ELECTRICAL APPARATUS. A Defcription of all the machinery that has been used in electrical experiments, would fill a large volume, We shall here confine ourselves to fuch as are of general ufe, and neceffary, in particular, to the following recreations: fuch are the revolving globe and rubber, with its prime conductor, ufually called an electric machine; the electric battery; the fulmineous conductor; and the electrometer. The conftruction of the electric machine is as follows *. Let a a, Plate I. be two * A great number of different electrical machines have been contrived. This appears to be the invention of Dr. Prieftley, from whose History of Electricity we have extracted the defcription of the following apparatus, and the fubfequent recreations, fome few articles excepted; for that valuable work contains every material difcovery or improvement that has been hitherto made in this fcience. A machine of a more fimple conftruction will be defcribed further on." ftrong ftrong boards of mahogany, the lower about an inch on each fide broader than the other: they are to be an equal length, parallel with each other, and about four inches asunder. In the upper board is a groove, that goes almoft its whole length, The pillars are of baked wood: that marked b is immoveable, being fixed to the upper and lower boards; the other flides in the groove just mentioned, that it may receive globes or cylinders of different dimensions *, but is only neceffary when they have axes. In each of the pillars there are holes at equal diftances, by means of which the globes may be height adapted to their bulk. placed at a These pil *It is not yet determined what fort of glass is The moft proper for thefe globes and cylinders. beft flint is commonly used, but Dr. Priestley feems to think, that common bottle metal is the most eligible. Some operators line their globes or cylinders with wax, or fome other electric fubftance; which in large globes may be of good ufe; but when they are fall, no material advantage can be expected from any lining. lars lars are to be high enough to admit two or more globes at the fame time. If two globes be fixed on one axis, four of a moderate fize may be used, and the wheel may have feveral grooves for that purpose. When a globe with one neck is used, as in the plate, a brafs arm c, with an open focket, is neceffary to fupport the axis. beyond the pulley: this part is also contrived to put higher or lower, together with the brafs focket in which the axis turns. The axis d, is made to come quite through the pillar, that it may be turned by a handle, without the wheel, at the option of the operator. As the frame ferews to the table, it may be placed at different diftances from the wheel, in proportion to the length of the ftring, in different ftates of the air. The wheel is fixed in a feparate frame e, by which means it may be placed in any fituation, with respect to the pulley, and be turned to one fide, fo as to prevent the parts of the ftring from cutting each other. The back part of this frame is fupported by a separate foot *. The rubber f, confifts of a hollow piece of copper, ftuffed with horfe hair, It is fupported by a focket, that receives the cylindrical axis of a round, flat piece of baked wood g, the oppofite part of which is inferted into the focket of a bent steel and covered with a bafil skin. fpring ht. Thefe parts are easily separated; fo that the rubber, on the piece of wood by which it is infulated, may be * Some electric machines are turned by a brass toothed wheel and pinion, inclosed in a box, which has a more elegant appearance; but these wheels are fubject to accidents, which are not fo easily repaired as thofe that happen to a string. + If the rubber be very narrow, some parts of the globe will pafs without a fufficient friction: to remedy this inconvenience the hand, when dry, may be held to the globe, juft before the rubber, to fupply the deficiency. There fhould be no fharp edges or angles about the rubber, for they would make its infulation, which is a matter of great confequence, ineffectual. changed changed at pleasure. The position of the spring may be altered two ways: it may either be flipped along the groove, or moved in the contrary direction, fo as to give it every defireable pofition with regard to the globe or cylinder: and it is, befides, furnished with a fcrew i, which makes it prefs harder or lighter, as the experiment may require, The prime conductor k *, is a hollow yeffel of polished copper, in the form of a *For common purposes a fmall conductor is moft convenient; but when a strong spark is wanted, it is proper to have a large conductor ready, which may be placed in contact with the fmaller. But whatever be the fize of the conductor, that part which is most remote from the globe should be round, and much larger than the reft for the effort of the electric matter to fly off, is always the greatest at the greatest distance from the globe; and from that part the longest and ftrongest sparks may be drawn. The largest and most pungent fparks are drawn from any conductor along an electric fubftance. Thus if the conductor be fupported by pillars of glafs |