ELECTRICITY. 



417 



History. 



Franklin's 



discoveries. 



Identity of 

 lightning 

 and elec- 

 tricity. 



for the operation of the Leyden phial ; and it was not 

 till the Franklinian theory was discovered, that it re- 

 ceived a proper explanation. 



This explanation consists in supposing, that when 

 one side of the glass jar was electrified positively, the 

 other was electrified negatively, so that the charging of 

 the phinl consisted merely in diminishing the quantity 

 of the electric matter in one side of the glass, and ac. 

 cumulating electric matter in the other. Dr Franklin 

 had observed, that the outside coating was always ne- 

 gative, when the inside coating was positive ; and he 

 proved the truth of this, by the attraction and repul- 

 sion of a cork ball suspended by silk. When a cork 

 ball was suspended between the wires communicating 

 with each coating, and brought near one another, it was 

 alternately attracted and repelled by each wire, till the 

 phial was' discharged. In order to shew, that, in the 

 charging of a phial, the one side lost as much as the 

 other gained, he suspended a small linen thread near 

 the coating of a charged phial, and observed that the 

 coating attracted the thread whenever lie brought his 

 finger near the wire, the outside coating drawing in by 

 the thread the same quantity tliat was taken from the 

 inside by touching the wire. Several experiments ena- 

 bled Dr Franklin to prove, that, in the discharge of the 

 phial, the quantity of electricity which left the one side 

 of the phial, was equal to that which was received by the 

 other. In one of these, he insulated the rubber of his 

 machine, and found that a phial, suspended from the 

 conductor, could not be charged, even though his hand 

 was constantly applied to it ; for, though the electric 

 matter left the outside, there was none to be accumu- 

 lated on the inside. Upon removing his hand, and con- 

 necting the outside coating and the insulated rubber by 

 a conducting substance, the phial was easily charged 

 Hence it followed, that the very same electric matter 

 which left the external coating, was conveyed by the 

 way of the globe, the conductor, and the wire of the 

 phial, into the inside of the jar. 



3. Hitherto the science of electricity embraced no wider 

 range than the phenomena developed during the exci- 

 tation of glass and other electrics, and had therefore 

 not yet connected itself with any of the great events of 

 the material world. Astronomy had elevated the mind 

 to the contemplation of the most splendid and magnifi- 

 cent phenomena which the imagination could compre- 

 hend ; Optics had dared to investigate the properties 

 of that ethereal matter, which constitutes the very soul 

 of the visible world; and Magnetism had connected her 

 facts with the polar attraction of the great globe itself. 

 It had indeed been conjectured, that the shock and 

 spark of the electrical machine, were miniature effects 

 o*'a more tremendous agent ; but it was reserved for Dr 

 Franklin, not only to give a form and character to this 

 infant science, but to raise it to a higher rank among 

 the other great divisions of human knowledge. The 

 discovery of the identity of electricity and lightning was 

 the step by which this great change was effected. The 

 vulgar were astonished at the sight of fire brought down 

 from heaven ; and philosophers themselves startled at the 

 recollection, that they had been amusing themselves 

 with a thunderbolt in their hands, and trifling with that 

 terrible agent, which had so often alarmed and convul- 

 sed the physical world. Human genius indeed seems 

 on this occasion to have made an impious excursion be- 

 yond its mortal range, and one victim was demanded to 

 expiate the audacious attempt. 



uecting that the electric fluid was similar to that 



VOL. VIII. PART II. 



Franklin's 

 discoveries' 



which produced lightning, Dr Franklin drew up a state- History.^ 

 ment of the principal points in which these two agents ^~ 

 resembled each other. He found that flashes of light- 

 ning, like the electric spark, are generally seen crooked 

 and waving in the air ; that lightning, as well as elec- 

 tricity, strikes pointed objects in preference to all others; 

 that lightning and electricity take the readiest and the 

 best conductor ; that they both dissolve metals, and 

 inflame combustible substances; that they rend solid 

 bodies, strike persons blind, reverse the poles of a mag- 

 net, and destroy animal life. These points of resem- 

 blance appeared to Dr Franklin so very striking, that 

 he resolved to examine, by direct experiment, the truth 

 of his conjecture. For some time he waited for the 

 erection of a spire in Philadelphia, to assist him in his 

 views; but he afterwards thought of a more simple 

 method of carrying them into effect. Having extend- 

 ed a large silk handkerchief over two cross sticks, he 

 formed a kite, which, unknown to any person but his 

 son, he elevated during the first thunder storm, which 

 happened in the month of June 1752. The kite re- 

 mained a considerable time in the atmosphere without 

 any appearance of electricity. A cloud, which had the 

 appearance of being charged with lightning, passed 

 over it without producing any result ; and Dr Frank- 

 lin began to despair of success. His attention, how- 

 ever, was roused by the erection of some loose fibres 

 on the hempen cord, and on holding his knuckles 

 to the key upon the string, he received an electric 

 spark. Before the rain had wetted the string, other 

 sparks were obtained; but when the string was thorough- 

 ly wet, Dr Franklin collected the electric fire in great 

 abundance. About a month before Dr Franklin had 

 made these successful trials, the French philoso- 

 phers had obtained similar results. In order to shew, 

 by direct experiment, that the electricity collected 

 from the atmosphere had the same properties as thrft 

 which was generated by the friction of an electric, he 

 erected an apparatus in his house at Philadelphia, con- 

 sisting of an insulated iron rod connected with two 

 bells, which indicated by their ringing that the rod was 

 electrified. After numerous trials, he found that the 

 natural and artificial electricities were in every respect 

 the same ; that the clouds were sometimes negatively, 

 and sometimes positively electrified ; and that some- 

 times, in the course of one thunder storm, they changed 

 several times from positive to negative. On one oc- 

 casion, when there was no thunder at all, he found 

 the air to be strongly electrified during a fall of snow. 



The great practical application of the preceding dis- 

 coveries to the protection of buildings from the de- 

 structive effects of lightning, contributed most essen- 

 tially to extend the fame of Dr Franklin's discove- 

 ry ; and every conductor that has been reared for this 

 purpose, m;iy l>e regarded as a monument to the genius 

 of the Ameri' in philosopher. We are disposed to 

 doubt, however, whether this application has been as 

 useful as it at first promised. When a thunder cloud 

 is directly above, and not far distant from, a conductor, 

 the electric fluid will certainly be conveyed into the 

 earth by tile conducting power of the iron rod ; but 

 there have been numerous instances where one end of 

 a house has been destroyed by lightning, when there 

 v ;n a conductor placed at the other. 



A great variety of miscellaneous and important ex- 

 periments were made about this time by Le Monnier nosby Le 

 the younger, Mr Smeaton, and Dr Miles. Le Monnier Monnier. 

 made a great number of experiments with a very power- 



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