Electrome ter. Fig. 8. The following account is given of this electrometer, in a letter from Mr Lawson to the editor of the Philosophical Magazine. "Some time ago it struck me that some additions to Brooke's electrometer might be made, so as to fit it for a good discharging electrometer to measure the repulsion between two balls (of a certain size) in grains, and also effect the discharge of a battery at the same time. The instrument known by the name of Cuthbertson's discharging electrometer, (See ELECTRICITY, N° 203.) was at that time the best, and indeed the only instrument for discharging batteries or jars by its own action, then made; but I think this will be found, in the essentials, and in the theory and use, a more perfect instrument. "On the basis (fig. 8.) is fixed the glass pillar G, supporting the hollow brass ball B. I is a light graduated brass tube, divided (from the weight W towards the ball B) into 30 parts, representing grains. W is a sliding weight. L, a light brass ball screwed to the end of the tube I. On the other end of which tube adjusts the heavy counterbalance ball C, the tube I and its two balls being suspended at their common centre of gravity by a silk line in the centre of the ball B, the mechanism of which is shewn in fig. 9. The brass ball Fis stationary, and of the same size as the ball L; and is fixed by, and adjusts close to, the ball L, or at any lower station between that and the ring r. The brass tube to which the ball A is fixed is divided into inches, halves, and quarters: (a more minute division is unnecessary and improper.) The divisions begin, or the line o is marked on the said tube at the ring r, when the three balls A, L, F, are close together. The ring serves as an index, as the divisions pass in succession into the glass tube P on lowering the ball A. The hook H is screwed into the base of P. The quadrant, or Henley's electrometer, Q, is supported in a long brass stem, to keep it out of the atmosphere of the lower part Fig. 9. of the instrument. Fig. 9. shows the internal construction of the ball B, fig. 8. In the first place the ball screws in half, horizontally. The light tube I passes through the ball, and is suspended nearly in the centre of it by some silk twist, s, which small silk twist is fixed into the eye of the adjusting wire, a, part of which wire is filed square and goes through the square hole h. The nut n screws on a, and serves to adjust the light tube I vertically. The light plates PP are of copper, and move freely on the wire w w somewhat like a hinge, and rest on the copper wires CC, serving to make the direct communication between the inside and out of the battery or jar. NN are notches serving to let the tube I descend when the discharge is made. Into the tube Z the glass pillar is ground. Note, that at the bottom of the notch N is a piece of brass filled with a Y, and so placed as to keep the centres of the balls L and F, fig. 8. under each other when they come close toge ther. "When the instrument is adjusted, which is done by placing the weight W, fig. 8. at o on the line of grains, and then screwing or unscrewing the counterbalance ball C, till the tube I rises slowly into its horizontal position; then set the ball A at the distance from the ball L that you choose, and the weight W placed at the division or number of grains that you wish the repulsive power of the electricity to arrive at before the discharge ter. is made: this being done, connect the battery or jar Electrome. with the ball B, by means of the wire y, the end of which goes into B at the hole X, and should stand at right angles to B, the ball of y resting on the battery: then connect the outside of the battery or jar with the hook H. As the battery charges, the electrometer Q continues to rise; and when it is so highly charged that the repulsive power between the balls L and F is equal to the number of grains at which the weight W was placed, the ball L will descend, and deliver the charge of the battery to the ball A. The substance or thing through which the shock is intended to be passed, must form part of the communication between the hook H and the outside of the battery or jar. V. Hauch's ELECTROMETER. Fig. 10. contains a re- Fig. 1c. presentation of this electrometer, and the different parts of which it consists. OP is a board of dry mahogany, twelve inches in length and four in breadth, which serves as a stand for the instrument. In this board are fastened two masɛy glass pillars, M and N, which support the two brass caps or rings GG, with the two forks of tempered steel KK screwed into them. The two rings GG are well covered with varnish. In the ring is fastened a brass rod, which terminates in a ball E of the same metai, and an inch in diameter. The length of the rod and ball together is four inches and a half. A very delicate beam AB, the arms of which are of unequal length, moves on a short triangular axis (a knife edge) of well tempered steel, on the fork K of the pillar M. It is seventeen inches in length, and so constructed that the short arm forms a third, and the long one twothirds of the whole beam. The short arm of brass furnished with the ball B, exactly of the same size as the ball E, is divided into forty-five parts equivalent to grains. The long arm A is of glass covered with copal varnish, and ends in an ivory ball A, into which is fitted an ivory hook R, destined to support the ivory scale H. In order to render the insulation more complete, this scale is suspended by three hairs. A very delicate beam CD, eleven inches in length, moves on an axis like the former, on the pillar N, though not here shewn. This beam is proportioned in the same manner, one arm being a third and the other two-thirds of the whole length. The long arm of brass is furnished at the end with a ball D, and divided into thirty parts corresponding to grains. The short arm of glass terminates in a long roundish plate C, covered with copal varnish. The steel forks are shewn by the sections of the two brass caps FF, as are also the two knife edges L, L. By these caps the escape of the electric matter is partly prevented. A brass ring Q, capable of being moved along the short arm of the upper beam AB, shews by means of marks determined by trial and cut out on the beam, the number of grains which must be placed in the small scale to restore the equilibrium of the beam,. at each distance of the ring Q from the point of suspension. On the long arm CD of the lower beam there is also a moveable ring S, which, like the ring Q, shews in grains, by its distance from the point of suspension, the power requisite to overcome the preponderance of LD in regard to LC. The power necessary for this purpose will be found, if the ter. Electrome-the shell H, which weighs exactly fourteen grains, be suffered to sink down on the glass plate C, and the ring s be pushed forwards till both the arms of the beam are in equilibrium. The part of the beam on which the rings has moved, is divided into fourteen parts, so that O marks the place where the ring s must stand when the beam, in its free state, is in equilibrium; and 14 stands at the place where the rings again restores a perfect equilibrium when the shell H is laid on the glass plate C. Each of these parts, which are divided into quarters, indicates a grain. The lower divisions of the scale will be found with more accuracy, if quarters of a grain be put in succession, into the shell H (after it has been laid on the plate C), and the ring s be moved between each quarter of a grain until the perfect equilibrium be restored. This place on the beam is then to be marked, and you may continue in this manner until the 30th part of a grain be given. Both scales, for the sake of distinctness, are only divided so low as quarters of a grain; though the instrument is so delicate, and must absolutely be so, that 1-20th of a grain is sufficient to destroy the equilibrium. The two glass pillars M and N, together with the steel forks affixed to them, are so fitted into the stand, that both the beams lie parallel to each other as well as to the rod GE. In this position of the beams AB, the balls B and E are just in contact. The smallest glass pillar N is of such a height that the ball of the beam CD stands at the distance of exactly four lines from the ring G, and cannot move without touching the latter. The small shell H is suspended in such a manner that there is a distance of exactly two lines between it and the shell C. In each of the brass rings GG is a small hole, that the instrument may be connected with the two sides of an electric jar. I is a brass wire, with a hollow bit of ivory, a, destined to support the beam CD, which is necessarily preponderate at D, in order to prevent oscillation between the discharges to be examined by the instrument. It may be readily comprehended that, when the beam AB has moved, A must pass over twice the space that B does; and that in the beam CD, the case is the same în regard to C and D. If AB be therefore connected with the external, and CD with the internal side of a battery, but in such a manner that the instrument is at a sufficient distance beyond the electric atmosphere; and if the battery be charged, the repulsive effect of the electric power will oblige the ball B to separate from the ball E; the shell H must therefore naturally sink down with double velocity, so that when the ball B rises a line, the shell H must sink two when it reaches this depth it will touch the shell C, and the latter, by the power excited in it, will be obliged to sink, by which D must naturally again ascend in a double proportion to the sinking of C; so that when C has fallen two lines, D must have ascended four, and D that moment touches the ring by which the two sides of the battery are connected with each other, and dis charges the battery. But as the attractive electric power between unlike atmospheres, under like circumstances, is at least as strong as its repulsive power between like atmospheres, it would thence follow, that the electric power, instead of repelling the ball B from the ball E, would rather attract D, and by its contact with G, promote the dis4 charging; by which the instrument would fail of its Elec object, and be subjected to the temperature of the atmosphere like all other electrometers; and, besides this, the electric power could no longer be determined by weight. To obviate this inconvenience, the instrument, in all electrical experiments, must be applied in such a manner that the power with which the ball D is attracted by AB may exceed in strength the power required to repel the ball B from the ball E. For this purpose the ring s must always be removed two divisions farther on CD, towards D, than the ring Q is shifted on AB towards B. If, for example, an electric force were required equal to eight grains, according to this electrometer, the ring Q must be removed to the place where 8 stands, and the ring s to the place marked 10. The repulsive power will then naturally repel the balls B and E before G is in a condition to attract the ball D, as a power of two grains would be necessary for this purpose, besides that of the eight already in action. The shell H with its weight of fourteen grains, will easily overcome the preponderance of LD over LC, as it amounts only to ten grains, and therefore nothing exists that can impede the discharging. When the rings, according to the required power, is removed so far towards D, that the shell H is not able by its weight to destroy the preponderance of LD in regard to LC, the active power of the shell H must be so far increased by the addition of weights, that it can act with a preponderance of four grains on the plate C. If, for example, an electric power of 14 grains be required, the ring s must be removed to 16, by which LD rests upon a, with a preponderance of 16 grains in regard to LC. Now, to make H act on the plate C with a preponderance of four grains, it must be increased to 20 grains, that is, six grains weight more must be added, as it weighs only 14; which six grains are again laid upon LB; and therefore the ring Q is shifted to 20, as the strength of the repulsive power is pointed out by 14 grains. If an electric power of 25 grains be required, the rings must be removed to 27, and the weight of 17 grains be put into the shell H, in order to produce a preponderance of four grains in regard to s. These 17 grains are added to the required power of 25 grains, and the ring Q is pushed to 42, &c. In this manner the repulsive power always acts before the attractive power can. It may be readily perceived that the faults and inconveniences common to all the electrometers hitherto employed, and which have been already mentioned, cannot take place here; because the discharging is performed by immediate connection between the positive and negative electricity in the instrument itself, without any external means being employed. One of the most essential advantages of this instrument is, the certainty with which the same result may be expected when the experiment is repeated. From the same degree of electric power, whatever be the temperature of the atmosphere, it will always be neces sary to commence the separation of the two balls B and E from each other, the quantity of coated glass and the distance of the ring Q from the axis L being the same. Another no less important advantage of this instrument is, that in an experiment where the same electric power, ter. Electrome- power, often repeated, is necessary to ascertain the result with accuracy; such, for example, as the charging a battery through acids, water, &c.; the same degree of precaution is not necessary as is indispensably so in any other electrometer, as the person who puts the machine in motion has nothing to do but to count how often the electrometer discharges itself; and the instrument may be inclosed in a glass case, or prevented in any other manner from external contact, or any other circumstances which might render the experiment uncertain. *Phil. Magaz. vol. iv. Fig. 11. Fig. 12. Fig. 13. "I flatter myself (says M. Hauch), that the simplicity of the construction of this instrument, the facility with which it may be made at a very small expence, and the certainty that two instruments, prepared according to the same scale, with a like quantity of coated glass, must exactly correspond with each other; but above all, that the certainty and accuracy by which experiments may be made with it, and by these means be accurately described, are advantages which will not be found united in any of the electrometers hitherto invented." * We shall close this account of electrometers with describing the construction and use of M. Coulomb's electrometer, or, as he calls it, Electrical Balance. ABDC (fig. 11.) represents a glass cylinder, twelve inches in diameter and the same in height, covered by a glass plate fitted to it by a projecting fillet on the under surface. This cover is pierced with two round holes one inch and three-fourths in diameter. One of them fis in the centre, and receives the lower end of the glass tube fh, of twenty-four inches beight, 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. 12. N° 3.) bored truly cylindrical with a small shoulder, which rests on the top of the tube. This collar is fastened with cement, and receives the bollow cylinder (fig. 12. N° 2.), to which is joined the circular plate a b, 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. 12. N° 1.) having a milled head b, and furnished with an index i o, whose point is bent down so as to mark the divisions on the circle a b. This pin turns stiffly in the hole G, and the cylinder moves steadily in the collar H. To the lower end of the centre pin is fastened a little pincer, q, formed like the end of a port-crayon, and tightened by the ring 9, so as to hold fast the suspension wire, the lower end of which is grasped by a similar pincer, Po (fig. 13.) tightened by the ring . The lower end is cylindrical, and is of such a weight, as to draw the wire perfectly straight, but without any risk of breaking it. It may be made equal to half of the weight that will just break it. This pincer is enlarged at C, and pierced with a hole, which tightly receives the arm g C q of the electrometer. This arm is eight inches long; and consists of a dry silk thread, or a slender straw completely dried, and dipped in melted lac or fine sealing-wax, and held perpendicularly before a clear fire, till it be come a slender cylinder of about one-tenth of an inch in diameter. This occupies six of the eight inches, from g to q: the remaining two inches consist of a fine thread of the lac or sealing-wax, as it drains off in forming the arm.. At a, is a ball of pith or fine cork, ter. one-fourth or one-half of an inch in diameter, made very Electrome smooth, and gilded. It is balanced by a vertical circle of paper g, of large dimensions, made stiff with varnish. The resistance of the air to this plane soon checks the oscillations of the arm. The whole instrument is seen in its place in fig. 11. 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 x o q, drawn on a slip of paper, and made to adhere to the glass by 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 lac m, inclosing a dry silk thread. This stalk is grasped by a clamp of cleft deal, or any similar contrivance, which is made to lie 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 to o on the graduated circle. In order to electrify the ball t, we are to employ the insulating handle, fig. 14. which is a slender stick Fig. 14. of sealing-wax or lac, holding a metal wire that carries a small polished metallic ball. This is to be touched with some electrified body, such as the prime conductor of a machine, the knob of a jar, &c. This electrified ball is to be introduced cautiously into the hole m, and the ball t is to be touched with it. The ball a is immediately repelled 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. This is the process for examining the law of electric action. When it is desired to examine the action of different bodies in different states, another apparatus is wanted. This is represented by the piece c Ad (fig. 15.) consisting of a plug of sealing-wax A, fitting Fig. 15. tightly into the hole m, and pierced by the wire cd, hooked at c, to receive a wire to connect it occasionally with an electrified body, and having below a polished metal ball d. The instrument is fitted for observation in the following manner: The milled button b is turned at top, till the twist index io is at the mark o of the twist circle. Then the whole is turned in the collar H, till the ball a stand opposite to the mark o of the paper circle az o Q, and at the same time the ball t or d is touched. The observation is thus made. The ball t is first electrified as just described, and thus a is repelled, and retiring twists the wire, settling, after a few oscillations, at such a distance as is proportional to the repulsion. The twistindex is now turned so as to force a nearer to t. The repulsion thus produced is estimated by adding the motion of the index to the angle at which the ball first stopped. Giving the index another turn, we have another repulsion, which is estimated in a similar way, and thus we obtain as many measures as required.. It is not necessary to make this instrument of very large dimensions; one 14 inches high, and five in diameter, of which the arm a g should occupy two inches and a half, will be sufficiently large for most purposes. The diameter of the glass cylinder must always be double the length of the arm a g, that the position of this may not be disturbed by the action of the glass. Dr Robison considered this electrometer as one of the The bottom should be furnished with a round hole, admitting the lower end of the cylinder Co belonging to the lower pincer (when the wire is strained at both ends) to hang freely, by which means much tedious oscillation will be prevented. It is much more convenient to have the suspension wire strained at both ends; and it should extend as far below the arms as above it, and the lower extremity should be grasped by a pincer that turns by a milled head in a hole at the end of a slender spring. The instrument may then be speedily adjusted by placing the twist index at o, and gently turning the lower button till the ball a point exactly at o on the paper circle. ed by the different states of humidity of the air. In the El scale of Saussure's hygrometer, the relation to the quantity of water which a cubic foot of air is capable of holding in solution is distinctly marked; the relation of this solution to the dissipation of electricity in Coulomb's experiments may hence be seen in the following table, the first column of which marks the degrees of Saussure's hygrometer, the second how many grains of water are dissolved in a cubic foot of air at each degree, and the third column column shews the corresponding dissipation per minute. The instrument will be greatly improved, if, in place make The outside of the tube must also be varnished, and It was by means of this incomparable instrument, that M. Coulomb made the valuable experiments, to which we alluded in the article ELECTRICITY, when treating of the law of action of the electric fluid. By means of this electrometer, he also made his experiments on the dissipation of electricity into the air, and along imperfect conductors. He ascertained the law of dissipation into the air from bodies in contact, and the relation which this bore to the original repulsion, by first observing the gradual approach of the ball a towards t, in proportion as the electricity dissipated from both, and then slackening the twist index till the ball a resumed its original situation. The following was the general result of Mr Coulomb's experiments. That the momentary dissipation of moderate degrees of electricity is proportional to the degree of electricity at the moment. He found that the dissipation is not sensibly affected by the state of the barometer or thermometer; nor is there any sensible difference of bodies of different sizes or different substances, or even different figures, provided that the electricity is very weak. But he found that the dissipation was greatly affect 7,197 6,180 3,61; or at a me make;= 9,240 The immediate object that M. Coulomb had in view in his experiments, was to ascertain the diminution of repulsion. He found that this, in a given state of the air, was a certain proportion of the whole repulsion taken at the moment of diminution, which is double the proportion of the density of the fluid; for the repulsions by which we judge of the dissipation are reciprocal, being exerted by every particle of fluid in the ball t of the electrometer, on every particle of fluid in the ball a. The diminution of repulsion is therefore proportional to the density of the electric fluid in each ball; and, as during the whole dissipation, the densities continue to have their original proportion, and as the diminution of repulsion is directly proportional to the diminution of the products of the densities, it is consequently directly proportional to the square of either. If we put d for the density, the mutual repulsion will be represented by d, and its momentary diminution by the fluxion of d3, or 2 d d=2 dxd. But 2 dxd: d'=2d: d. The diminution of the repulsion observed by experiment will be to the whole repulsion, in double the proportion that the diminution of density, or the dissipation of fluid will have to the whole quantity of fluid at the moment of observation. Let us, for instance, suppose the observed diminution of repulsion to be ; we may conclude, that the quantity of fluid lost by dissipation is. M. Coulomb did. not examine the proportion of the dissipations from bodies of various sizes. But we know, that if two spheres communicate by a very long canal, their superficial densities, and the tendencies of fluid to escape from them, are inversely as the diameters of the spheres. Now, in a body that has twice the diameter of another body, the surface of the former is quadruple of that of the latter; and though the tendency of fluid to escape from the former is only the half of its tendency to escape from the latter, yet the greater surface of the former may so far make up for its smaller density, that the Electrome- the dissipation of fluid from a large sphere may in fact ter. be greater than that from a small one in the same given time. We have remarked above, that these experiments were made in a particular state of the air; and the law of dissipation ascertained by them is of course adapted only to that given state. In a different state of the air, even if this should be impregnated with the same pro· portion of moisture, the law of dissipation may be different. The inference which M. Coulomb expected to draw from his experiments was, that the ratio of dissipation would prove to be less than the cube of the quantity of water held in solution, except when that quantity of water was what the air was capable of holding in solution at the given temperature. This is agreeable to observation; for we know that air which is considered as dry, that is, when it is not nearly saturated with moisture, is the most favourable to electrical phenomena. Such is the general result of Coulomb's experiments on the dissipation of electricity into the air. The method in which M. Coulomb examined the dissipation along imperfect conductors, by means of this instrument, was, by completely insulating the ball t, and then after observing the loss sustained by a body in contact with it from the air, sliding a metallic rod down the insulating stalk, till the dissipation began to exceed what took place only by the air. From his experiments respecting the dissipation along imperfect conductors, he found that this took place in a different manner from that in which electricity escaped by communication with the contiguous air. The electricity seems to be diffused chiefly along the surface of the insulator, and appears principally to be produced by the moisture that is more or less attached to it. M. Coulomb illustrates this in the following manner. Water is found to adhere to the surface of all bodies from which it is prevented by adhesion from escaping when the bodies are electrified, and is thus rendered capable of receiving a greater degree of electric power. Let us suppose that the particles of moisture are disposed uniformly over the surface, with intervals between them; the electricity that is communicated to one particle, must acquire a certain degree of density, before it can fly from this particle to the next, across the intervening insulating space. When an imperfect conductor of this kind is electrified at one extremity, the communicated electricity, in passing to the other extremity, must be weakened every step in passing from particle to particle. Suppose we have three adjacent particles, which we may call a, b, and c; we infer from N° 374. of the article ELECTRICITY, that the motion of b is sensibly effected, only by the difference of a and e: and therefore the passage of electric fluid from b to c, requires that this difference be superior, or at least equal to the force necessary for clearing this coercive interval. Let a particle pass over. The density of fluid of the particle b is diminished, while the density of the particle on the other side of a remains as before. Therefore some fluid will pass from a to b, and from the particle preceding a to a; and so on, till we come to the electrified end of this insulator. It is plain, from this consideration, that we must at last arrive at a particle beyond c, where the whole repulsion of the preceding VOL. VIII. Part I. + ter particle is just sufficient to clear the coercive interval. ElectroneSome fluid will come over; and the repulsion of this, acting now in the opposite direction, will prevent any fluid from coming to supply its place in the particle which it has just quitted; the transference of fluid will therefore stop here, and beyond this point the insulation will be complete. Hence we perceive that there is a mathematical relation between the insulating power, and the length of the canal; and this may be ascertained by the theory which we adopted in the article ELECTRICITY. We shall here give an instance of this investigation; and, for the sake of simplicity, we shall take a very probable case, viz. where the insulating interval, or, as we may more properly call it, the coercive interval, is equal in every part of the canal. Let R represent the coercive power of the insulator, or the degree of force required to clear the coercive interval between two particles. Suppose a ball C, fig. 16. suspended by a silken thread AB; and let us de- Fig. 16. note the quantity of a redundant fluid in the ball by C, and let the densities at the different points of the canal be denoted by AD, P d, &c. ordinates to some curve DdB, cutting the axis in B, the point where the thread AB begins to insulate completely. Let Pp be an element of the axis; draw the ordinate pf, a tangent to the curve dfF, the normal d E, and draw fe perpendicu lar to Ÿ d. lar to Pd. Suppose AC=r, AP=x, and P day. Then we shall have Ppx, and de--y. It was shewn in N° 374. of the article ELECTRICITY, that the only sensible action of the fluid on a particle at P is yy when the action of the redundant fluid in the globe on the particle at P, having the density y, is denoted by Cy yy Therefore is R, the coercive power of (r+x)2° Pdxde Pr the thread, which is supposed to be constant, is therefore equal to some constant line R. But Pp (or fe): de Pd: PE. The subnormal, PE, is therefore a constant line. But as this is the property of a parabola, the curve of density D d B must be a parabola, of which 2PE 2R, is the parameter. perfect insulator are in the subduplicate ratio of their COR. 1. The densities at different points of an imdistances from the point of complete insulation: for Pd: AD-BP: BA. different densities of the electric fluid are in the dupliCOR. 2. The lengths of canal requisite for insulating cate ratio of their densities; for AB= is a constant quantity. AD❜ 2PE' and PE COR. 3.-The length of canal requisite for insulation is inversely as its coercive power, and may be represented by For AB = D1 R' DA D' 2PE 2R = |