262

ELECTRICITY.

quantity, and thus it is positive; but after the charge the first sidecontinuing to give out the fluid, because of its redundancy, is stillpositiv<, but as this is given out, the pressure is taken "from theother side, and hence its deficiency begins to be supplied, or itbegins to receive the fluid, or is negative.

Prop. XI. While an electric plate is charging from a negativeconductor, both sides are negative, hut after the charge the sidewhich was presented is negative and the other positive. For thefluid is taken from one part of the side presented, and hence thereis a deficiency as far as the coating extends, and hence it begins toreceive, or is negative, and the pressure being taken from the op-posite side it will also receive or be negative; and after the chargethe side which had been presented will continue to receive, orwill still be negative, and hence the pressure will return to the otherside which will begin to give out what it had received, thatis.itwill be positive, however in a weak charge the positive electricity isscarcely perceptible, because in this case it had received very lit-tle during the charge.

Prop. XII. When a pointed body is strongly electrified posi-tively the electric fluid is rendered visible in a diverging streamfrom the electrified point, when negatively electrified the ap-pearance is that of a small star. The electric fluid,when sufficientlydense ant! in rapid motion through the air, exhibits the electric light,and the air can oppose little resistance to a point; hence the fluidw ill move with rapidity from the positively electrified point, andfrom its elasticity will tend to diverge, it is immediately followedby other fluid from the point, which forces it in ramificationsth'rough the air and renders the streams visible to some distance.The case is different at the negative point, for since the fluid is enter-ing from the air the fluid is not impelled by that w hich follows,and hence will become visible only near the point, and at an equaldistance from all parts, which will give the appearance of the smallstar. Wc have thought proper to explain the above pluvnomenato elucidate, and shew the utility of our theory, but our limits willnot permit us to proceed thus to expatiate upon the very numerouselectrical appearances which have been observed, we shall there-fore notice the principal of them, and leave the explanation for themost part to the ingenuity of the reader.

PRACTICE OF ELECTRICITY.

Of the Electrical Apparatus lit General.

The practical part of electricity naturally divides itself into twobranches, viz. the apparatus employed, and the experiments madewith it; but the use and application of the former being best illus-trated by the latter, we thought it improper to separate them. .Mr.Cavallo subdivides the former into three classes, viz. 1. The in-struments necessary to produce electricity: 2. Those pro-

per to accumulate, retain, and employ it: and 3. Those necessaryto measure its quantity, and ascertain its quality. The instrumentsprincipally used to produce electricity are called by way of em-phasis, Electrical Machines. They are made in so many diflerentforms, that it would be tedious and difficult to give only a veryshort description of them all. We shall therefore first lay downthe most necessary rules for constructing electrical machines ingeneral; and then give a particular description of those which aremost generally useful, and contain the latest improvements. Theprincipal parts of the electrical machine are the electric, the mov-ing engine, the rubber, and the prime conductor, i. c. an insu-lated conductor, which immediately receives the electricity fromthe excited electric. Electrics were formerly used of different sub-stances, as glass, rosin, sulphur, sealing-wax, &c. Their formswere also various, as globes, cylinders, spheroids, See. The rea-son of this variety was, 1st, that it was not then ascertained whatsubstance acted most powerfully; and 2dly to produce a positiveor negative electricity at pleasure. At present, globes, cylinders,and plates of smooth glass, are chiefly used; for when the machinehas an insulated rubber, the operator may produce positive or ne-gative electricity at his pleasure, without changing the electric ;which (dearly proves the falsity of those systems of electricity,which are founded upon the supposition of a specific differencebetween vitreous and resinous electricity, as arising from the na-ture of the different substances producing them. The best ce-ment for electrical purposes is made with two parts of rosin, two•f bees-wax, and one of the powder of red ochre. These ingre-

d-ents are melted, and mixed together in any vessel over the fire ;and afterwards kept for use. This kind of cement -ticks very fast,and is much-preferable to rosin only, as it is not so brittle, and atthe same time insulates equally well. The cylinders are madewith two necks: they are used to the greatest advantage withoutanv axis; and their common size is from four inches diameter andeight inches long, to twelves inches diameter and two feet long.Very large cylinders should be strengthened by a glass axis, viz. asolid stick of glass from the one cap to the other. The glass ge ■nerallv used is the best flint; though it is not absolutely deteimin-ed which kind ot metal is the best for electrical machines. Thethickness of the glass seems immaterial, but perhaps the thinne. t ispreferable. It lias often happened that glass globes and cylinders,

: in whirling, have burst in pieces with great violence, and with nosmall danger to the by-standers. Those accidents, are supposedto happen when the globes or cylinders, after being blown, aresuddenly cooled. The workmen should therefore he enjoined tolet them pass gradually from the heat of the glass-house, to thetemperature of the atmosphere. Electricians are not agreed, whe-ther a lining of some electric substance, as rosin, turpentine, Ac.on the inside surface of the glass, has any effect to increase itselectrical power; but it seems pretty well determined, that if it donot increase the power'of a glass globe, or cylinder, at least it con-siderably improves a bad one. The most improved compositionfor this purpose is made with four parts of Venice turpentine, onepart of rosin, and one part of bees wax. This composition mustbe boiled fbr about two hours over a gentle fire, and stirred veryotten : afterwards it is left to cool, and reserved for use. When aglobe or cylinder is to be lined with this mixture, a sufficientquantity of it i- to be broken into small pieces, and introduced intoglass; then, by holding the glass near Ihe fire, the mixture ismelted, and equally spread over all its internal surface to aboutthe thickness of a sixpence. I n this operation, care must be takenthat the glass he made hot gradually, and be continually turnedso that it may be healed equally in all parts, otherwise it is apt tobreak in the operation. As to the engine which is to give motionto the electric, multiplying wheels have been generally used,which, properly adapted, might give the electric a quick motion,while tuey are conveniently turned by a winch. However someturn the machine simply with a winch, without any acceleratedmotion: and this method, on account of its simplicity and easyconstruction, may in most cases be preferred.

"l'he Rubber, as it is now made, (says Mr. Cavallo,) is nothingmore than a silk cushion stuffed with hair; and over this is put apiece of leather, on which some amalgam lias been rubbed, so asto stick as fast as possible to the leather. Sometime ago it wasgenerally used, and it is now customary also, to make the rubberof red Basil skin stuffed with hair; blit the silk one, as above de-scribed (which is an improvement of Dr. Month's) is much pre-ferable. If this silk cushion, on account of adapting it to the sur-face ofthe glass, is to be fixed upon a metal plate, then care shouldbe taken to make the plate five from sharp points, edges, or cor-nel's, and it should be as much as possible concealed, or coveredwith silk. In short to construct the rubber properly, it must bemade in such a manner, that the side o( it, which the surface of theglass enters in whirling, may be as perfect a conductor as it can bemade, in order to supply electricity as quick as possible; and theopposite part should be as much a non-conductor as possible, thatnone of the fluid may return back to Ibe rubber; which has beenfound to he the case, when the rubber is not made in a propermanner; for which reason a piece of silk is also added to the ex-tremity of the leather. Mr. Nairne’s rubber consists of silk onlyput over the leather cushion, and very little amalgam is used withit.” 'Ihe following is still more simple; let apiece of wood befitted to the surface intended to lie subjected to friction, and cover-ed first with two or three thicknesses of elastic cloth,, then withsmooth leather, and lastly with a flap of silk, pasted upon the edgeat which the glass in its rotation arrives, and let it pass loose overthe face of the cushion, and thence upon the surface of the glass,as far as the commencement ol the'primc conductor. To the lineof contact of the rubber and electric, a little amalgam ought tobe applied, since it renders the excitation much more active.Any metal dissolved in quicksilver, will do for an amalgam,but the one that has been generally used is made witli two parts ofquicksilver, and one of tin-foil, with a small quantity of powdered

1 chalk