MACHINE, electric. The electric ma chine consists of three parts, the electric body, which is rubbed ; the rubber, which is a compounded conductor ; and the prime conductor, which is destined to re ceive and convey the electricity, in mak iog experiments. The first electrical ap paratus consisted of a tribe of glass, or a stick of sealing-wax, rubbed by the hand. Class globes, whirled quickly on an axis, were substituted as an improvement, and the rubber was still the hand : but subse quently a round concave cushion. These were succeeded by glass cylinders, which are cheap, sate, and considerably power ful, but the present fashion determines in favour of flat glass plates, on account of the advantage of a large surface, rub bed by two or more pairs of cushions, and the equality of pressure, which causes the supply of electricity to be steady and without undulation, as to its quantity. Machines of very great power (see " Ni• cholson's Journal," quarto), have been made by M. Walckie•s, consisting of an endless web or jack-towel of silk, passing between two pair of cylindrical rubbers, aced with cat.skin ; the electricity being communicated to a prime conductor, ly ing between the parallel pieces of the silk.
The robber is usually a piece of wood fitted to the surface intended to be sub jected to friction, and covered first with two or three thicknesses of elastic cloth, then with smooth leather, and lastly with a flap of silk, pasted upon the edge at which the glass m its rotation arrives, and passing loose over the Face of the cushion, and thence upon the surface of the glass, as far as the commencement of the prime conductor. Its use has been explained under the article EXCITATION.
In fig 1, Plate Machine Electric, A re presents the glass cylinder of a machine, turned by, B, the handle. Its surface rubs against, C, the cushion, which pro ceed, 9, the sill flap, and the electricity is conveyed to E, the prime conductor. The supports of the cylinder, the cushion, and (indispensably) of the prime con ductor, are made of glass Or baked wood, in order that the electricity may not be conveyed to the earth, unless when the operator chooses to make the communi cation by some conducting body. A prime conductor is sometimes applied to the rubber.
Though we have produced as strong an excitation as we have ever heard of; by the amalgam of mercury and zinc, with a little tallow, as mentioned under the article ExCITA T ION, yet as many electricians, particularly the experienced Mr. Cuthbertson, prefers tin and zinc, and it is probable that this mixture may afford a speedier oxydation, we shall give his receipt.
Melt two parts of tin and zinc in a cru cible, and pour them on two parts of trier eery in a wooden box made for that pur pose, which close and agitate till the metals are cold. Then pulverize the granulated mass very finely, and make it into a paste with h ng's lard Fig. 2, shows the plate machine, with
Nicholson's cylinder improvements for changing the two states at pleasure, as adapted by Dr. Von Marum, of' Hacrlem.
The glass plate, G G, is fisstened to the axis, B B, by means of a screw on the axis passing through a hole in the centre of the plate, and secured by a nut, C, on the opposite side. The axis is supported by a single pillar, A, which for this pur pose is provided with a bearing piece, K, on which are two brass collar pieces, that early the axis; and on the end of the axis, opposite the glass, is a counterpoise, 0, of lead, to prevent too great a friction in the collar nearest the handle. The arc of the conductor, E E, which carries the two small receiving conductors, Fr, is fixed to an axis turning in the ball, H. On the other side of the plate is the other arc, I, of brass wire, fixed in the hearing piece, K, but so as to admit of being turned round like the arc EE. P is a copper tube, moving like a radius on the stem of the ball, S, which, being screwed into the conductor, H, serves to confine the arm, P, in any position that may be required. The dissipation of electricity along the glass supports is prevented by a kind of cap, T, of mahogany, which affords an electrical well or cavity under neath, and likewise effectually covers the metallic cap into which the glass is ce mented, The lower extremity of the pil lar is guarded in the same manlier by a hollow piece or ring of mahogany, V. The three glass pillars are set in sliding pieces, WWW, adjustable by screws ; at each extremity of the horizontal diame ter of the plate are two rubbers, N, one on each side, pressed regularly and uni formly against the plate by means of a spring, Y, the force of pressure of which is regulated by means of a screw. To these rubbers are attached silk flaps, ZZ, of one pair of rubbers descending, and those of the other pair ascending, in the direction in which the plate is work ed. A piece of fine dry writing paper, as long as the rubber, and half an inch broader, so as to cover the seam that Cis tens the silk to the leather, allows greater pressure to be employed, by diminishing the friction, and prevents both the glass and silk from being soiled by the amal gam, so that the excitement is more powerful, and the amalgam requires to be renewed less frequently. As the semi circular branch of the prime conductor is moveable, it may be made to exhibit the electricity of the rubber at any time, by placing the cylindrical ends in contact with the cushions, the semicircular wire, I, being at the same time turned so as to cross it at right angles, which insulates the cushions. When the conductor is re quired to give electricity from the glass, the arc 1 must be in contact with the cushions, and the arc EE perpendicular to the horizon.