Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/16

Rh 6 ELEGTBICIT Y [HISTOKY. matter from them. In the case of two insulated persons, the one in contact with the rubber and the other with the conductor, he observed that either of them would communi cate a much stronger spark to the other than to any by stander. The electricity of the one, he says, became more rare than it is naturally, and that of the other more dense, so that the density of the electricity in the two insulated persons differed more than that between either of them and a bystander. A variety of interesting experiments were made about this time by Le Monnier, Nollet, Winckler, Ellicott, Jallabert, Boze, Menon, Srneaton, and Miles. In 1746 Le Monuier confirmed the result previously obtained by Gray, that electricity is communicated to homogeneous bodies in proportion to their surfaces only. Boze dis covered that capillary tubes which discharged water by drops afforded a continuous stream when electrified. The Abbe Nollet (Essai sur VElectricite, 1746; liecherches, 1749; Lettres, 1753), the friend and coadjutor of Dufay, ascertained that electricity increases the natural evapora tion of fluids, and that the evaporation is hastened by placing them in non-electric vessels. Jallabert confirmed the result previously obtained by Watson, that electricity passes through the substance of a conducting wire, and not along its surface. Smeaton found that the red hot part of an iron bar could be as strongly electrified as the cold parts on each side of it. Dr Miles kindled com mon spirits by a stick of black sealing-wax excited by dry flannel. Ellicott conceived that the particles of the electric fluid repel each other, while they attract those of all other bodies. Mowbray concluded that the vege tation of two myrtles was hastened by electrifying them, a result which Nollet confirmed in the case of vegetating seeds. The Abbe Menon found that cats, pigeons, spar rows, and chaffinches lost weight by being electrified for five or six hours, and that the same result was true of the human body ; and hence it was concluded that electri city augments the insensible perspiration of animals. Frankliu A. high place in the history of electricity must be [I 7. 06 allotted to the name of Dr Benjamin Franklin of Phila delphia. His researches did much to extend our theoreti cal and practical knowledge of electricity, and the clearness and vigour of his style made his writings popular, and spread the study of the subject. One of the first labours of the American philosopher was to present, in a more distinct form, the theory of posi tive and negative electricity, which Sir W. Watson had been the first to suggest. He showed that electricity is not created by friction, but merely collected from its state of diffusion through other matter by which it is attracted. He asserted that the glass globe, when rubbed, attracted the electrical fire, and took it from the rubber, the same globs being disposed, when the friction ceases, to give out its electricity to any body which has less. In the case of the charged Leyden jar, the inner coating of titifoil had received more than its ordinary quantity of electricity, and was therefore electrified positively or plus, while the outer coating of tinfoil having had its ordinary quantity of electricity diminished, was electrified negatively or minus. Hence the cause of the shock and spark when the jar is discharged, or when the superabundant plus electricity of the inside is transferred by a conducting body to the de fective or minus electricity of the outside. This theory of the Leyden phial Franklin established in the clearest manner, by showing that the outside and the inside coat ing possessed opposite electricities, and that, in charging it, exactly as much electricity is added on one side as is subtracted from the other. The abundant discharge of electricity by points was observed by Franklin in his ear liest experiments, and also the power of points to conduct it copiously from an electrified body. Hence he was fur nished with a simple method of collecting electricity from other bodies ; and he was thus enabled to perform those remarkable experiments which we shall now pro ceed to explain. Hawksbee, Wall, and Nollet had succes sively suggested the similarity between lightning and the electric spark, and between the artificial snap and the natural thunder. Previous to the year 1750 Franklin drew up a statement, in which he showed that all the general phenomena and effects which were produced by electricity had their counterpart in lightning. After wait ing some time for the erection of a spire at Philadelphia, by means of which he thought to bring down the electricity of a thunder-storm, he conceived the idea of sending up a kite among the clouds themselves. With this view he made a small cross of two small light strips of cedar, the arms being sufficiently long to reach to the four corners of a large thin silk handkerchief when extended. The corners of the handkerchief were tied to the extremities of the cross, and when the body of the kite was thus formed, a tail, loop, and string were added to it. The body was made of silk to enable it to bear the violence and wet of a thunder-storm. A very sharp pointed wire was fixed at the top of the upright stick of the cross, so as to rise a foot or more above the wood. A silk ribbon was tied to the end of the twine next the hand, and a key suspended at the junction of the twine and silk. In company with his son, Franklin raised the kite like a common one, in the first thunder-storm, which happened in the month of June 1752. To keep the silk ribbon dry, he stood within a door, taking care that the twine did not touch the frame of the door ; and when the thunder-clouds came over the kite he watched the state of the string. A cloud passed without any elec trical indications, and he began to despair of success. He saw, however, the loose filaments; of the tw y ine standing out every way, and he found them to be attracted by the ap proach of his finger. The suspended key gave a spark on the application of his knuckle, and when the string had be come wet with the rain, the electricity became abundant; a Leyden jar was charged at the key, and by the electric fire thus obtained spirits were inflamed, and all the other electrical experiments performed which had been formerly made by excited electrics. In subsequent trials with another apparatus, he found that the clouds were some times positively and sometimes negatively electrified, and so demonstrated the perfect identity of lightning and elec tricity. Having thus succeeded in drawing the electric fire from the clouds, Franklin conceived the idea of protecting buildings from lightning by erecting on their highest parts pointed iron wire or conductors communicating with the ground. The electricity of a hovering or a passing cloud would thus be carried off slowly and silently ; and if the cloud was highly charged, the lightning would strike in pre ference the elevated conductors. The most important of Franklin s electrical writings are his Experiments and Observations on Electricity made at Philadelphia, 1751-54 ; his Letters on Electricity, and various memoirs and letters, Phil. Trans., 1756, 1760, ifec. About the same time that Franklin was making his kite experiment in America, D Alibard and others in France had erected a long iron rod at Marli, and obtained results agreeing with those of Franklin. Similar investigations were pursued by many others, among whom Father Bec- caria deserves especial mention. These experiments were often dangerous, and in one case Deal: a fatal accident occurred. Professor Kichman of St Peters- Rick burg had erected on his house an iron rod to collect the an electricity of thunder-clouds. On the 6th August 1753, during a thunder-storm, he was observing, along with his friend Sokolow, the indications of an electrometer which