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When it rains or snows, the small cover, fig. 9, "in diameter. The other extremity D passes is to be screwed on the top of the instrument, as through a cork ball, so as to move with a slight by this its insulation is preserved. This indicates friction. not only the electricity of fogs, but also that of 165. The construction of this part of the elecserene weather, and enables the observer to dis- trometer is such that when FE lies perpendicucover the kind of electricity which reigns in the larly to the horizon, and the stalk BD, with its atmosphere; and, in some degree, to form an balls, is allowed to hang freely, the ball B just estimate of its quantity, and that under two dif- touches the ball A, as represented in fig. 11. ferent points of view, the degree of intensity, and The ball F is fixed to one end of a glass rod FI the distance from the earth at which it first passing perpendicularly through the centre of a begins to be sensible. A conductor exhibits signs graduated circle GHO, and furnished at the of electricity only when the electric fluid is more opposite extremity I with a knobbed handle of or less condensed in the air than in the earth. boxwood. HK is the stand of the electrometer, Though the air resists the passage of the electric in the head of which is a hole in which the rod fluid, it is not absolutely impermeable to it; it F I slides smoothly but not easily. There is also suffers it to pass gradually, and generally with adapted to the glass rod FI an index NH that more ease in proportion as its mass is less. It turns round it. This index is placed so as to be is, therefore, interesting to discover at what height parallel to a line LA drawn through the centre it is necessary to be elevated, in order to find a of the ball A. And, as the circle is divided into sensible difference between the electricity of the 360 rees, o being marked above, and 90 earth and that of the air.
on the right hand ;, the index will point out the 163. Mr. Brooke of Norwich constructed an angle which the line LA makes with the vertical electrometer of a very ingenious description, and line. It is convenient to have another index on certainly valuable in its application, of which he the rod FI turning with some friction round it, has given a full account in his Miscellaneous and extending considerably beyond the circle Experiments; the limits, however, within which GHO. our article must be confined, will not allow of 166. When this electrometer is used for comour giving his lengthened description of the puting the quantum of electricity belonging to instrument. We shall, therefore, present our any body, it must be connected by means of a readers with a short account of another electro- wire with the substance to be examined : the meter, the invention of professor Robison of wire must be fixed into the hole of the ball F, Edinburgh, which in our opinion is, in its chief which is in part filled by the needle; and indeed it essentials, much superior to that of Mr. Brooke. must come in contact with the needle. The index This beautiful and delicate instrument is repre- must now be turned round by the handle I until sented by fig. 10, where A, a finely polished brass it stand at 90°. In this position C B is horizonball of a quarter of an inch diameter, is fixed on tal, and the ball B contiguous to A. If in this the point of a common sewing needle about position the balls B and A are electrified, they three inches long, and as slender as can be pro- do not separate till the index be turned back cured of that length. On the other end of the towards o.' In some part of this space they will needle is fixed a ball of amber, glass, or other separate, and the point must be carefully noted, Tion-conducting substance, of about for ths of as this is the measure of their electric power an inch in diameter. This ball is so fixed as while in contact. By turning still more towards that the needle does not quite reach to its surface, O, the separation is increased. An assistant must though the ball F must be completely perforated. now turn the long index till it be parallel to the From the electric ball there passes a slender glass other, and consequently to CD. On loading Tod, F, E, L, bent at right angles at E, so that the cork ball D with grain weights till B D be the part F E may be about three inches long, and nicely balanced in a horizontal position, and the other extremity L, immediately opposite to computing for the proportional length of CB the centre of the ball A. A piece of amber C, and CD, we obtain an exact measure in grains so cut as to have two parallel cheeks, is fixed on of the electric force with which the balls B and the extremity L of the glass rod. For the prin- A separate in this position. cipal part of the instrument, a strong dry silk 167. The electrometers already described have thread is to be prepared by dipping it perpen- all been found highly useful to the practical dicularly in melted sealing-wax, till it be fully electrician, according to the peculiar nature of penetrated by the wax, so as to retain a thin the course of his experiments; but for ascercoating of it.
taining the actual repulsive and attractive powers 164. The thread, thus coated, must be kept of very faintly electrified bodies, they are perextended, so that it may be quite straight; it haps all excelled by the ingenious invention of must be made perfectly smooth by holding it M. Coulomb, a French philosopher. The conbefore a fire, and rolling it on a smooth table. struction of this delicate instrument, which is It is then to be passed through a small cube of called the torsion balance, will be best unamber, that has two holes drilled in two of its derstood by a reference to the plate, where we opposite faces, perpendicularly to the stalk. By have given a representation of it in its complete these holes the cube is suspended, so as to move state, and of some of its parts on an enlarged readily, on two fine brass pins, between the scale. cheeks of the piece of amber at L. The waxed Fig. 1, Plate IV., is a cylinder of flint glass thread is about six inches long, and is equally twelve inches in diameter, and twelve in height, divided by the amber cube. To the end B is covered with a plate of glass, which is made to fixed a ball of some conducting substance, as of fit to it by a projecting fillet on the lower surpolished metal, or gilt cork, a quarter of an inch face and having in it four round perforations, of
an inch and three-fourths in diameter, one of 170. We shall here add, as an illustration of which is in the centre f, and receives the glass the method of using this ingenious instrument, tube fh, which is two feet in height, and is fixed an account of Coulomb's method of determining in the glass plate with cement. Into the top of by it the electrical repulsion of the two balls a this tube is inserted the brass piece H, fig. 2, and b when electricity was communicated to No. 3, which is perfectly cylindrical, and having them. Having electrified the brass pin, fig. 5, it a small shoulder which rests on the top of the was introduced through the hole m, in the top tube into which it is cemented. This brass piece of the cylinder, and made to touch the ball b, in is made to fit by means of a screw on the hollow contact with the ball a, thus communicating its cylinder, No. 2, fig. 2, to which is joined the electricity to the two balls, which consequently circular plate ab, divided into 360 degrees, and became electrified with the same electricity; and, having a hole G in its centre for admitting the on this pin being withdrawn, a mutual repulsion cylindrical pin i, No. 1, fig. 2. This pin is sur took place between the two balls, a being driven mounted with a milled head b, from which an from b to a distance easily measured on the graindex io projects, having a point turned down- duated scale, and obviously regulated by the wards at o, to mark the divisions on ab. This resistance of the wire to farther torsion. After a pin moves with some friction in the hole G, few slight oscillations the arm will rest and the while the cylinder moves steadily in the brass degree of repulsion may be accurately noted. piece H. To the lower end of the pin there The index io is now to be turned backwards attached the pincer q, resembling the end of a till the ball a come to its former position, by solid port crayon, and capable of being tightened which movement the silver wire will be twisted, by a sliding ring : so as to seize a fine silver and a force produced proportional to the angle wire, while its lower end is held by a similar of torsion which is required to bring the ball a pincer, shown by Po, fig. 3, and tightened by the in contact with the ball b. By this means M. sliding ring r.
Coulomb ascertained the distance at which dif168. The stalk ro is cylindrical, and is made so ferent angles of torsion bring the balls in conheavy as to keep the wire quite straight without tact after mutual repulsion; by comparing the breaking it. Fig. 3 exhibits this pincer with the forces of torsion with the corresponding distances arm z C 9 passing through it. The length of this of the balls, he obtained a measure of their arm is eight inches, and it is formed of a stout repulsive force. silk thread, or a fine round straw coated with 171. The following are given by Dr. D. wax, or with lac; it is about one-tenth of an Brewster, as the results of some experiments inch diameter, and six inches long, and is ter- with this curious instrument, by its inventor. minated by two inches of wax drawn out into a (1.) The two balls being electrified with the fine thread. The end q carries a pith-ball a turned head of the pin, and the index of the micrometer very smooth, and gilded, and the other end is a being set to zero, the ball a was repelled by the small circular plane of paper covered with var- ball 6 to the distance of 36o. nish, and of sufficient weight to counterpoise the (2.) The silver wire being twisted by turning pith-ball a, which is from one-fourth to one-half the index of the micrometer 126°, the ball a of an inch in diameter.
approached to the ball b, and stopped at the 169. Fig. 1 represents the whole of the parts distance of 18° from it, having moved backof the instrument in their combined form. The wards through an arc of 18°. arm is represented as hanging horizontally in the (3.) Having again twisted the silver wire middle of the glass cylinder, so as to admit of its through an arc of 567°, the two balls apturning freely round its centre, in a circle de- proached, and stopped at the distance of 8° 30'. scribed on the glass by a graduated slip of paper 172. Now, as the force of torsion, or the force divided into 360°. When a is opposite to o on which is capable of keeping a thread twisted to this graduated circle it is contiguous to the ball a certain degree, so as to hinder it from turning b, which hangs within the cylinder by a silk round its axis, and recovering its natural state, thread covered with lac, and kept steady by the has been shown by Coulomb to be proportional piece of wood, which is seen lying on the cover, to the angle of torsion, or the arc through which from which it is suspended. This position of it has been twisted, we have in the first experithe instrument is produced by turning the milled ment such a force, equal to 36°; and in the headb, which carries the index io, called the second experiment, when the distance of the twist inder, round until it points to o on the balls was 18°, the angle, and consequently the graduated circle ab, and the whole is then turned force of torsion, was 126° +18=144°; hence in the brass socket H, till the ball a stands at () the repulsive force, at the distance of 36°, was on the graduated paper circle Q. Fig. 4 repre- 36°: and the repulsive force, at the distance of sents a cylindrical stick of sealing wax in the 18°, was 144°, or quadruple at half the distance. one end of which is inserted a fine brass wire In the third experiment, when the distance of terminated by a smooth ball. Fig. 5 is another the balls was 8° 30', the force of torsion was part of this apparatus shown on an eniarged 567°; so that, at a quarter of the distance, the scale; it consists of a plug of sealing-wax A, repulsive force was nearly eight times as great. which is made to fit tightly into the upper part from this it follows, that the repulsive force of of the instrument; through this plug of wax a two small globes electrified either positively or wire c, hooked at the top, passes perpendicu- negatively, is in the inverse ratio of the squares Jarly, and terminates in the finely polished me- of the distance of the centres of the two globes. tallic ball d; its use is to connect the electro 173. In these experiments, the wire P l was meter with other bodies.
twenty-eight inches long, and of a grain in weight,
and the force necessary to twist it through an put into its place in B, it will cause B to rest angle of 300°, when at the distance a P, was upon b, with a pressure equal to that weight, so sks of a grain, as calculated from the formulæ that more electricity must be communicated than given by M. Coulomb in his Memoir on the formerly, before the balls will separate ; and, as force of Torsion. Hence the real forces in the the weight in B is increased or diminished, a preceding experiments were,
greater or less quantity of electricity will be reDistances of Angles and Forces Absolute Forces
quired to effect a separation. the Balls. of Torsion.
176. When this instrument is to be used with
in grains. 36° 36 stooth of a grain.
a jar, or battery, one end of a wire, L, must be 18 144 ਤੇਰ
inserted into the ball b, and the other end into a 81 576 ata
hole of any ball proceeding from the inside of a
battery, or jar, as M;k must be screwed upon c, 174. An electrometer has been constructed by with its index towards A; the reason of this inMr. Cuthbertson, which, we think, in point of strument being added, is to show, by the index real utility, to the practical electrician, is equalled continuing to rise, that the charge of the batby none with which we are acquainted. Those tery is increasing, because the other part of the only who have to go through the operation of electrometer does not act till the battery has reinelting wires before a public audience, can duly ceived its required charge. appreciate the value of this incomparable instru 177. It was formerly observed that the unment. We have given in fig. 6 a representation coated part of a jar might be too dry to admit of this discharging compound electrometer, as of its being highly charged: the following expeconnected with a single jar, which, if it be about riment will illustrate the truth of this, as well as six inches in diameter and twelve inches high, show the use of the electrometer just described will be sufficient to fuse four inches of fine pen- in fusing wires. Every thing being prepared, as dulum wire. A description of it may be of use represented in the figure, with the jar M anto the inexperienced electrician. The base G H nexed to the electrometer, which jar may conis an oblong square piece of mahogany of about tain 168 square inches of coating put into B, the eighteen inches long, and six in breadth: in this pin marked 15, take two inches of watch penare three glass supports, D, E, F, mounted with dulum wire, fix to each end a pair of small brass balls, a, b, c. Under the brass ball a there pincers, as is represented at G m, hook one end is placed a brass hook; the ball c is made of to m, and the other to the wire N, communitwo hemispheres, the under one being fixed to cating with the outside of the jar; let the unthe brass mounting, and the upper turned with coated part of the jar be made very clean and a groove to shut upon it, so that it may be taken dry; and let the prime conductor of the maoff at pleasure. The ball b has a brass tube chine, or a wire proceeding from it
, touch the fixed to it, about three inches long, cemented to wire L; if then the machine be put in motion, the top of F; and a hole at the top, of about the jar and electrometer will be charged, as will half an inch in diameter, corresponding with the be seen by the rising of the index k, and, when inside of the tube. A B is a straight brass wire, charged high enough, B will be repelled by b; with a knife-edged centre in the middle, placed A will descend and discharge the jar through the a little below the centre of gravity, and equally wire, which was confined in the pincers, and balanced with a hollow brass ball at each end, the wire will be fused and run into globules. the centre, or axis, resting upon a properly formed 178. Repeat this experiment with the followpiece of brass fixed in the inside of the ball c; ing variations, viz. instead of two inches of wire that side of the hemisphere towards c is slit take eight; and instead of loading the ball with open, to permit the end c A of the balance to fifteen, insert the pin weighing thirty grains ; descend till it touches the ball a, and the upper charge, as before, and a spontaneous explosion hemisphere C is also so opened to permit the will take place between the coatings of the jar end i B to ascend; i is a weight of a certain without moving the electrometer, and consenumber of grains, and made in the form of a quently the wire will remain as it was. But let pin with a broad head ; the ball B has two holes, the uncoated part of the jar be now a little one at the top, and the other at the bottom; the moistened by breathing on it through a glass upper hole is so wide as to let the head of the tube; charge the jar again, the electrometer will pin pass through it, but to stop at the under operate, the charge pass through the circuit, and one, having its shank hanging freely in b; the whole length of the wire will appear redseveral such pins are made to each electrometer hot, and be instantly fused into globules. of different weights; k is a Henley's quadrant 179. A very useful instrument, called the unielectrometer, and when in use it is screwed versal discharger, was invented by Mr. Henley. upon the top of c.
It is shown in fig. 7, and consists of a maho175. It is obvious, from the construction, that gany base, A A, fourteen inches long, and about if the foot stand horizontally, and the ball B be four broad. B, B, are two glass pillars, cemented made to touch b, it will remain in that position in two holes upon the board A, and furnished at without the help of the weight i; and if it should top with brass caps, each of which has a turning receive a low charge, the two balls b, B, will joint, and supports a spring tube, through which repel each other; B will begin to ascend, and, the wires D, D, slide. Each of the caps is comon account of the centre of gravity being above posed of three pieces of brass, connected so that the centre of motion, the ascension will continue the wires D, D, besides their sliding through the till A rest upon a. If the balance be again set Sockets, have a horizontal and vertical motion. horizontally, and a pin i, of any small weight, be Each of the wires, D, D, is furnished with an