318 



ELECTRICITY. 



Phil. Trant. 1791, p. 185; 1792, p. 85. Cavallo, 

 Pnil. Tra>u. 1776, p. 407. Singer". Element of Elec- 

 tricily, p. 281. 



Seer. III. On Co*dclart for the pretfrvalion of 

 Building*. 



, As soon as Dr Franklin established the identity of 



tori*faMhe ''P ntn ' n g nl "l electricity, he applied this discovery to 

 _j.u_ the great practical purpose of' defending buildings 

 tiooof from the effects of lightning. With this \iewhepro- 

 buiUing*. posed to erect metallic roil, pointed at each extre- 

 mity, nt tin- side of the building, so that it should rise 

 to a considerable height above the highest part of 

 it, and descend lower than it* foundation, either into 

 water, or the nearest conducting substance. I It-nee, if 

 a thunder cloud is immediately above the building, its 

 charge w.ill be drawn off by the pointed conductor, and 

 be transmitted silently to the earth ; whereas, had it 

 struck the unprotected building, the impcrfccL eon- 

 ducting bodies of which it is composed, would have 

 exposed such a resistance to the passage of the light- 

 ning, that it would either have been considerably in- 

 jured or destroyed. 



When this instrument was first proposed, a dispute 

 arose among the members of the Royal Society of Lon- 

 don, respecting the propriety of making the conductors 

 terminate in acute points. The subject was long and 

 keenly agitated ; but the party, headed by Mr Benja- 

 min Wilson, who insisted upon the necessity of furnish- 

 ing the extremities of the conductors with round halls, 

 was ultimately baffled in the contest, and the use of 

 pointed conductors now universally prevails. The ad- 

 vantages of conductors in the protection of buildings, 

 have been generally illustrated by the experiments of 

 the powder-house, the thunder-house, and the pyramid, 

 of which we shall give a short explanation, for the sake 

 of our young readers. 



Thunder. The thunder house is represented in Fig. 14s of 

 '"'"* Plate CCXLVIII. where a thick piece of l>oard A, re- 



PLATE presenting the end of a house, is fastened into the base 

 CCXLVIII. B. About 8 inches from A is fixed a glass pillar C, 

 Tig. 14. movcahle alxmt its axis, from the top of which pro- 

 ceeds a bent wire EF, having a spring socket F, through 

 which a double knobbed wire FG is moveable. In the 

 piece of wood A is fixed a wire HL, with a knob H at its 

 extremity, and another wire MN bent upwards at X. 

 Into a hole LMK, about -J of an inch deep, and nearly 

 one inch wide, is loosely fitted a square pieceof wood, of 

 the same shape, which can be taken out at pleasure. A 

 diagonal wire IK is fastened to the piece of wood. Let 

 a charged jar J be now placed near the apparatus, so 

 that its inside coating communicates with the ball E by 

 means of a chain, and ils outside coating with the wire 

 N by means of another chain. Having placed the balls 

 G, H at a considerable distance, take out the piece of 

 wood ILKM, and put it in so that the wire IK lies 

 in the direction ML, and forms a communication with 

 the wires HL and MI ; then bring the balls G, U gra- 

 dually nearer one another, and at a certain distance 

 the jar will explode, and no particular effect will be pro- 

 duced : the explosion being carried off by the continu- 

 ity of the conducting wires. Let the wire IK be now 

 placed as in the Figure, so that there is an interruption 

 in the conducting wire HLMN, and let the jar be ex- 

 ploded as before. The pieceof wood 1LMK will now 

 be throw n out of it* place to a considerable distance, as 

 the electricity does tiot find a ready passage from L to 

 M, In this experiment, the explosion of the jar may 



represent thunder cloud above the chimney of the Puttied 

 house A, which is saved when protected by a continu- ^*- 

 ous conductor, but thrown down when the lightning S< "V^"' 

 does not find a ready passage. 



The powder-house, asvonstmctt-d by Mr Cuthbert- Powder. 

 on, is represented in Fig. 15. Plate CCXLVIII. The home. 

 brass ball A communicates with L by a brass wire ; L PLATE 

 C(iini7iunicatei< with the ivory piece i/by a brass chain; CCXLVIII. 

 and the ivory piece it communicates with the brass tube Fig. 14. 

 c, standing in the brass dish c. This again communi. 

 cates with the brass pipe k, which is connected with 

 the hollow arm L. The tube c, and the ivory piece d, 

 are then filled with loose gunpowder, and the brass 

 pin is stuck into it within a quarter of an inch of the 

 iMittoiu. Alcohol is poured into the ili.sh c ; wettish 

 gunpowder is put into k, and L is filled with gunpow- 

 der hard pressed. The house being then shut up, and 

 its roof placed upon it, the outside of a Ley den phial 

 is made to communicate with a hook below L ; and as 

 soon as the communication is complete, by making the 

 inside of the jar communicate with the ball A, the ex- 

 plosion of the gunpowder in c and d will blow off the 

 roof, and, inflaming the alcohol, will set that part 

 of the house on lire, and, alter burning some time, 

 it will kindle the gunpowder in the tube k, and when 

 it is consumed, the powder in the arm L will be set on 

 fire, and, after a loud explosion, the house will be blown 

 up, and fall to pieces. 



The pyramid consists of several pieces of wood pla- Pyramid. 

 ced on one another, the lowest piece forming a square 

 base, and the pyramidal part resting upon this base by 

 three small brass balls A piece of wood is then loose- 

 ly fitted into a square hole on one side of the base, and 

 one of the brass balls is made to rest upon this piece of 

 wood, so that the upper p:irt of the pyramid will stand 

 when this piece of wood is in its place, but will fall 

 when it is thrown out. The piece of wood is so con. 

 strueted, that it can be made either to complete or to 

 interrupt the communication between the upper and the 

 lower wire, as in the thunder- house. When the inter- 

 ruption is completed, and an explosion sent along the 

 wires as in the tliunder-housc, no effect will be produ- 

 ced ; but, by interrupting the circuit, the piece of 

 wood will be thrown out, and the pyramid will fall to 

 pieces. 



The effect of lightning nj>on ships may be shewn by 

 the following pretty experiment by Mr Cuthbertsoii. 

 InFig.l6.PlateCCXI.Vl II. where A 15 is a trough filled Fig. 1. 

 with water, C a ship swimming in it, so that the top 

 of the mast may nearly reach the bnll L. Place the 

 ship as in the Figure at the end of the trough, and, 

 by means of a thread T attached to it, draw the ship 

 under the ball L, connected with the two charged jars 

 D, F, and the charge flying out of L will strike the 

 mast, and break the ship in pieces. When the ship is 

 repaired, unscrew the round piece of brass from the 

 top of the spindle, and hang the chain q r upon it, 

 having the star A screwed upon the top. Bring the 

 ship, as formerly, with considerable rapidity under the 

 ball L, and it will be struck by the shook, the fire be- 

 ing seen to pass along the chain, without touching the 

 mast. But if the ship H drawn slowly forward be- 

 neath the ball L, no discharge will be heard, as the 

 electricity of the jar is drawn slowly off by the points 

 of the star A. 



Conductors for the preservation of buildings should c nductu 

 be formed of a copper or iron rod, about ^ or J of an for build- 

 inch thick, made sharp at the point, and rising to the ">& 

 height of four or five feet above the highest part of the 



