Animal Electricity. See Galvanism, in this Supplement.
Electrometer.
is an instrument which measures the quantity of electricity in any electrified body. The most common electrometers are described in the article ELECTRICITY (Encycl.), no 27, and 182—233. A very valuable one is likewise described in no 85. of the article ELECTRICITY in this volume; but there are still two electrometers, of which we have hitherto given no account, though they are of such value, that to pass them unnoticed would be unpardonable. The first, which is incomparably the most accurate and delicate instrument of the kind that we have seen, was invented by Mr Coulomb, and is adapted to ascertain the smallest quantity of redundant electricity. The second is a late invention of Mr Cuthbertson, the ingenious improver of the air-pump, and is employed only to measure the charge of large jars and batteries.
Electrometer, by Mr Coulomb of the Royal Academy of Sciences at Paris, described in the Memoirs for 1785.
Mr Coulomb had made some experiments in examination of Dr Hooke's theory of springs "ut tensio sic vis;" and found, that it was surprisingly exact, in regard to the force necessary for twisting elastic wires. Having suspended a nicely turned metal cylinder by a fine wire in the direction of its axis, and having given it several turns, and left it to regain its natural position, he observed, that it performed all its revolutions of untwisting and twisting in times precisely equal, whether these oscillations were of a few degrees, or consisted of several revolutions. He thence concluded, that the force with which the wire endeavoured to regain its natural position was exactly proportional to its distance from it. Engaged, soon after, by order from the Minister of Marine, in an examination of the phenomena of the mariner's compass, he took this method of suspending his needles, in order to obtain exact measures of the forces which caused them to deviate from the magnetic meridian. He made some observations with needles so suspended; which are highly valuable to the philosopher engaged in that study. When his success in this research had fully gratified his wishes, he turned his thoughts to the examination of the law of electric action by the help of an electrometer suspended in the same manner. It is constructed as follows:
ABDC (fig. 1.) represents a glass cylinder, 12 inches in diameter and in height. This is covered by a glass plate fitted to it by a projecting fillet on the under surface. This cover is pierced with two round holes of 1 inch in diameter. One of them is in the centre, and it receives the lower end of the glass tube fh, of 24 inches height, which is fixed in the hole with a cement made of sealing wax, or other electric substance. The top of this tube receives the brass collar H (fig. 2. n° 3.), bored truly cylindrical, and having a small shoulder, which rests on the top of the tube. This collar is fastened with cement, and receives the hollow cylinder P (fig. 2. n° 2.), to which is joined the circular plate ab, divided on the edge into 360 degrees. It is also pierced with a round hole G in the centre, which receives the cylindrical pin i (fig. 2. n° 1.), having milled head b, and an index i o, whose point is bent down, so as to mark the divisions on the circle ab. This pin turns flilly in the hole G, and the cylinder Φ turns readily in the collar H. To the lower end of the centre pin is fastened a little pincer g, formed like the end of a port-crayon, and tightened by the ring r, so as to hold fast the suspension wire, the lower end of which is grasped by a similar pincer Po (fig. 3.), tightened by the ring s. The lower end s o is cylindrical, and it is of such weight as to strain the wire perfectly straight, but without any risk of breaking it. It may be made half of the weight that will just break it.
This pincer is enlarged at C, and pierced with a hole, which receives tightly the arm g C q of the electrometer. This is eight inches long, and consists of a dry silk thread, or slender straw of some grass completely dried, and dipped in melted gum lac or fine sealing wax, and held upright before a clear fire, till it form a slender cylinder of about 1/8th of an inch in diameter. This occupies six of the eight inches, from g to q; the remaining two inches is a fine thread of the lac or sealing wax, as it drains off in forming the arm. At a is a ball of pith of elder or fine cork, one-fourth or one-half of an inch in diameter, made very smooth, and gilded. It is balanced by a vertical circle g of paper, of large dimensions, stiffened with varnish. The retinence of the air to this plane soon checks the oscillations of the arm.
The whole is seen in its place in fig. 1, where the arm hangs horizontally about the middle of the height of the great cylinder. In its oscillations the ball a moves round in a circle, whose centre is in the axis of the whole instrument. Its situation is indicated by a graduated circle Z O Q, drawn on a slip of paper, and adhering to the glass with varnish. The electrified body, whose action is to be observed, is another small ball of cork t, also gilt, or a brass ball well polished. This is carried by a stalk of gum lac m r, including a dry silk thread. This stalk is grasped by a clamp of cleft deal, or any similar contrivance which lies firm on the glass cover. When this ball is let down through the hole m, it stands so as to touch the ball a on the arm when that ball is opposite o on the graduated circle.
To electrify the ball t, we employ the inflating handle, fig. 4, which is a slender stick of sealing wax or lac, holding a metal wire that carries a small polished metal ball. We touch with it some electrified body, such as the prime conductor of a machine, the knob of a jar, &c. Introduce this electrified ball cautiously into the hole m, and touch the ball t with it. The ball a is immediately repelled, and goes to a distance, twisting the suspension-wire, till the force of twist exerted by the wire balances the mutual repulsion of the balls t and a.
Such is the process for examining the law of electric action. But when we would examine the action of different bodies in different states, another apparatus is wanted. This is represented by the piece c A d (fig. 5.), consisting of a plug of sealing wax A, which fits tight into the hole m, and is pierced by the wire c d, hooked at e, to receive a wire connecting it occasionally with an electrified body, and having below a polished metal ball d.
The instrument is fitted for observation in the following manner: Turn the milled button b at top, till the twist-index i o is on the mark o of the twist circle. Then turn the whole in the collar H, till the ball a stands opposite to the mark o of the paper circle z O Q, and at the same time touches the ball t or d.
The observation is made thus: The ball t is electrified as already said, and a is repelled, and retires from t, twisting the wire, and, after a few oscillations, settles at a distance corresponding to the repulsion. Now turn the twist-index, so as to force the ball a nearer to t. We estimate the force of this new repulsion by adding the motion of the twist-index to the angle at which the ball first retired. By turning the twist-index still more, we bring the balls still nearer, and have a measure of another repulsion.—And thus may we obtain as many measures as we please.
In this way Coulomb ascertained the relation between the repulsion and the distance to be the inverse duplicate ratio of the distances. He discovered the law of dissipation by air in contact, and the relations which this bears to the primitive repulsion, by observing the gradual approach of a to t as the electricity dissipates from both, and by slackening the twist-index till the ball a retires to its primitive distance. He ascertained the dissipation along imperfect conductors, and the length necessary for insulation, by completely insulating the ball t, and observing the loss by air in contact with it, and then sliding a metal rod down the insulating stalk, till the dissipation began to exceed what took place by the air alone. He examined the proportion of redundant fluid in communicating bodies, by connecting them alternately with the pieces, fig. 5., as also by electrifying one ball, and observing its repulsive force, and then sharing its electricity with another, and observing the diminution. He examined the graduation of his electrometer, by sharing the electricity of one ball with an equal ball, which gave him the position that indicated one-half; and, by repeating this, for one-fourth, &c. in the same manner as we practised and related in Electricity (Suppl.), n° 141, &c.
An example of one or two of these trials will give a clear conception of the conclusions deduced from these observations.
The ball t was introduced and electrified; a was repelled, and settled at 40°; the index was twisted 140°, which brought a to 20°; and the time was noted. The electricity gradually dissipated, and a came nearer to t. The index was untwisted 30°, and a retired a little beyond 20°; but on waiting a few seconds, it flood exactly at 20°. The time was again noted. The interval was exactly three minutes. The conclusion from the experiment was as follows:
When the ball was brought to 20°, the repulsion was evidently 140 + 20, or 160. Three minutes afterwards it was 110 + 20, or 130; and 30° were lost in three minutes, or 10° per minute. The mean force was 145. Therefore the mean loss per minute was 12°. Observe also, that the primitive force corresponding to the distance was 40°; and the force corresponding to 20° was 160, or inversely as 20° to 40°.
But observe, that the distances were not measured by the angles, but by the chord of the angles. The obliquity of action must also be accounted for; and the real lever is less than the arm, in the proportion of radius to the cosine of ½ the angle.
The wire used by Coulomb in his first experiments on the law of action was of such strength, that ¼ th of a French grain, applied at the point a, held it fast till Electricity (Suppl.) has the advantage; but in every other respect, Mr Coulomb's is the finest electrometer that has yet been published, giving absolute measures, and this with great accuracy. The Hon. Mr Cavendish has employed the construction in his most valuable experiments on the force of gravity (Phil. Trans. 1798, Part II.) an experiment which Newton would have been delighted with observing.
Coulomb's Electrometer is thus described by himself in the last number of the second volume of Nicholson's Philosophical Journal. GH (fig. 6.) is an oblong piece of wood, about 18 inches in length and six in breadth, in which are fixed three glass supports, D, E, F, mounted with brass balls, a, b, c. Of these supports E and F are exactly of the same length; but D is four inches shorter. Under the brass ball a is a long brass hook; the ball c is made of two hemispheres, the under one being fixed to the brass mounting, and the upper turned with a groove to fit upon it, so that it can be taken off at pleasure. The ball b has a brass tube fixed to it, about three inches long, cemented on the top of F, and the same ball has a hole at the top, of about one-half inch diameter, corresponding with the inside of the tube. A B is a straight brass wire, with a knife-edged centre in the middle, placed a little below the centre of gravity, and equally balanced with a hollow brass ball at each end, the centre, or axis, resting upon a proper shaped piece of brass fixed in the inside of the ball c; that side of the hemisphere towards c is cut open, to permit the end c A of the balance to descend till it touches the ball a; and the upper hemisphere C is also cut open to permit the end c B to ascend; i is a weight, weighing a certain number of grains, and made in the form of a pin with a broad head; the ball B has two holes, one at the top, and the other at the bottom; the upper hole is so wide, as to let the head of the pin pass through it, but to stop at the under one, with its shank hanging freely in b; k is a common Henry's quadrant electrometer; and when in use it is screwed upon the top of c.
It is evident, from the construction, that if the foot stand horizontal, and the ball B be made to touch b, it will remain in that position without the help of the weight i; and if it should by any means receive a very low charge of electric fluid, the two balls b, B will repel each other; B will begin to ascend, and, on account of the centre of gravity being above the centre of motion, the ascension will continue till A rests upon a. If the balance be set again horizontal, and the pin i be put into its place in B, it will cause B to rest upon b, with a prelude equal to that weight, so that more electric fluid must be communicated than formerly before the balls will separate; and as the weight in B is increased or diminished, a greater or less quantity of electric fluid will be required to effect a separation.
When this instrument is to be applied to a jar, or battery, one end of a wire L must be inserted into a hole in b, and the other end into a hole of any ball proceeding from the inside of a battery, as M. A chain, or wire, or any body through which the charge is to pass, must be hung to the hook at m, and carried from thence to the outside of the battery, as is represented by the line N. k must be screwed upon c, with its index towards A. The reason of this instrument being added, is to show, by the index continuing to rise, that the charge of the battery is increasing, because the other... It is almost needless to observe that this instrument consists of three electrometers, viz. Henley's electrometer, Lane's discharging electrometer considerably improved, and Brooke's fleecy yard electrometer improved likewise. By this combination and these improvements, we possess all that can be required in an electrometer for batteries and large jars; for, by k, we see the progress of the charge; by the separation of B b, we have the repulsive power in weight; and by the ball A, the discharge is caused when the charge has acquired the strength proposed.
In the journal from which this abstract is taken, the reader will find some curious experiments made with batteries by means of this electrometer; but one will be sufficient to explain its use. Prepare the electrometer in the manner shown in the figure, with the jar M annexed, which contains about 168 square inches of coating. Take out the pin in B, and observe whether the ball B will remain at rest upon b; if not, turn the adjusting screw at C till it just remains upon A. Put into B the pin, marked j, weighing 15 grains; take two inches of watch pendulum wire, fix to each end a pair of spring tongs, as is represented at G m, hook one end to m, and the other to the wire N, communicating with the outside of the jar; let the uncoated part of the jar be made very clean and dry; and let the prime conductor of an electrical machine, or a wire proceeding from it, touch the wire L; then, if the machine be put in motion, the jar and electrometer will charge, as will be seen by the rising of the index of k; and when charged high enough, B will be repelled by b, and A will descend and discharge the jar through the wire which was confined in the tongs, and the wire will be fused and run into balls. The ingenious author, by breathing through a glass pipe into the jar, damped it a little in the inside. Then loading B with a pin of 30 grains, he obtained such a charge as fused eight inches of watch pendulum wire, disposed exactly as the two inches were disposed in the former experiment. By repeating and varying his experiments, he found that double quantities of electrical fluid, in the form of a discharge, will melt four times the length of wire of a certain diameter.