832 



ELECTRICITY. 



crystals, when covered by a plate of metal, render it 

 positive, the crystals themselves becoming nega- 

 tive. 



XI. The resemblance between the electric, spark, 

 and more especially the explosive discharge of the 

 Leyden jar, and atmospheric lightning and thunder, 

 struck the mind of Dr Franklin with so much force, 

 that he was determined, if possible, to verify their 

 identity by experiment. Having constructed a kite, 

 by stretching a large silk handkerchief over two 

 sticks in the form or a cross, on the appearance of 

 an approaching storm, he went into a field in the 

 vicinity of i'lii!aclr!|>lii;i and raised it, taking care to 

 insulate it by a silken cord attached to a key, witii 

 which the hempen string terminated. No sooner 

 had a dense cloud, apparently charged with light- 

 ning, passed over the spot on which he stood, than 

 his attention was arrested by the bristling up of some 

 loose fibres on the hempen string : he immediately 

 presented his knuckle to the key, and received an 

 electric spark. The rain now fell in torrents, and, 

 wetting the string, rendered it conducting in its 

 whole length ; so that electric sparks were now 

 collected from it in great abundance. This grand 

 experiment was made in June, 1752 ; and, al- 

 though the same idea which led Franklin to institute 

 it had occurred to other philosophers, yet to him be- 

 : ongs exclusively the glory of the discovery. The 

 discovery of Franklin immediately engaged the at- 

 tention of European philosophers ; one of whom, 

 professor Richmann, of St Petersburg, fell a victim to 

 his attempt to draw down the electric fluid from the 

 clouds. He had constructed an apparatus for obser- 

 vations on atmospherical electricity, and was attend- 

 ing a meeting of the academy of sciences, when the 

 sound of distant thunder caught his ear. He imme- 

 diately hastened home, taking with him his engraver, 

 Sokolow, in order that he might delineate the appear- 

 ances that should present themselves. While intent 

 upon examining the electrometer, a large globe of 

 fire flashed from the conducting rod, which was insu- 

 lated, to the head of Richmann, and, passing through 

 his body, instantly deprived him of life. A red spot 

 was found on his forehead, where the electricity had 

 entered ; his shoe was burst open, and part of his 

 clothes singed. His companion was struck down, 

 and remained senseless for some time ; the door-case 

 of the room was split, and the door itself torn off its 

 hinges. 



The atmosphere is very generally in an electrical 

 state. This is ascertained by employing a metallic 

 rod, insulated at its lower end, elevated at some 

 height above the ground, and communicating with an 

 electroscope. In order to collect the electricity of 

 the higher regions of the air, a kite may be raised, 

 in the string of which a slender metallic wire should 

 be interwoven. The atmosphere is almost invariably 

 found to be positively electrified ; and its electricity 

 is stronger in the whiter than in the summer, and 

 during the day than in the night. From the time 

 of sunrise, it increases for two or three hours, and 

 then decreases towards the middle of the day, being 

 generally the weakest between noon and four o'clock. 

 As the sun declines, its intensity is again augmented, 

 till about the time of sunset, after which it dimin- 

 ishes, and continues feeble during the night. In 

 cloudy weather, the electrical state is much more un- 

 certain ; and when there are several strata of clouds, 

 moving in different directions, it is subject to great 

 and rapid variations, changing backwards and for- 

 wards in the course of a very few minutes. On the 

 first appearance of fog, rain, snow, hail, or sleet, the 

 electricity of the air is generally negative, and often 

 highly so ; but it afterwards undergoes frequent 

 transitions to opposite states. On the approach 



of a thunder storm, these alternations of the electric 

 condition of the air succeed one another with re- 

 markable rapidity. Strong sparks are sent out in 

 great abundance from the conductor ; and it becomes 

 dangerous to prosecute experiments with it in its 

 insulated state. Thunder is merely the noise pro- 

 duced by the motion of the lightning * 



The protection of buildings from the destructive 

 effects of lightning is the most important practical 

 application of the theory of electricity. The con- 

 ductors, for this purpose, should be formed of metallic 

 rods, pointed at the upper extremity, and placed so 

 as to project a few feet above the highest part of the 

 building they are intended to secure ; they should be 

 continued without interruption till they descend into 

 the ground below the foundation of the house. Cop- 

 per is preferable to iron as the material for their 

 construction, being less liable to destruction by rust, 

 or by fusion, and possessing also a greater conduct- 

 ing power. The size of the rods should be from half 

 an inch to an inch in diameter, and the point should 

 be gilt, or made of platina, that it may be more ef- 

 fectually preserved from corrosion. An important 

 condition in the protecting conductor is, that no in- 

 terruption should exist in its continuity from top to 

 bottom ; and advantage will result from connecting 

 together by strips of metal all the leaden water pipes, 

 or other considerable masses of metal in or about the 

 building, so as to form one continuous system of 

 conductors, for carrying the electricity by different 

 channels to the ground. The lower end of the con- 

 ductors should be carried down into the earth, till it 

 reaches either water, or at least a moist stratum. 

 For the protection of ships, chains, made of a series 

 of iron rods linked together, are most convenient, 

 on account of their flexibility. They should extend 

 from the highest point of the mast some way into the 

 sea, and the lower part should be removed to some 

 distance from the side of the ship, by a wooden spar 

 or outrigger. 



The utility of conducting rods for buildings is well 

 exhibited in an experiment with the following piece 

 of apparatus : In Fig. 12, g represents a board, 

 shaped like the gable end of a house, and three 

 quarters of an inch in thickness. It is fixed perpen- 

 dicularly into the board, a b, into which also is fixed 

 the glass pillar, f. In the middle of the wooden 

 gable there is a square hole^f, about an inch wide, 

 and a quarter of an inch deep, into which a square 

 piece of wood is introduced a little smaller than the 

 hole itself, so that it may fall out when the instru- 

 ment is shaken. On this small piece of wood a 

 small piece of wire is placed, so tliat it may reach 

 from one corner to the other diagonally. At the top 

 of the gable there is a ball, d, which terminates a 

 wire that passes down the gable, and reaches the cor- 

 nej of the hole, g. The wire stops here ; but there 

 is another which connects the bottom of the gable 

 with the corner of the li ttle hole next to a, that is, in 

 a diagonal with the termination of the upper wire. 

 From the top of the glass pillar, e, there proceeds a 

 bent brass wire, e c, which carries a perpendicular 

 wire, having a brass ball at each end ; this wire is 

 capable of being moved up or down in a spring 

 socket in the end of the wire, e c, so that the under 

 ball may be made to come nearer to or farther from 

 the ball, d. For a similar reason, the glass pillar, /, 

 is not fastened tightly into the board, but may be 

 made to move upon its axis. Let now the wire, e c, 

 be connected by a chain with the wire of the jar, A, 

 and also the outside coating of the jar is in like man- 

 ner to be connected with the wire at the bottom of 



* 1'he air of close rooms, vitiated by respiration, u found 

 to be negatively electrified. 



