ELECTRICITY. 



Mbounof 



..'. > '. 



- i.Uo a belter cetuluctor than fresh water. From 



"" "V^"* several i \|>ciiiuci:U. Mr Cu\ctulish likewise determined, 



that the quantity of electricity which LIM of 



different shape* and magnitudes will ..cq'.iiie from the 



trilicalion, is directly as tin- area of 



1 inversely as the thickness of the glass. 



.i)l. IxvL 



Mr Cavendish was OIK- of the first persons who su- 

 oessfully emploved electricity as a chemical agent, and 

 hid the merit of beginning those brilliant investigations 

 which have since presented chemistry with many inte- 

 resting result*. He succcedc 1 in decomposing atmo- 

 spheric air, by means of lh<- electric spark, lie formed 

 pure water by exploding a mixture of hydrogen and 

 \ygen gas ; and he produced nitric acid, by exploding 

 .-ares of oxygen with three measures of nitro- 

 gen. A full account oi these experiment- will be found 

 in iv 



The science of electricity was now destined to n . 



t brilliant accessions from the genius of M. 



Born 17:{G. Coulomb, to whom many of the physical sciences owe 

 JbOtJ. the deepest obligations. By means of aij ingenious in- 

 struiueiit invented by himself, called a torsion balance, 

 Coulomb was enabled to measure small forces with a 

 degree of accur.-u.-y hitherto unknown in physics. In 

 order to ii\.iil himself of this contrivance, be investiga- 

 ted, both theoretically and experimentally, the laws of 

 the force of tor.-imi, (or the force with which a body 

 in; >!*,) relative to the length, the thickness, 



apd the nature of the metallic wires which he employ- 

 ed, and he has applied these laws with the utmost 

 address, and in the most successful manner, to the 

 most delicate researches in electricity, magnetism, and 

 hydrodynamics. In the theories of JEpinus and Ca- 

 vendish, the action of the electric fluid, in produ- 

 cing attraction or repulsion, was considered merely as 

 diminishing with the distance; and in consequence of 

 the law being undetermined, several problems received 

 only an approximate solution. By the contrivance how- 

 ever already mentioned, of which we shall give a minute 

 account in another part of tin's article, * M. Coulomb 

 discovered that the electrical force was, like that of 

 etavity, in the inverse ratio of the squares of the dis- 

 tances ; a conclusion the more interesting, as Newton 

 and other natural philosophers had believed that the 

 electrical and magnetical forces followed the inverse 

 nitio of the cube, or even some higher power of the 

 distance. The experiments from which this result 

 was obtained, he varied in different ways, and always 

 luund them conformable to the preceding law ; and in 

 another memoir, he has explained different means by 

 which he had obtained a similar result. The labours of 

 Coulomb, however, were not confined to the determi- 

 uation of the law of electrical action ; he found that the 

 momentary dissipation into the air of moderate de/ 

 jjrees of electricity, is proportional to the degree of elec- 

 tricity at the time ; that the dissipation is not sensibly 

 changed by any variation in the temperature or weight 

 uf the air ; and that when the electricity was very 

 weak, there was no perceptible difference in bodies of 

 different kinds, or differing in shape or magnitude. 

 The dissipation, however, was greatly affected by the hy- 

 grometrical state of the atmosphere, and was very near- 

 ly as the cube of the moisture of the air. M. Cou- 

 lomb likewise examined the dissipation which took place 

 along imperfect insulators, in which case the electricity 



to l>e conveyed along the surface of the insula- " 

 tor chiefly by the moisture which adheres to it, and V "^"Y"^'' 

 obtained the following rc-idts: 1. The ' 



Cerent points of an imperfect in-ulator are in the sub- t'oulomb. 

 duplicate ratio uf their di.-tance from the point of com- 

 plete insulation. _'. The K-rgths of canal requisite to 

 in-ulati- electricity of different densities', is in the du- 

 plicate ratio of the det.-itics. And, 3. The length of 

 canal necessary for insulation, is in the inverse ratio oi 

 the coercive or insulating force of the canal. M. Cou- 

 lomb likewise found, that the most perfect of all insu- 

 lators i> a thread of gum lac, which, measuring its ex- 

 cellence by its .-l"irtni'!-, insulates ten times better than 

 a silk thread as dry | the silk t! 



'\<rcd with fine sealing-wax, it had the same 

 power of insulation a* gum lac, when it was four times 

 us long. The dissipation idong these insulating bodies 

 i^ not wholly owing to moisture, but to a small degree 

 of conducting pov < 



The next object of M. Coulomb was to ascertain the 

 distribution of the electric fluid in an overcharged body. 

 I le found, that if \ve bring an electrified conducting bo- 

 dy iiito contact with another in its natur.d slati 

 electricity conveyed to the latter will depend solely on 

 the shape of the two bixlics, and in no rc.-pcct upon th 

 nature of the body itself; and that the fiuid diffuses it- 

 self along the surface without penetrating the body. 

 This result Coulomb at fir.-t obtained for bodies simi- 

 lar in form, and equal in surface : but he afterward* ex- 

 tended his enquiries to spherical bodi'-s having sur; 

 of different magnitudes ; and the general ret-ult 

 that when the surfaces of the unequal sphere-, wt 

 the numbers 1,4, 1 6,0'i, in/iaitr, then the duisi:; 

 the small globe was as the numbers 1, l.(H, 1.3, 

 2, but never obtained the magnitude 2. The density 

 of the fluid on different parts of the surface of two equal 

 spheres in contact, he found to be as the numbers 0, 1, 

 4, 5, 6, at the angles 0, 30, 60, 90', 180, being al- 

 most uniform from 9 to 18C 3 . M. Coulomb has 

 likewise determined the mode in which the electric 

 fluid is distributed among globes placed in a row, and 

 also over different points-of the surface of a cylinder. 

 In all his calculations, he has employed the hypo- 



of the electric fluid being ODnpOMd of two 

 fluids, which are neutralised in the ordinary state of bo- 

 dies, and disengaged when the body exhibits signs of 

 electricity. The particles of each fluid mutually r*. 

 pel each other, and attract ti\>se of the other fluid. 

 These two fluids have been called the fluid of tilienus, 

 and the fluid of rrsinnus electricity. 



There are few philosophers of the present day to DiscoToiig 

 whom electricity owes so much as M. Volta, professor of Volta. 

 of natural philosophy, at Como, in Italy. He iinented 

 an elegant instrument for collecting electricity, called 

 an electrophorus, t the first hint of which is said to 

 have been given by jDpinus ; and another instrument, 

 called a condenser, the object of which is to accumulate 

 and render visible the smallest quantities of natural or 

 artificial electricity. The Galvanic discoveries of Volta 

 have raised him to a high rank among the discoverers 

 of the present day; and under the article GALVANISM 

 we shall have frequent occasion to follow him in his 

 career of discovery. 



In the year 1781, MM. La Place, Lavoisier, and 

 Volta, discovered, that solid and fluid bodies passing 

 into the gaseous state, gave unequivocal marks of ne- 



S Put I. Chp. I. Sfrt. IX. pp. 441, 450, 441. 



-j The name of elccuophsrus u given to this inHrumeni from itt rrtauriog or curving its elettxic virtue for a long time* Sec p. 515. 



