Exp. 3. If the first hall 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. 12. When 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 rendered very bright. On some occasions, the positive spark strikes the nega tive ball at a distance from the negative spark.
E-xP. 5. Take two conductors P, M, about three quar ters of au 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 M, as in Fig. 13. When M is negatively 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 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 elec tricity constantly passing between the ends of the conduc tors, as is shewn in Fig. 15.
Exp. 6. Fix a fine point upon the extremity b of a stem b c, and upon this stem place a brass ball a, about six inches in diameter, in such a manner, that, by turn ing round the ball A, the point may be made either to withdraw itself within the surface of the ball, or protrude itself beyond it. The point will not act when it is be neath the surface of the ball, and the ramified spark will be seen between the two balls, as in Fig. 16 ; but in pro portion 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. When it pro
jects to a sufficient distance from the ball, it acts as if no ball were present. Mr Nicholson, to whom we are in debted for the preceding experiments, remarks, " that the effect of a positive surface appears to extend further than those of a negative one, for the point acts like a ball when considerably more prominent, if it be positive, than it will if negative." Mr Nicholson has applied the preceding principle to the construction of a simple instrument for distinguish ing positive from negative electricity, when they are suf ficiently strong to afford sparks. Two metallic balls, A, B, Fig. 17. may be placed at different distances, by means of a joint at C. The arms CA, CB, arc made of glass, and covered with varnish, and a short point it projects from the ball B towards the ball A. This simple appa ratus is now to be placed near any electrified body, so, that the electric power may pass through the bails ; then if the electricity passes from A to 13, there will be a cer tain distance of the balls, at which a spark will pass be. tween them, and this distance will be much shorter %Olen the electricity is passing from B to A.
7. Hold an uninsulated sheet of paper near a positively electrified conductor, and the electrical light will form a beautiful star, about four inches in diameter, consisting of distinct radii, but not ramified. Perform the same experiment with a negatively electrified con ductor, and instead of forming a star, it will throw out many pointed brushes of light to the paper.
Exp. 8. Present the point of a needle to a body posi tively electrified, in the dark, and the point will be il luminated with a star ; but when the body is negatively electrified, it will exhibit a pencil or brush of light.
From the two preceding experiments, it becomes very easy to distinguish positive from negative elect•i city, by means of the different characters of the electric Exp. 9. It appeared from experiment 6, that the ef fect of a point was destroyed, when it was withdrawn in to a large brass ball, and sparks were produced as if the point had been wholly removed. The same effect will be obtained if the point is inclosed in a glass tube, or if it is placed between two balls.