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When it rains or snows, the small cover, fig. 9, is to be screwed on the top of the instrument, as by this its insulation is preserved. This indicates not only the electricity of fogs, but also that of serene weather, and enables the observer to discover the kind of electricity which reigns in the atmosphere; and, in some degree, to form an estimate of its quantity, and that under two different points of view, the degree of intensity, and the distance from the earth at which it first begins to be sensible. A conductor exhibits signs of electricity only when the electric fluid is more or less condensed in the air than in the earth. Though the air resists the passage of the electric fluid, it is not absolutely impermeable to it; it suffers it to pass gradually, and generally with more ease in proportion as its mass is less. It is, therefore, interesting to discover at what height it is necessary to be elevated, in order to find a sensible difference between the electricity of the earth and that of the air. 163. Mr. Brooke of Norwich constructed an electrometer of a very ingenious description, and certainly valuable in its application, of which he has given a full account in his Miscellaneous Experiments; the limits, however, within which our article must be confined, will not allow of our giving his lengthened description of the instrument. We shall, therefore, present our readers with a short account of another electrometer, the invention of professor Robison of Edinburgh, which in our opinion is, in its chief essentials, much superior to that of Mr. Brooke. This beautiful and delicate instrument is represented by fig. 10, where A, a finely polished brass ball of a quarter of an inch diameter, is fixed on the point of a common sewing needle about three inches long, and as slender as can be procured of that length. On the other end of the needle is fixed a ball of amber, glass, or other non-conducting substance, of about , or oths of an inch in diameter. This ball is so fixed as that the needle does not quite reach to its surface, though the ball F must be completely perforated. From the electric ball there passes a slender glass Tod, F, F, L, bent at right angles at E, so that the part FE may be about three inches long, and the other extremity L, immediately opposite to the centre of the ball A. A piece of amber C, so cut as to have two parallel cheeks, is fixed on the extremity L of the glass rod. For the principal part of the instrument, a strong dry silk thread is to be prepared by dipping it perpendicularly in melted sealing-wax, till it be fully penetrated by the wax, so as to retain a thin coating of it. 164. The thread, thus coated, must be kept extended, so that it may be quite straight; it must be made perfectly smooth by holding it before a fire, and rolling it on a smooth table. It is then to be passed through a small cube of amber, that has two holes drilled in two of its opposite faces, perpendicularly to the stalk. By these holes the cube is suspended, so as to move readily, on two fine brass pins, between the cheeks of the piece of amber at L. The waxed thread is about six inches long, and is equally divided by the amber cube. To the end B is fived a ball of some conducting substance, as of
Polished metal, or gilt cork, a quarter of an inch .
in diameter. The other extremity D passes
through a cork ball, so as to move with a slight friction. 165. The construction of this part of the electrometer is such that when FE lies perpendicularly to the horizon, and the stalk BD, with its balls, is allowed to hang freely, the ball B just touches the ball A, as represented in fig. 11. The ball F is fixed to one end of a glass rod FI passing perpendicularly through the centre of a graduated circle G H O, and furnished at the opposite extremity I with a knobbed handle of boxwood. H K is the stand of the electrometer, in the head of which is a hole in which the rod FI slides smoothly but not easily. There is also adapted to the glass rod FI an index N H that turns round it. This index is placed so as to be parallel to a line L.A drawn through the centre of the ball A. And, as the circle is divided into 360 degrees, 0 being marked above, and 90 on the right hand; the index will point out the angle which the line LA makes with the vertical line. It is convenient to have another index on the rod F I turning with some friction round it, and extending considerably beyond the circle G H O. 166. When this electrometer is used for computing the quantum of electricity belonging to any body, it must be connected by means of a wire with the substance to be examined : the wire must be fixed into the hole of the ball F, . which is in part filled by the needle; and indeed it must come in contact with the needle. The index must now be turned round by the handle I until it stand at 90°. In this position C B is horizontal, and the ball B contiguous to A. If in this position the balls B and A are electrified, they do not separate till the index be turned back towards 0. In some part of this space they will separate, and the point must be carefully noted, as this is the measure of their electric power while in contact. By turning still more towards 0, the separation is increased. An assistant must now turn the long index till it be parallel to the other, and consequently to CD. On loading the cork ball D with grain weights till BD be nicely balanced in a horizontal position, and computing for the proportional length of CB and CD, we obtain an exact measure in grains of the electric force with which the balls B an A separate in this position. 167. The electrometers already described have all been found highly useful to the practical electrician, according to the peculiar nature of the course of his experiments; but for ascertaining the actual repulsive and attractive powers of very faintly electrified bodies, they are per
haps all excelled by the ingenious invention of
M. Coulomb, a French philosopher. The construction of this delicate instrument, which is called the torsion balance, will be best understood by a reference to the plate, where we have given a representation of it in its complete state, and of some of its parts on an enlarged scale. Fig. 1, Plate IV., is a cylinder of flint glass twelve inches in diameter, and twelve in height, covered with a plate of glass, which is made to fit to it by a projecting fillet on the lower surface and having in it four round perforations, of
an inch and three-fourths in diameter, one of which is in the centre f, and receives the glass tube fli, which is two feet in height, and is fixed in the glass plate with cement. Into the top of this tube is inserted the brass piece H, fig. 2, No. 3, which is perfectly cylindrical, and having a small shoulder which rests on the top of the tube into which it is cemented. This brass piece is made to fit by means of a screw on the hollow cylinder, No. 2, fig. 2, to which is joined the circular plate ab, divided into 360 degrees, and having a hole G in its centre for admitting the cylindrical pin i, No. 1, fig. 2. This pin is surmounted with a milled head b, from which an index io projects, having a point turned downwards at 0, to mark the divisions on ab. This pin moves with some friction in the hole G, while the cylinder moves steadily in the brass piece H. To the lower end of the pin there is attached the pincer q, resembling the end of a solid port crayon, and capable of being tightened by a sliding ring : so as to seize a fine silver wire, while its lower end is held by a similar pincer, shown by Po, fig. 3, and tightened by the sliding ring r. 168. The stalk ro is cylindrical, and is made so heavy as to keep the wire quite straight without breaking it. Fig. 3 exhibits this pincer with the arm z C q passing through it. The length of this arm is eight inches, and it is formed of a stout silk thread, or a fine round straw coated with wax, or with lac; it is about one-tenth of an inch diameter, and six inches long, and is terminated by two inches of wax drawn out into a fine thread. The end q carries a pith-ball a turned very smooth, and gilded, and the other end is a small circular plane of paper covered with varnish, and of sufficient weight to counterpoise the pith-ball a, which is from one-fourth to one-half of an inch in diameter. 169. Fig. 1 represents the whole of the parts of the instrument in their combined form. The arm is represented as hanging horizontally in the middle of the glass cylinder, so as to admit of its turning freely round its centre, in a circle described on the glass by a graduated slip of paper divided into 360°. When a is opposite to o on this graduated circle it is contiguous to the ball b, which hangs within the cylinder by a silk thread covered with lac, and kept steady by the }. of wood, which is seen lying on the cover, rom which it is suspended. This position of the instrument is produced by turning the milled head b, which carries the index io, called the twist inder, round until it points to o on the graduated circle ab, and the whole is then turned in the brass socket H, till the ball a stands at O on the graduated paper circle Q. Fig. 4 represents a cylindrical stick of sealing-wax in the one end of which is inserted a fine brass wire terminated by a smooth ball. Fig. 5 is another part of this apparatus shown on an eniarged scale; it consists of a plug of sealing-wax A, which is made to fit tightly into the upper part of the instrument; through this plug of wax a wire c, hooked at the top, passes perpendicularly, and terminates in the finely polished metallic ball d, its use is to connect the electrometer with other bodies.
170. We shall here add, as an illustration of the method of using this ingenious instrument, an account of Coulomb's method of determining by it the electrical repulsion of the two balls a and b when electricity was communicated to them. Having electrified the brass pin, fig. 5, it was introduced through the hole m, in the top of the cylinder, and made to touch the ball b, in contact with the ball a, thus communicating its electricity to the two balls, which consequently became electrified with the same electricity; and, on this pin being withdrawn, a mutual repulsion took place between the two balls, a being driven from b to a distance easily measured on the graduated scale, and obviously regulated by the resistance of the wire to farther torsion. After a few slight oscillations the arm will rest and the degree of repulsion may be accurately noted. The index i o is now to be turned backwards till the ball a come to its former position, by which movement the silver wire will be twisted, and a force produced proportional to the angle of torsion which is required to bring the ball a in contact with the ball b. By this means M. Coulomb ascertained the distance at which different angles of torsion bring the balls in contact after mutual repulsion; by comparing the forces of torsion with the corresponding distances of the balls, he obtained a measure of their repulsive force. 171. The following are given by Dr. D. Brewster, as the results of some experiments with this curious instrument, by its inventor. (1.) The two balls being electrified with the head of the pin, and the index of the micrometer being set to zero, the ball a was repelled by the ball b to the distance of 36°. (2) The silver wire being twisted by turning the index of the micrometer 126°, the ball a approached to the ball b, and stopped at the distance of 18° from it, having moved backwards through an are of 18°. (3.) Having again twisted the silver wire through an arc of 567°, the two balls approached, and stopped at the distance of 8° 30'. 172. Now, as the force of torsion, or the force which is capable of keeping a thread twisted to a certain degree, so as to hinder it from turning round its axis, and recovering its natural state, has been shown by Coulomb to be proportional to the angle of torsion, or the arc through which it has been twisted, we have in the first experiment such a force, equal to 36°; and in the second experiment, when the distance of the balls was 18°, the angle, and consequently the force of torsion, was 126° + 18- 144°; hence the repulsive force, at the distance of 36°, was 36°: and the repulsive force, at the distance of 18°, was 144°, or quadruple at half the distanceIn the third experiment, when the distance of the balls was 8° 30', the force of torsion was 567°; so that, at a quarter of the distance, the repulsive force was nearly eight times as greatFrom this it follows, that the repulsive force of two small globes electrified either positively or negatively, is in the inverse ratio of the squares of the distance of the centres of the two globes 173. In these experiments, the wire Pl was twenty-eightinches long, and kofagrainin weight
174. An electrometer has been constructed by Mr. Cuthbertson, which, we think, in point of real utility, to the practical electrician, is equalled by none with which we are acquainted. Those only who have to go through the operation of melting wires before a public audience, can duly appreciate the value of this incomparable instrument. We have given in fig. 6 a representation of this discharging compound electrometer, as connected with a single jar, which, if it be about six inches in diameter and twelve inches high, will be sufficient to fuse four inches of fine pendulum wire. A description of it may be of use to the inexperienced electrician. The base GH is an oblong square piece of mahogany of about eighteen inches long, and six in breadth: in this are three glass supports, D, E, F, mounted with brass balls, a, b, c. Under the brass ball a there is placed a 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 shut upon it, so that it may be taken off at pleasure. e ball b has a brass tube fixed to it, about three inches long, cemented to the top of F; and a hole at the top, of about half an inch in 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 properly formed piece of brass fixed in the inside of the ball c : that side of the hemisphere towards c is slit open, to permit the end cA of the balance to descend till it touches the ball a, and the upper hemisphere C is also so opened to po the end c B to ascend; i is a weight of 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, having its shank hanging freely in b: several such pins are made to each electrometer of different weights; k is a Henley's quadrant electrometer, and when in use it is screwed upon the top of c.
175. It is obvious, from the construction, that if the foot stand horizontally, 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 receive a low charge, 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 rest upon a. If the balance be again set horizontally, and a pin i, of any small weight, be
put into its place in B, it will cause B to rest upon b, with a pressure equal to that weight, so that more electricity 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 electricity will be required to effect a separation. 176. When this instrument is to be used with a jar, or battery, one end of a wire, L, must be inserted into the ball b, and the other end into a hole of any ball proceeding from the inside of a battery, or jar, as M ; 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 part of the electrometer does not act till the battery has received its required charge. 177. It was formerly observed that the uncoated part of a jar might be too dry to admit of its being highly charged : the following experiment will illustrate the truth of this, as well as show the use of the electrometer just described in fusing wires. Every thing .." prepared, as represented in the figure, with the jar M annexed to the electrometer, which jar may contain 168 square inches of coating put into B, the pin marked 15, take two inches of watch pendulum wire, fix to each end a o of small pincers, 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 the machine, or a wire proceeding from it, touch the wire L.; if then |. machine be put in motion, the jar and electrometer will be charged, as will be seen by the rising of the index k, and, when charged high enough, B will be repelled by b; A will descend and discharge the jar through the wire, which was confined in the pincers, and the wire will be fused and run into globules. 178. Repeat this experiment with the following variations, viz. instead of two inches of wire take eight; and instead of loading the ball with fifteen, insert the pin weighing thirty grains; charge, as before, and a spontaneous explosion will take place between the coatings of the jar without moving the electrometer, and consequently the wire will remain as it was. But let the uncoated part of the jar be now a little moistened by breathing on it through a glass tube; charge the jar again, the electrometer will operate, the charge P. through the circuit, and the whole length of the wire will appear redhot, and be instantly fused into globules. 179. A very useful instrument, called the universal discharger, was invented by Mr. Henley. It is shown in fig. 7, and consists of a mahogany base, AA, fourteen inches long, and about four broad. B, B, are two glass pillars, cemented in two holes upon the board A, and furnished at top with brass caps, each of which has a turning joint, and supports a spring tube, through which the wires D, D, slide. Each of the caps is composed of three pieces of brass, connected so that the wires D, D, besides their sliding through the sockets, have a horizontal and vertical motion. Each of the wires, D, D, is furnished with an