EXCITATION OF ELECTRICITY. 



motion of the non-conducting body car- 

 ried it off to a situation where it lends to 

 its former state, and consequently advan- 

 ces in a current towards such parts as al- 

 low of the restoration of that state. It 

 seems reasonable to conclude, that the 

 di&turbances of the ehe'vic state or equi- 

 librium, and the currents by which they 

 are restored, are in most natural opera- 

 tions performed r hrough very short and 

 good conductors; so thru, though in all 

 probability they may contribute to very 

 important results, *he immediate changes 

 eludt. our observation, except in a few 

 instances, such us that of lightning and 

 luminous meteors. And it seems from 

 the facts to be nearly decided, that we 

 should never have had it in our power to 

 exhibit the phenomenon of the electric 

 spark, which is electricity producing ig- 

 nition by breaking through a non-con- 

 ductor, if we had not fortuitously experi- 

 mented in circumstances, where the elec- 

 tricity is first made to take the. form of a 

 charge, and afterwards brought into a 

 state of considerable intensity, by sepa- 

 rating those bodies from each other, 

 which produced the compensation by 

 their opposite states. Thus in the elec- 

 trical machine, (see Nicholson, in the 

 Philos. Trans. 1789,) little or no electric 

 signs are produced by a cylinder rubbed 

 by a very flat amalgam ed leather, termi- 

 nating in a neat line of contact. But this 

 rubber and cylinder will, without any al- 

 teration, afford electricity, if a flat piece 

 of metal, or the hand, or any other flat 

 conductor, be held over that part of the 

 cylinder which is in the act of receding 

 from the cushion, even though this con- 

 ductor be held at the distance of an inch 

 or more, without touching either the 

 cylinder or its rubber. It is proved from 

 experiment, that the conducting body 

 thus presented acquires the opposite 

 state, and enables the cylinder to carry 

 off a greater quantity of electricity in the 

 form of a charge, the interposed air being 

 the electric. 



When the cushion is thick and round- 

 ed, as is the case with the human hand, 

 which was first used for this purpose, the 

 rounded part opposite the receding sur- 

 face of the cylinder, performs the office 

 of compensation ; and the best applica- 

 tion, which has yet been made for this 

 purpose, is that of a flap of silk proceed- 

 ing from beneath the cushion, which as- 

 sumes the negative state, so as to com- 

 pensate the positive state on the cylinder, 

 in a very considerable charge, which is 

 conveyed by the rotation to the farther 



end of the silk, where it becomes uncom- 

 pensated electricity upon the naked sur- 

 face, at an intensity which could not 

 otherwise have been produced. 



It has not been determined yet what 

 are the conditions and circumstances of 

 the change which takes place by the ac- 

 tion of the air at the face of the rubber, 

 nor why the surface of the glass should 

 become" positive when rubbed with one 

 kind of rubber, as for example the human 

 hand; and negative, if rubbed with ano- 

 ther kind, such as cat-skin, or flannel; 

 nor why glass, deprived of its polish, be- 

 comes negative with rubbers, which 

 would have rendered smooth glass posi- 

 tive. The most rational conjecture seems 

 to be, that the surface which is most 

 heated in consequence of its roughness, 

 or the relative smallness of its dimen- 

 sions, acquires the negative state. 



There is a certain velocity of rotation, 

 which is about five feet per second, at 

 which the excitation of electricity by a 

 cylinder nearly vanishes; but it returns 

 again the moment the velocity is dimi- 

 nished. Some, who maintain the exis- 

 tence of a material cause of heat, or 

 caloric, are disposed to consider electri- 

 city as one of the states of caloric, in 

 which the matter of heat can pass through 

 bodies without raising their tempera- 

 ture, and with much greater velocity 

 than that by which temperature is com- 

 municated. 



From the imperfect knowledge we pos- 

 sess respecting excitation, it is very diffi- 

 cult for the most experienced electrici- 

 ans to excite a cylinder with certainty and 

 power. If the cylinder be greased all 

 over with tallow, and then turned for 

 sometime in contact with the cushion, 

 the silk flap being thrown back, and an 

 amalgamed leather be applied and rubbed 

 about upon the surface of the cylinder in 

 motion, electric sparks are soon produced 

 in abundance ; and if the silk be then 

 thrown again into contact with the cylin- 

 der, the excitation will, in general, be 

 strong ; but it is seldom so strong at the 

 first time of exciting, as it proves to be 

 after the expiration of a day or more. It 

 seems as if the amalgam and tallow re- 

 quired a considerable time of working 

 to be brought into the best state for ex- 

 citation. 



In order to judge of the degree of in- 

 tensity of an excited cylinder, we must 

 have recourse to some standard of the 

 quantity of effect produced by the fric- 

 tion of a given surface. It has not been 

 shewn that much, if any thing, depends 





