591 



ELECTRICITY. 



Volta'scon- 

 denscr. 



pccu-d with 

 n rlectro- 



till it strikes the Iwll P. If the arm OH be again set 

 horizontal, and the slider g at 10 grains on the c;de, 

 the ball C! will rc-t upon F with n pressure equal to 10 

 grains ; so that a certain quantity of electricity will l>e 

 necessary, in order thnt the balls may -i- -arate. Hence 

 the number of grains to which the index s is set, will 

 always bean exact measure in ^r:iin- of the electricity 

 which separates the kills (! ami I'. This in-trument is 

 n combination of Henley's, Lane's, and Brooke's elec- 

 trometer. Henlcv'- electrometer is of use to shew the 

 progress of the charge. See Cuthbertson's Practical 



icitfi, p. 177. 



For an account of a numlx-r of other electrometers, see 

 Darcy '> l-'.lectrometer. M,-in..t ,nl. I',i> . I 7 H>, p. <>-'* Rich- 

 man's Electrometer, A'oc Cum. Pclrop. iv. p. 301. Co- 

 mus' Electric PlatomcU -r. I!o/ier'- Journal, vii. p. .'.'JO. 

 Terry'- Electrometer, Ro/ier, xxiv. p. SIS. Id. xxv. 

 228. P>ii\cr Brim'.- Electroscope for a Conductor, Ro- 

 zier, xxviii. p. 183. De Luc's Fundamental Electro- 

 meter, Iilft-s stir Mftforologie, rccxcvu. Chappe's 

 Electrometer, Rozier, xxxiv. p. 'J~0. Vassal!, Mem. 

 Turin. 17JX>, v. p. ~)~. Cadet's Electrometer, Annales 

 tie Cliimie, xxxvii. p. 6$. Nicholson's Journal, v. p. 31. 

 Marechaux's Electrometer, Nicholson's Journal. Law- 

 son's Discharging Electrometer, Phil. Mag. xi. p. 251. 



BOOK IV. 



ON INSTRUMENTS FOR CONDENSING, DOUBLING, AND 

 MuLTii'LYiNu ELECTRICITY. 



TIIF. electrometers described in the preceding Book, 

 though extremely delicate, are unable to indicate the 

 presence of weak degrees of electricity. In order to 

 remedy this defect, several ingenious instruments have 

 been employed for condensing, doubling, and multiply- 

 ing small degrees of electricity. The most important 

 of these, we shall now proceed to describe. 



CHAP. I. 



Instruments for Condensing Electricity. 



SECT. I. Volta's Condenser. 



Tins instrument is nothing more than an applica- 

 tion of the electrophorus already described. Having 

 constructed an electrophorus with a very thin resinous 

 plate, and having its electricity completely extinguish- 

 ed, lay the upper conductor upon this resinous plate in 

 full contact, taking care that it does not touch the lower 

 conductor. A wire is now brought from the place 

 where the weak electr.city exists to the upper conduc- 

 tor alone. The apparatus being left a certain time in 

 this situation, till the upper conductor has accumulated 

 a sufficient quantity of electricity, remove the conduct- 

 ing wire, and raise the upper conductor from the resi- 

 TIOII- plate by the insulating handle, and it will be found 

 to have condensed so much electricity as to aflect the 

 electrometer most powerfully, and even to give sparks. 

 As the thin resinous cake may receive some electricity, 

 Volta substituted in place of it an imperfectly conduct- 

 ing substance, such as a clean and dry marble slab, a 

 plate of wood clean and dry or covered with a coat of 

 varnish, or even a conducting body covered with a 

 piece of dry tattety or other dry silk. 



i Comdtiuer connected with an Elect ronnitr. 



Tliis instrument, wliich Volta has employed for dc- 



termining the effects of Galvanic electricity, is rrpre- Prurtio! 

 sciited in Plate ( ( XI IV Fig. IS. The electro. 

 consists i,f two slijr- of straw ./r, HX, perfectly even and ^ 

 rtraight. These straws are suspended by means of two 

 metallic wires terminating in hooks, which move free- 

 ly in two small holes mnde in the lower end of a pi. re 

 of metal, whose upper extremity is soldered beneath 

 the stopple of the jar f /it. Above this stopple is ce- 

 mented the collecting plate or disc of brass nl, from 

 the lower surface of which proceeds a metallic wire 

 tipt with a metallic globule g. Above this plate is 

 another plate nli. with an in-ulating glass handle mn, 

 and communicating with surrounding bodies by a slip 

 of metal //ir, bent in such a manner as not to. 

 near the collectim,' plate. The lower surface of rt'i, 

 and the upper surface of erf, are both varnished. A 

 graduated scale tz is placed on the out-ide of tl -.. 

 for estimating the angular distance of the straws whin 

 in a state of divergency. The electrified body is 

 brought repeatedly in contact with a small globi 

 and after a certain number of contact.", the upper pi.ite 

 ab is lifted up, and the straws immediately diverge. 



SECT. II. Cavallo't improved Condenser. 



IT occurred to Mr Cavallo, that if the condensing Cvallo'i 

 plate of the electrophorus indicated small degree-, of improved 

 electricity that could not otherwise be observed, another ^ink-user. 

 smaller plate, or small condensing apparatus, might be 

 employed, to condense the weak electricity of the large 

 plate. He therefore constructed a small plate about the 

 size of a shilling, having a glass handle coated with 

 sealing-wax, and when the large metal plate w.-r, 

 scarcely able to aflect an electrometer, he placed the 

 small plate upon the inferior plane, and touched it with 

 the edge of the large plate. He then removed the large 

 plate, and upon presenting the small plate to the elec- 

 trometer, the balls diverged to their utmost limits. 



ECT. III. Cavallo's Condenser, or CoUcctor of 

 Electricity. 



THIS instrument is represented in Plate CCXLIX. Cavallo'a 

 Fig. 16. where GHII.KM. and NOPV, are two frames ci.mk-nscr, 

 of wood, fastened to a woollen base by brass hinges, so or collector 

 that they can be either placed vertically, as in the 1 i- ".' 

 gure, or laid out in a horizontal position, so as to be 

 supported on the table on which the instrument stands. F'8- 1 "- 

 The inner surfaces of these frames above their middle, 

 are covered either with gilt paper, cr with very flat tin 

 plates. These two frames are kept in a vertical posi- 

 tion, by me.ins of two flat pieces of wood S and T, con- 

 nected by a hook H. Between these two vertical frames 

 there stands aflat tin plate, l.'i inches long, and 8 inches 

 broad, soldered to two tin tubes A, A, only one of which 

 i-^ seen in the Figure. These tin tubes being support- 

 ed by glass rods D, I), coated with sealing-wax, only 

 one of which is seen. When the instrument is ready 

 for use, as in the Figure, the surfaces of the gilt paper 

 are parallel to the tin plate, but not in contact with it. 

 An electrometer E is connected with one of the tin 

 tubes A, A, while the other tin tulw is connected by a 

 wire with the body, whose electricity is to be condi u- 

 sed. When the tin plate apparatus has stood for a short 

 time to accumulate the electricity, the hook II is loosen- 

 ed, and the wooden frames are thrown back into a hori- 

 zontal position, and the electrometer will then diverge 

 with the condensed electricity. The condensing power 

 of this instrument increases with the proximity of the tin 



