Exp. 16. Paste upon a tube of glass a number of small round pieces of tinfoil, as in Plate CCXLIII. Fig. IS. the distance between each being about the 30th of at inch. \Vhen this tube is held near the prime conduc tor, a brilliant line of light will appear to surround it, a spark being visible between each of the small circles of tinfoil.
.Exp. 17. Let several of those spiral tubes be placed on a board, as in Fig. 19. and round a central glass pillar supporting a wire a b, having a ball a and b at each ex tremity, and capable of turning round an axis. Com municate several sparks from a conductor to the ball C, and pushing the wire a b gently round, each tube will receive a spark from the balls, and will be illuminated as in the preceding experiment.
In like manner, luminous words may be made, as in Fig. 20. and a similar effect may be produced, by dis charging a jar coated in the manner represented in Fig. 21. where there are on the outside five rows of square pieces of tinfoil,with eleven in each row, and the same Dumber on the inside of the jar, making in all 110 square pieces. The diagonals of the square pieces are in a ho rizontal and a vertical position, and there is a round hole in the centre of each. Every adjacent pair on the out side jar, have two of their square points meeting in the centre of the round hole in the intervening pieces of tin foil on the inside, so that 110 sparks will be seen at once in a horizontal direction upon the discharge of the jar, and I 1G sparks in a vertical direction.
It was imagined by some of the early electricians, that the electric light contained no prismatic- colours. Dr Priestley observed, however, that this was a mistake, and a similar result has been more recently obtained by Dr Wollaston.t In every respect, indeed, the light of the electric spark comports itself like the direct light of the sun, or of a candle. The writer of this article has found, that it is capable of being polarised like other lights, either by transmission through a doubly refract ing crystal, or by reflexion at the polarising angle from the surface of a transparent body, or by oblique refrac tion through a bundle of glass plates.
IN the Section upon electric light, we have already seen the effect of obtuse bodies, such as halls, in carrying off the electricity of a conductor by a succession of bril liant sparks; and we had occasion to observe, that when a ball was thus receiving sparks, any pointed body pla ced at double the distance of the ball, prevented any sparks from passing between the conductor and the ball, and drew off the whole electricity of the conductor.
This singular property of points drawing off the electri city of any body to which they arc presented, and the corresponding property of discharging the or any body in which they are inserted, may be illustrated by t:le fallowing experiments.
Exp. I. Bring a metallic point near the prime con ductor after it has been charged with electricity, and it will draw off the whole of its electricity without any noise, so that no spark will he received either by pre senting, the knuckle, or any metallic body. In the dark, there will be seen at the point a plume of light if the con ductor be negatively electrified, and a simple point of light if it be positively electrified.
Fix a metallic point in the prime conductor, and when it is electrified by the action of the machine, a hissing noise will be heard, arising from the rapid dis charge of the electricity from the pointed wire. By holding the hand near it, a strong blast of air will be felt as proceeding from the point, and in the dark it will emit a beautiful plume of light if the conductor is positively electrified, but will exhibit a luminous point if the conductor is negatively electrified. This current of air is so strong, as to be able to give a rotatory motion to pointed bodies balanced upon a pivot.
Exp. 8. If when the electricity is drawing rapidly off by means of a point, we present several points together, a spark will pass from the conductor to the points, so that the effect of one point is diminished by the presence of another, and when the points are numerous, they pro duce the same effects as a round body.
Exp. 4. If a man, insulated by standing upon a stool with glass feet, holds in his hand a rod, with a ball at one end and a point at the other, he will be able to electrify himself, so as to give sparks when he holds the point at a short distance from the prime conductor; but when he holds the round end at the same distance from the conductor, he will not be able to draw any electricity from the conductor.