ELECTROMETER

, is an instrument that measures the quantity, and determines the quality of electricity, in any electrified body. Previous to the invention of instruments of this kind, Mr. Canton estimated the quantity of electricity in a charged phial, by presenting the phial with one hand to an insulated conductor, and giving it a spark, which he took off with the other; proceeding in this manner till the phial was discharged, when he determined the height of the charge by the number of sparks. Electrometers are of 4 kinds: 1, the single thread; 2, the cork or pith balls; 3, the quadrant; and 4, the discharging Electrometer.

The 1st, or most simple Electrometer, is a linen thread, called by Dr. Desaguliers, the thread of trial; which, being brought near an electrified body, is attracted by it: but this does little more than determine whether the body is in any degree electrified or not; without determining with any precision its quantity, much less the quality of it. The Abbé Nollet used two threads, shewing the degrec of electricity by the angle of their divergency exhibited in their shadow on a board placed behind them.

Mr. Canton's Electrometer consisted of two balls of cork, or pith of elder, about the size of a small pea, suspended by fine linen threads, about 6 inches long, which may be wetted in a weak solution of salt. See sig. 7. If the box containing these balls be insulated, by placing it on a drinking glass, &c, and an excited smooth glass tube be brought near them, they will first be attracted by it, and then be repelled both from the | glass, and from each other; but on the approach of excited wax, they will gradually approach and come together; and vice versa. This apparatus will also serve to determine the electricity of the clouds and air, by holding them at a sufficient distance from buildings, trees, &c; for if the electricity of the clouds or air be positive, their mutual repulsion will increase by the approach of excited glass, or decrease by the approach of amber or sealing-wax; on the contrary, if it be negative, their repulsion will be diminished by the former, and increased by the latter. See Philos. Trans. vol. 48, part 1 and 2, for an account of Mr. Canton's curious experiments with this apparatus.

If two balls of this kind be annexed to a prime conductor, they will serve to determine both the degree and quality of its electrification, by their mutual repulsion and divergency.

The Discharging Electrometer, sig. 3, plate ix, was invented by Mr. Lane. It consists of brass work G, the lower part of which is inclosed in the pillar F, made of baked wood, and boiled in linseed oil, and bored cylindrically about two-thirds of its length; the brass work is fixed to the pillar by the screw H, moveable in the groove I; and through the same is made to pass a steel screw L, to the end of which, and opposite to K, a polished hemispherical piece of brass, attached to the prime conductor, is fixed a ball of brass M well polished. To this screw is annexed a circular plate O, divided into 12 equal parts. The use of this Electrometer is to discharge a jar D, or any battery connected with the conductor, without a discharging rod, and to give shocks successively of the same degree of strength; on which account it is very fit for medical purposes. Then, if a person holds a wire fastened to the screw H in one hand, and another wire fixed to E, a loop of brass wire passing from the frame of the machine to a tin plate, on which the phial D stands, he will perceive no shock, when K and M are in contact; and the degree of the explosion, as well as the quantity of electricity accumulated in the phial, will be regulated by the distance between K and M. Philos. Trans. vol. 57, pa. 451.— Mr. Henley much improved Mr. Lane's Electrometer, by taking away the screw, the double milled nut, and the sharp-edged graduated plate, and adding other contrivances in their stead. Mr. Henley's discharger of this kind has two tubes, one sliding within the other, to lengthen and accommodate it to larger apparatus.

The Quadrant Electrometer of Mr. Henley, consists of a stem, terminating at its lower end with a brass ferrule and screw, for fastening it upon any occasion; and its upper part ends in a ball. Near the top is fixed a graduated semicircle of ivory, on the centre of which the index, being a very light rod with a cork ball at its extremity, reaching to the brass ferrule of the stem, is made to turn on a pin in the brass piece, so as to keep near the graduated limb of the semicircle. When the Electrometer is not electrisied, the index hangs parallel to the stem; but as soon as it begins to be electrisied, the index, repelled by the stem, will begin to move along the graduated edge of the semicircle, and so mark the degree to which the conductor is electrified, or the height to which the charge of any jar or battery is advanced.

Mr. Cavallo has also contrived several ingenious Elec- trometers, for disserent uses; as may be seen in his Treatise on Electricity, pa. 370, &c, and in the Philos, Trans. vol. 67, pa. 48 and 399.

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Entry taken from A Mathematical and Philosophical Dictionary, by Charles Hutton, 1796.

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