. The science-history of the universe . l demonstra-tion of these laws. Coulomb used for this purpose a simi-lar apparatus to the magnetic balance. From the figure itwill be seen that it consisted of two spheres so arrangedthat they could be charged and the force of repulsion be-tween them balanced by the torsion of the suspension. By means of this instrument Coulomb was able to provethe two laws of electrical attraction: 1. The repulsion between two electrified bodies chargedwith the same electricity varies inversely as the square ofthe distance between them. 2. The attractions and repulsions
. The science-history of the universe . l demonstra-tion of these laws. Coulomb used for this purpose a simi-lar apparatus to the magnetic balance. From the figure itwill be seen that it consisted of two spheres so arrangedthat they could be charged and the force of repulsion be-tween them balanced by the torsion of the suspension. By means of this instrument Coulomb was able to provethe two laws of electrical attraction: 1. The repulsion between two electrified bodies chargedwith the same electricity varies inversely as the square ofthe distance between them. 2. The attractions and repulsions vary in the ratio of i6o ELECTRICITY the products of the quantities of free electricity—that is tosay, of the electric charges of the two bodies. The action of points on metallic conductors in increas-ing the density of the charge at the point received the at-tention of Franklin. The following quotation from hisExperiments and Observations on Electricity, made atPhiladelphia, 1774, describes Franklins own experimentson this subject:. Fig. 5 —Coulombs Method of Proving Electrostatic Laws. Place an iron shot of three or four inches diameter onthe mouth of a clean, dry glass bottle. By a fine silkenthread from the ceiling, right over the mouth of the bottle,suspend a small cork ball, about the bigness of a marble;the thread of such a length as that the cork ball may restagainst the side of the shot. Electrify the shot, and theball will be repelled to the distance of four or five inches,more or less, according to the quantity of electricity ^When in this state, if you present to the shot the point ofa long, slender, sharp bodkin, at six or eight inches dis- ELECTROSTATICS 161 tance, the repellency is instantly destroyed and the corkflies to the shot. A blunt body must be brought within aninch and draw a spark to produce the same effect. To prove that the electrical fire is drawn off by thepoint, if you take the blade of the bodkin out of thewooden handle and fix it in a stick
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