ELECTRICITY. 



oille of the balance and the glol>c A ln-ing elect rU 

 ficd pusitivtly, the attractive force upon the needle of 

 ..be c was found to be exactly equal to the rcpul- 

 e force of the globe h. 

 An unin-u- Efp. 7. It we place an uninsulated cylinder at dif- 

 kid cyhn- fercnt distances from an electrified globe, so that its 

 */.'.; axis points U> the centre of the globe, we shall ha\e 

 1 Uie following result. The electrical density of the ex- 

 tremity of the cylinder nearest to the globe, will be a 

 little below the power | of the inverse ratio of the dis- 

 **. tanrc of this extremity from the globe. 



t'.ip. 8. In |ilacing *uccc-<-i\cly two cylinders of 

 different diameters at the same distance from an elec- 

 trified gloln', the electrical densities of the extremity of 

 the two cylinders were to one another nearly in the in- 

 verse ratio of the diameters of the cylinders, provided 

 that their diameters were much smaller than the dia- 

 meter of the gloln-. 



Exp. f). In placing an uninsulated cylinder of a 

 great length at a given distance from an electrified 

 globe, Coulomb found that the electrical density of dif- 

 ferent point* of the surface of this cylinder, were in- 

 versely as the square of the distance of these points 

 from the centre of the electrified globe. 



This law, however, does not hold upon a part of the 

 cylinder near the globe, equal to four or five diameters 

 of the cylinder. In this portion, the electrical density 

 increases towards the extremity of the cylinder in a ra- 

 tio much greater ; and, if the cylinder is terminated by 

 a hemisphere, the density at the extremity of the axis 

 nearest the globe is nearly double that of a point whose 

 distance from the extremity of the axis is equal to the 

 diameter of the cylinder. 



F.ifi. 10. If an uninsulated cylinder is placed at the 

 .same distance from the centre of two electrified globes 

 of different diameters, then, supposing the electrical 

 density of the globes to be the same, Coulomb found 

 that the density of points of the cylinder placed at the 

 same distance from the centre of the two globes, was as 

 the square of the radii of the globes. 



By combining the results in the four preceding ex- 

 periments, Coulomb has found, that the electrical den- 

 sities of a hemisphere which terminates different cy- 

 linders presented to an electrified globe, are of a 

 contrary nature to that of the globe, and in the di- 

 rect compound ratio of the density of the globe's sur- 

 face, and the square of the globe's diameter, and in 

 the inverse compound ratio of the power \ of the 

 distance of the centre of the globe from the extre- 

 mity of the cylinder, and of the radius of the cylinder. 

 Thus, if D be the positive electrical density on the sur- 

 face of the globe, whose radius is It ; r the radius of 

 the cylinder ; a the distance between the centre of the 

 globe and the extremity of the cylinder; then, if d be 

 the negative electrical density of the extremity of the 

 cylinder, we shall have 



._ wDR' 

 = 



Now, the constant quantity m wa found by experi- 

 ment to be t=2.0?V'(l inch) ; hence, if the values of 

 a, r and R be reduced to inches, we shall have 



2.07 PR* 

 = 



from experiment. U> th explanation of the effect* of 

 Conductor*, \\ ill be pointed out in the next Cha). 



At/I. 11. If a |il.-iinr, not insulated, U placed at any 

 di-t.mce from an electrified globe, so that the electricity 

 cannot l>e communicated from the one in the other but 

 across the stratum of air which separates them, then 

 Coulomb found that Uio electrical densities of a point 

 in the centre of the plane, is of a nature contrary to 

 that of the gli.be, and is inversely ;u the square of its 

 distance from tlie centre of the globe. 



CHAP. ii. 



On the PheiwmtM of Electricity produced 



The application of this formula, deduced directly 



IN the preceding Chapter we liave endeavoured to 

 give a succinct view of the lending phenomena of elec- 

 tricity as produced by friction : \Ve shall now proceed 

 to direct the attention of the reader to a scries of inte- without 

 resting electrical phenomena as produced without fric- exciutioa. 

 tion, either by a change of temperature, by a change of 

 form, or by the contact of dissimilar bodies ; or as ex- 

 hibited in the phenomena of the atmosphere, or in the 

 functions of living animals. 



SECT. I. On Electrical Phcnninvta produced by a cliange 

 of Teiajtcrature. 



TIIK property of exhibiting electrical phenomena 01. < 

 merely by an increase of tcni]>eraturc, without the aid <-l i*i.>- 

 of friction, is po-i-sscd only by regularlv crystallized " lt:na P 1 * 1 

 minerals. The tourmalin was, for a long tune, the '' 

 only substance which was known to be capable of this u 

 kind of excitation ; but the same property has since tucc. 

 been recognized in the topaz, in calannne or the oxide 

 of zinc, in the borate of magnesia, and in mesotype. 



1. On Ike Electrical Properties of the Tourmalin. 



The electrical properties of the tourmalin seem to On the drr . 

 have been known to the ancients. The Lyncurinm is trical pro- 

 mentioned by Theophrastus, as being a very hard body, I*"'" ' 

 as being usrd tor making .seals, as requiring a great la- llu! '" ur ~ 

 bour to polish it, and as possessing the same |TToi>erty 

 as amber of attracting light bodies : It is therefore 

 highly probable that this substance was the tourmalin 

 of modern mineralogists.* In the island of Ceylon, 

 where it is very common, it is known by the name of 

 iuiirnnmal; and the Dutch, who first became acquaint- 

 ed with it in this island, gave it the appellation of Asc/tcn- 

 trikker, from its property of attracting ashes when it i* 

 thrown into the fire. 



In the year 17I7i Mon. Lemery of the Academy of 

 Sciences, exhibited a stone from Ceylon, which he said 

 attracted and repelled different light bodies, such as 

 ashes, filings of iron, bits of paper, &c. in a manner dif- 

 ferent from a loadstone, t The experiments of Lemery 

 were merely noticed by Linna'iis in his Flora Zeylonica, 

 who mentions this stone by the name of lapis elect ricus. 



When the Duke de Noya was at Naples in the year 

 1 T-l-.'i, he was informed by the king's secretary, Count 

 Pichetti, that he had seen at Constantinople, a small 

 stone, called tourmalin, which had the singular faculty 



niahii. 



1 Se Dr Watson's Obimatimu relating to Ike Lyncurivm rftke Anettntt. Pkil Trant. 1759, vol. li. p. 39*. 



Phi* Paper h entitled OWvofwn* M r tuie pierre de l'l,U de Ceylon fui aUin <t repoustt different corps, nail fwte man/ere differ- 



