.ELECTRICITY. 



of Mr. Coulomb, that a thread of gum- 

 lac is the most perfect of all insulators, 

 and is ten times more effec-tual than a 

 silk thread as dry as it can he made ; 

 "for the former, when only one inch and 

 a halt' in length, insulated as well as a 

 fine silk thread of fifteen inches. When 

 the thread of silk was dipped in fine 

 sealing-wax, it was equal in power to a 

 thread of pure lac of six inches, that is, 

 of four times its length. Professor Ro- 

 bison found that the conducting power 

 of silk thread depends greatly on its 

 colour ; or, in other words, on the nature 

 of the drug with which it is dyed. When 

 of a brilliant white, or a black, its con- 

 ducting power is the greatest ; and a high 

 golden yellow, or a nut brown, renders 

 it the best insulator. Glass, even in its 

 dryest state, and in situations where it 

 was impossible that moisture could have 

 access to it, is stated by the same author 

 to insulate considerably better than silk ; 

 and when drawn into a slender thread, 

 and coated with gum-lac, it acted as 

 well as a thread of lac of one-third of 

 the length. It was found, how r ever, at 

 the same time, that extreme fineness 

 was requisite ; for it dissipated in pro- 

 portion to the square of its diameter. 

 The insulating power of glass is remark- 

 ably injured by having a bore, however 

 fine, unless that bore admits of being 

 also coated with lac. Human hair, 

 when completely freed from every thing 

 that water could wash out of it, and 

 then dried by lime, and coated with lac, 

 was equal to silk. Fir, cedar, larch, 

 and the rose-tree, when split into fila- 

 ments, and first dried by lime, and after- 

 wards baked in an oven, which just 

 made paper become faintly brown, 

 seemed scarcely inferior to gum-lac. 

 The white woods, as they are called, and 

 mahogany, were much inferior. Fir, 

 baked and coated with melted lac, seems, 

 therefore, the best support when strength 

 is required. The lac may be rendered 

 less brittle by a minute portion of pure 

 turpentine, which has been cleared of 

 water by a little boiling, without sensi- 

 bly increasing its conducting power. 

 Lac, or sealing-wax, dissolved in spirits, 

 is far interior, for these purposes, to 

 what it is when melted by heat. 



(26.) The laws which regulate the 

 gradual dissipation of electricity from 

 bodies in a state of imperfect insulation 

 have been investigated with great ability 

 by Coulomb. Three causes chiefiy 

 operate in depriving a body under these 

 circumstances of its electricity : First, 



the imperfection of the insulating pro- 

 perty in the solids by which it is sup- 

 ported. Secondly, the contact of suc- 

 cessive portions of air, every particle of 

 which carries off a certain quantity of 

 electricity. Thirdly, the deposition of 

 moisture upon the surface of.the insu- 

 lating body, which establishes commu- 

 nications with its remote ends, and may- 

 be considered as virtually increasing its 

 conducting power. 



(27.) With regard to the first cause, 

 Mr. Coulomb has completely ascertained 

 that for all fine cylindrical fibres, such 

 as hair, silk, filaments of gum-lac, &c. 

 if the nature of the substance, the dia- 

 meter of the fibre, and the dispersive 

 state of the air are supposed constant, 

 the length of the fibre requisite for the 

 complete insulation of a given intensity 

 of electricity, varies as the square of 

 that intensity. Theory, therefore, leads 

 to the conclusion that, however great 

 may be the intensity, there is always a 

 certain length beyond which a filament 

 of any of these bodies becomes a perfect 

 insulator ; and we find, in practice, that 

 by diminishing the intensity of the elec- 

 tricity, or increasing the length of the 

 substance it has to traverse, a sufficiently 

 accurate degree of insulation may be 

 obtained. With respect to the second 

 source of dissipation, it was found that 

 in a given state of the atmosphere, as 

 far as it could be determined by the 

 indications of the barometer, thermo- 

 meter, and hygrometer, the dissipation 

 at each instant of time, varied directly 

 as the intensity of the electricity. 



(28.) There is one very material cir- 

 cumstance relating to the dissipation of 

 electricity that should here be men- 

 tioned, although its explanation must 

 be deferred till the principles on which 

 it depends have been developed ; and 

 it is, that the power of retaining elec- 

 tricity in any body is much influenced 

 by its shape. The form most favourable 

 to its retention is that of a sphere ; next 

 to which is a spheroid, and a cylinder 

 terminated at both ends by a hemi- 

 sphere. On the other hand, electricity 

 escapes most readily from bodies of a 

 pointed figure, especially if the point 

 projects to a distance from the surface. 

 In such bodies it is scarcely possible, 

 indeed, to accumulate any sensible de- 

 gree of electricity, on account of its 

 rapid dissipation from the point. In 

 like manner pointed bodies receive elec- 

 tricity more readily than those of any 

 other form. 



