The electroscope is a handy instrument for estimating roughly medium poten tials. In one of the best of its forms a glass ball, about 4 in. in diameter, rests on a brass tripod, and its neck, about 1 in. in diameter, is inclosed by a brass collar fixed with shellac. A brass plate, with a hole of Ith of an in. in diameter in the middle of it, can be screwed air-tight into the collar. Before it is so fitted, a brass rod, ith of an in. in diameter, is fixed by shellac or sealing-wax into the hole in the middle, so as to be perfectly insulated from it. The upper end of the rod ends in a brass ball, and the lower end is filled on each side, to allow of two strips of gold•leaf, 1 in. in length, being attached to it. Before the plate and leaves are finally fixed, the interior of the ball is thoroughly dried, by passing hot dry air into it, so that the ball contains no moisture to carry away the charge of the leaves. When the plate is screwed to the collar, there is no communication between the included and external air. The insulation of the leaves is complete; and they keep their charge, in dry weather, for hours together. When the instrument is used, it may be charged directly, by contact being established with the ball and the body whose E. we would examine, or a charge may be carried to it by the proof plane, when the leaves diverge according to the charge communicated. When we would ascertain simply the kind of E. with which a body is charged, we proceed in the following way: A glass tube is rubbed, and brought into the neigh. borhood of the brass knob; the leaves diverge by induction, and, when so diverged, the knob is touched with the finger, and the leaves fall to their original position, for they are then out of the line of action. In this state, —E. is fixed by the action of the +E. of the tube on the side of the knob next it, and the corresponding + E. is lost in the ground. When the finger is removed, the + E. is cut off, while the — E. remains in the knob; and its presence is manifested by the leaves diverging permanently after the removal of the tube. If, now, a positively electrified body be brought near the knob, it draws away the — E. from tb.o leaves, and they consequently fall in; but if a negatively electrified body be brought near, it sends the — E. more to the leaves, so that they diverge further. We are thus enabled to distinguish between a -I- and a — charge. But it may be asked, why not charge the electrometer immediately with the glass ? There are two difficulties in the way of this. If the glass is powerfully electrified, it gives too great a charge; and if feebly, contact between the knob and the glass cannot be effected, although its E. acts powerfully by induction. We therefore bring the glass rod near the electrometer, and when the leaves diverge sufficiently, we touch the knob with the finger, and withdraw first the finger, then the rod, and the leaves diverge as before. For the more delicate use of the gold-leaf electroscope, see CONDENSER.
Coulomb's Torsion Balance has played an important part in examining the laws of electric forces. A glass canister is placed on a wooden frame, and is covered above by i a plate of glass or wood; in the middle of this plate a round hole is cut, over which fixed, by wooden fittings, a long glass tube having the graduated rim of a circle attached at its upper end. A circular plate, resting on this rim, closes the upper end of the tube; and when it is turned round, a mark upon it tells the number of degrees through which it has been moved. A cocoon thread or very fine wire is tied to a hook in the center of the lower side of this plate, and thence.descends to the body of the canister. It carries below a collar of paper or other light material, in which a needle of shell-lac is adjusted having a disk of gilt paper placed vertically, or a gilt pith-ball at its one end, and a counterpoise at its other. When the plate above is moved through any number of degrees, the needle below, impelled by the torsion of the thread, comes to rest at the same number on the scale below. This last consists of a strip of paper divided into degrees, pasted round the cylinder at the same height as the needle. In the cover of the canister there is another opening ,for the admission of a ball insulated at the end of a rod of shell-lac, and which, when supported by the cover, is on a level with the paper disk of the needle. When the instrument is adjusted for observation, the mark on the upper plate and the paper disk stand each at the zero-points of their respective scales, there being of course no torsion in the thread. The ball is removed, to receive a charge from the body under investigation, and is then placed in the cylinder, when the disk is first attracted, then repelled. Suppose that the disk be driven 40°, as shown by the
lower scale, from the ball, and that the upper plate has to be moved in the opposite direction, through 160° of the upper scale, to bring it back to the total degree of torsion is 160° .4- 40° = 200°. If the ball and disk be now discharged, and another charge be given to the ball, which requires 250° of torsion to place the disk at 10°, we have the relation 200 to 250, as that of the repulsive forces of the two charges, for the amount of torsion in degrees is proportional to the twisting force. Without entering further into detail, we may state the two laws that Coulomb established by this instru ment: The intensities of the mutual repulsion or attraction of two invariable quantities of electricity of the same or names, are in the inverse ratio of the squares of the distance at which these act. The intensities of the total repulsive or attractive action of two electrified bodies placed at an invariable distance, are proportional to the products of their electric charges.
. Electric Machine.—In the tube of glass and silk rubber of which we have made frequent mention, we have the embryo of the electric machine, viz., a body which, when rubbed, is positively electrified, and its rubber negatively. The first requisite we should expect in a machine of this nature is a large surface, to give a great amount of electricity. But there is another already casually referred- to: glass being a non-conductor, the E. formed on its surface has not a combined action, so that some arrangement is necessary to collect it, and render it available—to act, in fact, as its conducting reservoir. This portion of the machine is denominated the prime conductor. The rubbed surface of the electric machines is either a cylinder or plate of lass; hence we distinguish them into cylinder machines and plate machines. The former, from their more compact form, are the more manageable; and the latter, from both sides of the glass plate being rubbed, are the more powerful forms of the instrument. The description of ter's plate machine (fig. 1) will be quite sufficient to show the general requirements and construction of electric machines. It is one of the best existing forms of the machine. „The glass plate is turned on the axis ab by means of the handle c. The longer end of this axis, consisting of a glass rod, moves in the wooden pillar d, and the other rests in the wooden head of the glass pillar e. The plate is thus completely insulated, and little loss of its E. can take place through its supports. The two rubbers are triangular pieces of wood, covered with a ding of one or two layers of flannel, inclosed in leather, and they present a flat hard surface to the glass, so that friction between it and them takes place in every part. They are placed in a wooden frame on each side of the plate, and the pressure is regulated by metal springs, fixed to the outside between them and the frame. Before use they are covered with an amalgam of mercury, zinc, and tin, which is made to adhere with the aid of a little grease, and which increases immensely the production of electricity. The surfaces of the rubbers are therefore conducting, and are made to communicate by strips of tinfoil with the negative conductor, f (fig. 1). To limit the electric field in the neighborhood of the negative conductor, or, which is the same thing, to keep the potential of the glass from rising too high, so as to cause a discharge back into the rubbers, each rubber has a non-conducting wing fastened to it, which is made of several sheets of oiled silk, kept together by shellac varnish, beginning at the rubber with sev eral, and ending with one or two sheets. When the machine is in action, electrical attraction makes them adhere to the plate; but when it is out of action, they may be kept up by a split pin g. As the plate turns, the rubbers are kept in the frame by their ledges h. The whole frame-work of the rubbers and negative conductor is supported by the short glass pillar i, so that it can be insulated when required. The prime conductor, k, is a brass ball insulated on the long glass pillar 1, and to prevent the edges of the ball at the junction dissipating the E., the pillar enters the ball by a trumpet-shaped open ing. The collection of the E. from the glass is made by a row of points placed in the grooves, inside of two wooden rings, m, m, which are attached on each side of the plate to a piece of brass projecting horizontally from the ball of the conductor. The grooves are covered with tinfoil, which conveys the collected E. to the ball, and the points.are kept out of the way of injury by not projecting beyond the grooves.