ELECTRICITY. 



437 



Descriptive lost its charge ; for if new mercury or shot be put into the 

 Electricity, inside, and a new piece of tinfoil on the outside, the 

 >< i "V""*' shock will be felt as strongly as if no charge had taken 

 place. 



Dr Franklin likewise found, that when the glass was 

 gilt, a part of the gilding was torn off, as if the electri- 

 city had passed from the glass through the gilding; 

 and when a thin bottle was broken by the powerful at- 

 traction of the negative and positive electricities on each 

 side, the glass was always broken inwards, while the 

 gilding w.-is broken outwards. The theory by which 

 ])r Franklin explained these various phenomena, will 

 be fully explained in the third Part of the article. 

 Other elec- Other electric substances are capable of being charged 

 tries charged in the same manner as glass. Beccaria imparted a con- 

 like glass, giderable charge to a smooth plate of sealing-wax, made 

 by pouring it in a fluid state upon an oiled marble ta- 

 ble. He charged also a mixture of pitch and colopho- 

 ny, sulphur, and pitch, the magnitude of the charges 

 being in the order in which these substances are named. 

 In our history of electricity, we have already given a 

 sufficient account of the beautiful experiment in which 

 Wilke and /Epinus succeeded in charging a plate of air, 

 and taking shocks from it as if it were a plate of glass. 

 Lateral - We have already seen, that when ajar is discharged 

 plosion. jjy tne discharging rod, no effect is transmitted along 

 the glass handle. When the handle, however, is made 

 of brass, or any other metal, a small shock is transmit- 

 ted to the person that holds it, when the quantity of 

 electricity accumulated in the jar is considerable. In 

 like manner, it' a small chain is connected with the out- 

 side coating <>t a charged jar, it will appear luminous 

 when the jar is discharged in the dark by any conduct- 

 ing circuit, of which this chain does not form a part. 

 This effect has l>een named the lateral explosion, and 

 may be shewn most conspicuously by the following 

 method, which was pointed out by Dr Priestley. In- 

 sulate a thick metallic rod, and place it with one of its 

 ends contiguous to the external coating of a charged jar, 

 and at the distance of half an inch from its other ex- 

 tremity place a large body about G or 7 feet long, and a 

 few inches broad. Let a chain be now placed upon the 

 table, at the distance of 1 ^ inch from the outside coat- 

 ing, and apply one end of the discharging rod to the 

 other extrciiiit v of the chain. As soon as the opposite ex- 

 tremity of the discharging rod is made to communicate 

 with the knob of the jar, the discharge will take place, 

 and a brilliant spark will be seen between the insulated 

 metallic rod and tfe large body adjacent to it. This 

 spark does not change the electrical state of the body ; 

 and it is therefore imagined, that this lateral spark flies 

 from the coating of the jar, and returns to it IK t'.rc 

 an electrometer can be affected. This lateral spark has 

 the same length and brilliancy, whether it is received 

 on flat or smooth surface*, or on sharp points. In op- 

 position to the result of the preceding experiment, Dr 

 R obi son informs us. that he hut perceived a delicate 

 electrometer affected by the lateral explosion. 



SECT. VII. On the Eltctricnl Spur/:, and Electrical l.il,t. 



On the elee- THE light which always accompanies the excitation 

 trical upark, of an electric, was observed about the same time by ( )t- 

 idl^ht' to ( ' uericke "* Magdeburg, and our countryman Dr 



Wall, and attracted the particular attention of every 



subsequent electrician. 

 "During the excitation of a glass tube in the dark, 



small flashes of light, accompanied with a crackling 



noise, are always observed ; and in working an electri- 





Fig. 11. 



fying machine, streams of blue light are constantly flash- Descriptive 

 ing over the surface of the glass cylinder. The electri- Electricity^ 

 cal light, however, is more distinctly observed when any ^ *"Y"*" 

 round body is brought near the prime conductor of , 



the machine. A brilliant light, called the electric spark, 

 is seen between the body and the conductor, and ex- 

 hibits a great variety of appearances under different cir- 

 cumstances. 



Eip. 1. If we hold a brass ball, about three inches in The sparks 

 diameter, near the prime conductor of an electrical ma- 

 chine, sparks of bright white light will appear between 

 them, attended with a loud snapping noise. When the O f {] le con . 

 ball is very near the conductor, the sparks appear in ductor. 

 quick succession, and are then perfectly straight. 



Exp. 2. Having screwed a brass ball, about 2 inches 

 in diameter, into the prime conductor positively electri- 

 fied, so as to stand about three inches from it, hold ano- 

 ther ball near the first, and long ramified or zigzag sparks 

 will be obtained. This positive spark is represented 

 in Fig. 10. where not denotes the ball held in the hand, PLATE 

 and in a natural state of electricity, while pos is the CCXLIII. 

 positive ball connected with the conductor. When F '8- lu> 

 the ball on the conductor is made very small, the spark 

 becomes a faint divided brush of light. 



E.rp. 3. If the first ball in the preceding experiment 

 is electrified negatively instead of positively, the spark 

 becomes dense, straight, and more luminous than be- 

 fore. This is represented in Fig. 11. 



Exp. 4. If one of the balls is positively and the other 

 negatively electrified, the spark combines the appear- 

 ance both of Figs. 10. and 11. as in Fig. V2. When Fig. 18. 

 the distance between the balls is not too great, the 

 positive zigzag spark strikes the negative straight spark 

 generally at the distance of one-third of the length of 

 the latter from its point, the other two-thirds being ren- 

 dered very bright. On some occasions, the positive 

 spark strikes the negative ball at a distance from the ne- 

 gative spark. 



E.rp. 5. Take two conductors P, M, about three quar- 

 ters of an inch in diameter, with spherical ends of the 

 same diameter, and place them parallel to each other, 

 so that their distance is about two inches, and their ends 

 pointing in opposite directions, and six or eight inches 

 asunder. These conductors are to be successively elec- 

 trified. When P is positively electrified, it exhibits the 

 ramified zigzag spark, and strikes the side of the other 

 conductor^!, as in Fig. 13. When M is negatively Fig. 13. 

 electrified, P being now connected with the earth, the 

 sparks cease to strike as formerly ; but the end of the 

 electrified conductor M will exhibit the negative spark, 

 and strike the side of P, as in Fig. 14. When one of p; g . 14. 

 the conductors is electrified positively, and the other ne- 

 gatively, the ramified or positive spark appears at one 

 end, and the negative spark at the other, streams of 

 electricity constantly passing between the ends of the 

 conductors, as is shewn in Fig. 15. pjg, 14. 



I'.sp. (>. Fix a fine point upon the extremity 6 of 

 a stem l> c, and upon this stem place a brass ball a, 

 about six inches in diameter, in such a manner, that, by 

 turning round the ball A, the point may be made either 

 to withdraw itself within the surface of the ball, or pro- 

 trude itself beyond it. The point will not act when it 

 is beneath the surface of the ball, and the ramified 

 spark will be seen between the two balls, as in Fig. 16; pjg. ig. 

 but in proportion as the point is protruded beyond the 

 surface, it increases the transmitting power, and may be 

 made to have the same effect as any ball whatever, from 

 the smallest size to six inches in diameter. Wluii it 

 projects to a sufficient distance from the ball, it acts as 



