.-.28 



ELECTRICITY. 



> :..t,u:ty. 



Om Kin- 

 utnkj't 



fixed wire N, anil of course renders the plate B unin- 

 sulated. When tlie lexer Kl.. however, Ix-gins to move 

 towards X, the extremity m of the wire () m \* withdrawn 

 from N, Mid the plate B remains insulated ; nnd when 

 the lever has reached the )*<-itioii KX as far as it can 

 go, the wire m louche- the plate C, BO that the insula- 

 ted pUtes B and C' now communicate with e:i-h other. 

 When the plate* A anil B arc together, as in the 

 Figure, A ha it* capacity for electricity increased by 

 the pretence of the uninsulated plate H, and therefore it' 

 any weakly electrified body is made to tnueh A, it will 

 acquire a greater quantity of electricity from the con- 

 tact than it would otherwise have done. Let us sup- 

 pose that A acquires a small quantity of positive elec- 

 tricity, then B will acquire negative electricity. <>n 

 moving the lever KI, towards X. B remains insulated 

 and possessed of negative electricity. When I? touches 

 C by mean- of tin- wire viO, its negative electricity pas- 

 ses almost entirely to C, as the capacity of C for eiec- 



pames through a hole in the upper cap.and descend* into 



MIIII. the Uittom of the tul>e. 'I'he two hooked ''- Ic ' 



wire- 1 <i. II, slide through the caps, KI that the luass 1 7|"^'^"~ 

 knc.! > any distance. This iu.-trument ( ( ;. 



is placed upon a wooden stand C'l). If the knobs (>, I Fig. 4. 

 are brought into contact, and a charge sent through 

 them, by connecting the hooks F. I 1 ' with the outside 

 and inside of a I.eydenjar, no effect will he prod need; 

 but if the knobs are separated, so that the charge i. 

 sent through the intervening mass ofiiii, the air displa- 

 ced and raritied will press u|Kintlie water.it the bottom 

 of the tube, and raise it almost to the top of the small 

 tube All : it will then sink a little, and will afterward- 

 gradually subside into it- first position. 



SECT. II. folia's Hi/ilrogcn Lamp. 



THE facility with which hydrogt n gas i- inflamed, Volu'thjr- 

 i ven by a small electric spark, suggested to Mr \ olta < lr<> K cn 

 the construction of his hjdrogen lamp. A (piantity of 



tricity is greatly increased by the presence of the tin- hydrogen gas is put into 'a reservoir, and, by means of 

 insulated plate D. If the plate B is carried back to A, t |" K . j.re.-sii'rc ofacohu 



it will acquire a new quantity of negative electricity, 

 which in a similar manner may be communicated to C. 

 By a repetition of thii process, the electricity will be 

 multiplied or accumulated in C. The plate D is then 

 draw n out from C by means of the slider FP, and hence 

 .pacitv of C for electricity will be much diminish- 

 ed. Consequently, if an electrometer be brought into 

 contact w ith it, the negative electricity will be indi- 

 cated by the divergency of the gold leaves. Mr Cavallo 

 is of opinion, that the principal cause that renders this 

 instrument certain in its effects, is, that all the residuum 

 of elcctricitv which can remain upon the plate A, after 

 an experiment is performed, and after that plate is 

 touched, is too inconsiderable to induce a contrary elec- 

 tricity in B, the electricity which is originally commu- 

 nicated to A not being increased upon it in the course 

 of the experiment. The drawing is about 4d of the 

 real size of the instrument. See Cavallo's Treatise OH 

 Electricity, 1th edit. vol. iii. p. 98, and his Elementt of 

 yattiral Philotophy, vol. iii. p. 45 *29. 



BOOK V. 



ON INSTRUMENTS FOR GENERAL PURPOSES. 



SECT. I. On Kinncrsley's Electrical Air Thermometer. 



THIS instrument is represented in Plate CCL. Fig. 4. 

 where AB is a glass tube about 1 inches long and tw o 

 inches in diameter, and made air tight at both ends by 

 two brass caps. A small tube, HA, open at both ends, 



column of water, the gas is allowed to 

 from a small aperture by turning a stop-cock. 

 An electrophorus is placed in a box below this reser- 

 voir, and a wire passes through a glass tube from the 

 ti))|ier part of the box to the small aperture. The co- 

 ver of the clectrophorus being connected by a silk cord 

 with the handle of the i-top-cock, is raised when the 

 cock is opened, and the spark from this cover is con- 

 veyed by the insulated wire to the stream of gas, which 

 it instantly kindles, so that a candle may foe immedi- 

 ately lighted. As only a small quantity of the gas i> 

 consumed, a light may be procured above a hundred 

 times before all the gas in the reservoir is exhausted. 



SrtT. III. I'ulta's Etn-lric Pistol. 



Tin- instrument consists of n brass \cssel, of an ol>- 

 long spheroidal form, pierced at its two vertices. Into 

 one of these openings, a glass lube of the same diame- pistol. 

 in- i- introduced, which projects about four inches be- 

 yond the vessel, and is continued nearly to the middle 

 of the cavity. A metallic stem passes through this 

 tube, the outer end of which carries a brass ball, while 

 the other end extends beyond the inner extremity of 

 the tube. Through the other aperture in the sphe- 

 roidal vessel, is introduced an equal mixture of in- 

 flammable gas and atmospheric air, and the aperture i- 

 then closed by a cork. The vessel is now taken in the 

 hand, by the middle of its convexity, and a spark sent 

 from the prime conductor to the metallic ball, will 

 instantly inflame the gas, and drive out the cork with 

 a smart explosion. 



PART III. THEORETICAL ELECTRICITY. 



HAVING thus laid before our readers the great body of 

 which constitute the Science of Electricity, we 

 g| m ll now pr.K .c<l to that branch of the subject which 

 professes to connect together, by one general principle, 

 the various ,,) . hich we have described. 1 his 



principle, however, is nothing more than a hypothesis ; 

 and while our attention is occupied with the electric 

 fluid, and with the properties which it is assumed to 

 posne**, we must nc ver forget that we are speaking of a 

 .substance, the existence of which is yet undetermined, 

 v. hich we know merely from its effects. 



TliMrctiul 



Mcclrioitv. 



c HAP. i. r-' 



On the Theories of Electricity before the time of 

 JEpinns and Cavendish. 



IN our History of Electricity, we have already given <^ 

 a sufficiently fuU account of the crude hypotheses of | hoftc rf 

 Caba-us, Descartes, and Boyle; of the more rational _.*;,< ..u 

 views of Dufay, Nollct, Symmers, and Elhcott; and ol i:.,, 





