the theory of two kinds of electricity, Franklin claimed that all the phenomena connected with the subject could be explained by suppposing a body to contain more or less of electricity, and introduced the words postive  and negative  to illustrate the condition of a body containing more or less than its normal quantity, and suggested that the terms vitreous  and resinous  be supplanted by the words positive  and negative, a body being electrified positively when it received an addition of electricity, and negatively when some is taken from it; and these are the expressions that are now generally used in speaking of the different conditions.

Franklin's greatest contribution to this department of science, however, was the discovery that electricity and lightning are the same thing. The thought that this might be true was not strictly original with Franklin, as Gray and others had hinted it before, but he was the first to make the experiment which proved their identity beyond a doubt.

This discovery, which was destined to make the name of Franklin famous in the history of science, resulted from the simple experiment of drawing the lightning from the clouds by means of a silk kite, to which was attached a pointed wire—Franklin having demonstrated before this the power of points to attract electricity. The experiment was tried in the open field during a heavy thunder-shower, Franklin and his son standing under an open shed which afforded them a shelter from the rain. Franklin at first noticed that the fibres of the kite string which he held in his hand were separating, as in the passage of the electric current, and by means of a small metal key attached to the cord he obtained the electric spark and the shock, and charged a Leyden jar, as well as performing other electrical experiments.

The experiment was thus a complete success and established the identity of electricity and lightning beyond the shadow of a doubt. And although, when Franklin's paper on the subject was read to the Royal Society of London, the learned members greeted it with sneers and laughter, yet the scientists throughout the rest of Europe accepted its views with alacrity, and French, German, and Italian translations were eagerly sought for, and the name of the discoverer of this new secret of nature was spoken of everywhere with admiring praise.

Franklin's practical mind could not rest until he had found some means of applying this great discovery to the benefit of mankind, and the lightning rods which were before long erected on many buildings were among the results which followed his famous experiments; and had it not been for the engrossing political cares which occupied his mind during the long period of his country's need, it is probable that he would have made other inventions which, if not anticipating those at present in use, would at least have proven of much practical benefit in applying the powers of electricity to the concerns of daily life.

Next to the discovery of the law of gravitation, the discovery of the identity of lightning and electricity and its universality throughout nature, was perhaps the greatest truth of nature that had yet been grasped, and Franklin's work for science, though forming only an episode in his brilliant political career, was of such lasting importance as to place his name high on the list of the world's great discoverers.

He died in Philadelphia on April 17, 1790.