Peter Collinson, Fellow of the Royal Society of London, who had dealings with some Philadelphia merchants, was led to take an active interest in the library. This he did by sending over a number of books and papers relating to electricity together with an "electrical tube" with instructions for its use.
These literary and scientific contributions sent from London from time to time, excited much interest among the charter members of the Library Company, and principally that of Franklin himself. He had heard something of the new order of phenomena which was just then engaging the attention of European physicists. In the summer of 1746, while on a visit to Boston, his native place, he assisted at a lecture on electricity by a certain Dr. Spence, a Scotchman, who sought to illustrate the properties of electrified bodies by such experiments as could be made with glass tubes and suitable rubbers, the rudimentary apparatus available at the time. Franklin was impressed by what he saw and heard, even though he indulged in a little destructive criticism when he said that the experiments were "imperfectly made," because the lecturer was "not very expert." When Franklin wrote those words, he knew by repeated and painful experience the difficulty of getting satisfactory results from rubbing glass tubes or rotating glass globes, owing to the provoking attraction which plain, untreated glass has for moisture. Knowing this, he might have been less severe in his strictures on his friend, the peripatetic electrician.
It is evident, however, that the experiments which he witnessed surprised and pleased him, for, having shortly afterward received some electrical tubes together with a paper of instructions, from his London friend, Peter Collinson, he set to work for himself without delay. We may well say of him that what his right hand found to do, he did calmly, but with all his might. A twelve-month had not elapsed before he wrote: "I never was engaged in any study that so totally engrossed my attention and time as this has lately done; for, what with making experiments when I can be alone and repeating them to my friends and acquaintance who, from the novelty of the thing, come continually in crowds to see them, I have had little leisure for anything else." (1747.)
Here we see the calm, persistent character of the philosopher united with the affability and communicativeness of the gentleman.
For the sake of encouraging others as well, perhaps, as through a sense of personal relief, Franklin had a number of long tubes of large bore blown at the local glass-house, which tubes he distributed to his friends that they, too, might engage in research work. In this way, rubbing and rubbing of an energetic kind became quite an occupation in the Franklin circle. Kinnersley, whose name still survives in works on static electricity in connection with an electric "thermometer" which he devised, was among the band of ardent workers who ungrudgingly acknowledged Franklin's superior acumen, comprehensive grasp of detail and wondrous insight into the mechanism of the new phenomena. If we say that Franklin was not a genius, it is only for the purpose of adding that even in those early electrical studies he displayed an uncommon amount of the unlimited capacity for taking pains which is said to be associated with that brilliant gift. He tested all his results with great care and in a variety of ways before accepting any of them as final; and considered his explanations of them provisional, being ever ready to modify them or give them up altogether if shown to conflict with the simple workings of nature.
As early as 1733, the refined and tactful Dufay, in France, showed by numerous experiments on woods, stones, books, oranges and metals that all solid bodies were susceptible of electrification. This was a notable advance which swept away Gilbert's classification of bodies into electrics and non-electrics. The French physicist soon drew from his observations the conclusion that electrification produced by friction is of two kinds, to which he applied the terms vitreous and resinous, the former being developed when glass is rubbed with silk and the latter when amber or common sealing-wax is rubbed with flannel. He noticed, too, that silk strings repelled each other when both were touched either with excited glass or sealing-wax; but that they attracted each other when touched one with glass and the other with sealing-wax. From these observations, he deduced the electrostatic laws, that similarly electrified bodies attract while dissimilarly electrified bodies repel each other.
The law of distance was discovered later by Coulomb, who, in 1785, showed that the law of repulsion as well as of attraction between two electrified particles varies inversely as the square of the distance. In the year 1750, the law of the inverse square for magnets was stated by John Michell, who expressed it by saying that the "attraction and repulsion decrease as the square of the distance from the respective poles increases." Michell was fourth wrangler of his year (1748–9), Fellow of Queen's College, Cambridge, and inventor of the torsion balance, which, however, he did not live to use; but which, in the hands of Cavendish, yielded important results on the mean density of the earth. Coulomb probably re-invented the "balance" and applied the practical, laboratory instrument which he made it, to the study of the quantitative laws of electricity and magnetism.
To observe and correlate phenomena is the special work of the physicist; to speculate on ultimate causes is the privilege of the philosopher. Dufay was both. The theory which he offered was a simple one, even if untrue to nature. It was a good working hypothesis for the time being.
According to this theory, there are two distinct, independent electrical fluids mutually attractive but self-repelling. With that postulate, Dufay was able to offer a plausible explanation of a great many phenomena that puzzled the electricians of the time.
Franklin, however, held a different view; rejecting the dual nature of electricity, he propounded his one-fluid theory, which was found equally capable of explaining electrical phenomena. A body having an excess of the fluid was said to be positively charged, while one with a deficit was said to be negatively charged. The sign plus was used in one case and the sign minus in the other; and just as two algebraical quantities of equal magnitude but opposite sign give zero when added together, so a conductor to which equal quantities of positive and negative electricity would be given would be in the neutral state. The Franklinian theory was welcomed in England, Germany and Italy, but it met with opposition in France from the brilliant Abbé Nollet and the followers of Dufay.
Each of the rival theories affords a mental conception of the forces in play and also a consistent explanation of the resulting phenomena. Their simplicity, and, at the same time, the comprehensiveness of explanation which they afford, will continue to give them a place in our text-books for many years to come.
Efforts are being made to apply the electronic theory to the various phenomena of electrostatics, the electron being the smallest particle of electricity that can have separate, individual existence. It is many times smaller than the hydrogen atom, the smallest of chemical atoms, and it possesses all the properties of negative electricity. By the loss of one or more electrons, a body becomes positively electrified, whereas by the acquisition of one or more electrons it becomes negatively electrified. The electron at rest gives rise to the phenomena of electrostatics; in motion, it gives rise to electrical currents, electromagnetism and electric radiation.
We do not know what led Franklin to call positive the electrification of glass when rubbed with silk, and negative that of sealing-wax when rubbed with flannel. If he meant to imply that positive is the more important of the two, he erred, for many reasons can be given to show the preponderating influence of negative electricity; but it is too late now to change the terminology.
If asked to point out differences between the physical effects of positive and negative electrification, we would refer to the positive brush, which is finer and much more developed than the negative; to the Wimshurst machine, with its positive brushes on one side and negative "beads" on the other; to the positive charge acquired by a clean plate of zinc when exposed to ultraviolet light; to the ordinary vacuum tube in which there is a violet glow at the cathode end or negative terminal; to Crookes's tubes, X-ray