140 



ELECTRICITY. 



ced at double the distance of the ball, prevented any 



''i|fr; sparks from passing between the conductor and the ball, 



"""^ and drew off the whole elis-tricity of the conductor. 

 This singular property ofiioinU drawing off the elec- 

 tricity of any body to which they are presented, and the 

 corresponding property of di*chrging the electricity 

 of any body in which they are inserted, may be illus- 

 trated by the following experiments. 



Exp. I. Bring a metallic point near the prime con- 

 ductor after it has l>een charged with electricity, and it 

 will draw off the whole of its electricity without any 

 noise, so that no spark will be received either by prc'- 

 senting the knuckle, or any metallic body. In the 

 dark, there will l>c seen at the point a plume of light 

 if the conductor be negatively electrified, and a simple 

 point of light if it be positively electrified. 



Exp. 2. 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 mo- 

 tion to jMiinted bodies balanced upon a pivot. 



Exp. 3. 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 pre- 

 sence of another, and when the points are numerous, 

 they produce 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 l>e 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. 



Eip. 5. If a man, insulated as before, is electrified, 

 , the electricity will be rapidly discharged when he holds 



out any pointed body, and in the dark the light will be 

 seen streaming from the point. 



Exp. 6. If two cross wires ab, cd, Fig. 1. with a cape 

 at the point of intersection, and having 4 sharp points 

 as a, b, c, d, turned in the same direction, be nicely ba- 

 lanced upon a point at the top of the insulated rod or 

 stand ef, and if they are connected with an electrified 

 conductor by the chain g ft, they will immediately re- 

 ceive a rotatory motion. In the dark, a stream of light 

 will issue from each point, and, from the velocity of ro- 

 tation, these four streams will form a beautiful circle of 

 light. 



Exp. 1. Upon the preceding principle, an apparatus 

 called the electrical orrery is constructed. If a globe S 

 Fig.!. (Fig- 2-) representing the sun, has a projecting arm 

 S b r, and if upon the pointed extremity of this arm 

 is balanced another ball E, representing the earth car- 

 rying a wire M a, with a point a at one end, and at the 

 other a ball representing the moon; then, if a point 

 b is fixed in the middle of the arm S F, and if the 

 whole is balanced on an insulated stand, by loading 

 one side of the globe S, let the apparatus be connected by 

 a chain g h, with the prime conductor of an electrical 

 machine, and a rotatory motion will be given to both 

 the arms, so that the moon M will move round the 



PLATE 

 C'CXLIV. 

 1'ig. 1. 



earth F., while the earth performs its revolution round ncript! 

 the sun S. Eltv- 



s. In-i-rt five arms of wire, having their extre- """ ' 

 mities |x,jntc<l and turned in the Mime direction, into p,,,, 

 a piece of wood supported upon a point at the top of a iv\I.|V. 

 glass pillar, as shewn in Fig. .';. To one of tliev ; 3. 



which is longer than the rest, suspend a glass clapper 

 by a silk thread, and Miiml it a rod c d. Let 8 bells 

 be now placed upon the *tand. and it' a chain panes 

 from the upper point a to the prime conductor, the 

 points will move round, and thegL.-s cl.ipjx-r will strike 

 against the bells during it~ -mcivss^ > rcv.ii'r 



A'.r/i. !). Let an inclined plane lie Conned of two 

 straight parallel wires, AH, CD. and iiiMil.it.-d \\\ four 

 !. lass pillars A, 15, C, I) fixed in a stand; and" let a 

 small wire, m >i, with two little balls nt it-, extremi- 

 ties, be made so as to slide down the parallel wires by 

 the force of gravity. On a pivot in its centre, balance 

 a wire op with two points turned in the same direction. 

 When the wire m n is near the lower end of the in- 

 clined plane, connect the parallel wire with an electri- 

 licil conductor by means ot a chain. The wire op will 

 immediately turn round the pivot upon mn, and will 

 ascend the inclined plane. 



The great utility of points a conductors or thunder 

 rods for carrying off the electricity of the atmosphere, 

 and the theory of their mode of action, will be fully 

 explained in other parts of the present article. See 

 Si ( r. XI. Oil the Distribution of Electricity, Art. 3. p. 

 455. 



SECT. IX. On the Lair of Electrical Attraction and 

 Repulsion. 



Sir Isaac Newton, and some other philosophers, had 

 long ago imagined, that electrical and magnetical attrac- 

 tion followed the inverse ratio of the cube, or some high- 

 er power of the di-tancc. Kpimisand Cavendish, in 

 their theories of electricity, have assumed no particular 

 law, but merely that the action decreases n- the distance 

 increases, although the former had suspected that the 

 law was the same as that of gravity. 



Lord Stanhope had endeavoured to shew, that elec- 

 tric actions followed the inverse ratio of the square of the 

 distance, and his reasonings were by no means conclu- 

 sive, and the law was still considered as undetermined. 



Our countryman Dr Robison, more than 4t> years ago, 

 made a variety of experiments on this important sub- 

 ject, and determined that the law of electric attraction 

 and repulsion was nearly in the inverse duplicate ratio 

 of the distance. An account of his experiment- 

 read before a public society so early as 1769, but un- 

 fortunately he did not then regard them as of sufficient 

 importance to belaid before the public. As we conceive 

 that Dr Robison is entitled to a share of th:it honour 

 which is now given to Coulomb, we shall lay before our 

 readers an account of the method by which he ascer- 

 tained the law of electrical action. 



The instrument with which \)r Robison's experiments 

 were made, is represented in Figs. 5. and 6. 1 . 

 being a perspective view of the whole instrument. A 

 polished black ball A, ^ of an inch in diameter, is fixed 

 on the point of a very slender needle three inches in 

 length ; the other extremity of the needle passes through 

 a ball of amber or glass F, about three-fourths of an inch 

 in diameter, but though the ball is completely perfora- 

 ted, the end of the needle must not extend quite to the 

 surface. At right angles to the needle, a slender glass 

 rod is fixed into the bull F ; it is bent at right angles at 



On the law 

 of electrical 



attraction 

 aiitl n-jml- 

 sion. 



Dr Robi- 

 son's expe- 

 riment* on 

 the law of 

 electric 

 attraction. 



Instrument 

 med by Dr 



Robison. 



PLATE 



!< XI. |V. 



I ig. 5, 6. 



