« НазадПродовжити »
Electrome. The following account is given of this electrometer, is made : this being done, connect the batter
in a letter from Mr Lawson to the editor of the Philo. with the ball B, by means of the wire y, the sophical Magazine.
which goes into B at the bole X, and should “ Some time ago it struck me that some additions to right angles to B, the ball of y resting on the Brooke's electrometer might be made, so as to fit it for a then connect the outside of the battery or jar good discharging electrometer to measure the repulsion hook H. As the battery charges, the electro between two balls (of a certain size) in grains, and also continues to rise; and when it is so highly char effect the discharge of a battery at the same time. The the repulsive power between the balls L and F instrument known by the name of Cuthbertson's dis- to the number of grains at which the weight charging electrometer, (See ELECTRICITY, N° 203.) placed, the ball L will descend, and deliver the was at that time the best, and indeed the only in of the battery to the ball A. The substance strument for discharging batteries or jars by its own through which the shock is intended to be pass action, then made ; but I think this will be found, in form part of the communication between the the essentials, and in the theory and use, a more per
and the outside of the battery or jar. fect instrument.
V. Hauch's ELECTROMETER. Fig. 10. conta Fig. S. “ On the basis (fig. 8.) is fixed the glass pillar G, presentation of this electrometer, and the differe
supporting the hollow brass ball B. I is a light gra- of which it consists. OP is a board of dry ma duated brass tube, divided (from the weight W towards twelve inches in length and four in breadıl the ball B) into 30 parts, representing grains.
W serves as a stand for the instrument. In this b is a sliding weight. L, a light brass ball screwed to fastened two massy glass pillars, M and N, wl the end of the tube I. On the other end of which tube port the two brass caps or rings GG, with adjusts the heavy counterbalance ball C, the tube I and forks of tempered steel KK screwed into the its two balls being suspended at their common centre of two rings GG are well covered with varnish. gravity by a silk line in the centre of the ball B, the In the ring is fastened a brass rod, which te mechanism of which is shewn in fig. 9. The brass ball in a ball E of the same metai, and an inch in F is stationary, and of the same size as the ball L; and The length of the rod and ball together is fou is fixed by, and adjusts close to, the ball L, or at any and a half. lower station between that and the ring r. The brass A very delicate beam AB, the arms of whi tube to which the ball A is fixed is divided into inches, unequal length, moves on a short triangular axis balves, and quarters : (a more minute division is unne- edge) of well tempered steel, on the fork K oft cessary and improper.) The divisions begin, or the line M. It is seventeen inches in length, and so cor o is marked on the said tube at the ring r, when the that the short arm forms a third, and the long three balls A, L, F, are close together. The ring thirds of the whole beam. The short arm of ' serves as an index, as the divisions pass in succession nished with the ball B, exactly of the same sia into the glass tube P on lowering the ball A. The ball E, is divided into forty-five parts equi hook H is screwed into the base of P. The quadrant, grains. The long arm A is of glass cover or Henley's electrometer, Q, is supported in a long brass copal varnish, and ends in an ivory ball A, in
stem, to keep it out of the atmosphere of the lower part is fitted an ivory book R, destined to support Fig. 2. of the instrument. Fig. 9. shows the internal construc- scale H. In order to render the insulation m
tion of the ball B, fig. 8. In the first place the ball plete, this scale is suspended by three hairs.
small scale to restore the equilibrium of the
Electrome-the shell H, which weighs exactly fourteen grains, be charging ; by which the instrument would fail of its Electr
suffered to sink down on the glass' plate C, and the ring object, and be subjected to the temperature of the ats be pushed forwards till both the arms of the beam are mosphere like all other electrometers; and, besides this, in equilibrium. The part of the beam on wbich the the electric power could no longer be determined by rings bas moved, is divided into fourteen parts, so that weight. To obviate this inconvenience, the instrument, o marks the place where the ring s must stand when in all electrical experiments, must be applied in such a the beain, in its freè state, is in equilibrium ; and 14 manner that the power with which the ball D is attract. stands at the place where the ring s again restores à ed by AB may exceed in strength the power required perfect equilibrium when the shell H is laid on the glass to repel the ball B from the ball E. For this purpose
Each of these parts, which are divided into the ring s must always be removed two divisions fariher quarters, indicates a grain. The lower divisions of the on CD, towards D, than the ring Q is shifted on AB scale will be found with more accuracy, if quarters of a towards B. If, for example, an electric force were regrain be put in succession, into the shell H (after it has quired equal to eight grains, according to this electrobeen laid on the plate C), and the ring s be moved be nieter, the ring Q must be removed to the place where tween each quarter of a grain until the perfect equili. 8 stands, and the ring s to the place marked 10. The brium bė restored. This place on the beam is then to repulsive power will then naturally repel the balls B and be marked, and you may continue in this manner until E before G is in a condition to attract the ball D, as the 30th part of a grain be given. Both scales, for the à power of two grains would be necessary for this pursake of distinctness, are only divided so low as quarters pose, besides that of the eight already in action. The of a grain ; though the instrument is so delicate, and shell H with its weight of fourteen grains, will easily múst absolutely be so, that 1.20th of a grain is suffi- overcome the preponderance of LD over LC, as it cient to destroy the equilibrium.
amounts only to ten grains, and therefore nothing exists The two glass pillars M and N, together with the that can impede the discharging. steel forks affixed to them, are so fitted into the stand, When the ring s, according to the required power, that both the beams lie parallel to each other as well as is removed so far towards D, that the shell H is not to the rod GE. In this position of the beams AB, the able by its weight to destroy the preponderance of LD balls B and E are just in contact. The smallest glass in regard to LC, the active power of the shell H must pillar N is of such a height that the ball of the beam be so far increased by the addition of weights, that it CD stands at the distance of exactly four lines from the can act with a preponderance of four grains on the ring G, and cannot move without touching the latter. plate C. If, for example, an electric power of 14 The small shell H is suspended in such a manner that grains be required, the ring s must be removed to 16, there is a distance of exactly two lines between it and by which LÒ rests upon a, with a preponderance of the shell C. In each of the brass rings GG is a small 16 grains in regard to LC. Now, to make H act on hole, that the instrument may be connected with the the plate C with a preponderance of four grains, it must two sides of an electric jar. I is a brass wire, with a be increased to 20 grains, that is, six grains weight hollow bit of ivory, a, destined to support the beam more must be added, as it weighs only 14 ; which six CD, which is necessarily preponderate at D, in order grains are again laid upon LB; and erefore the ring to prevent oscillation between the discharges to be ex- Q is shifted to 20, as the strength of the repulsive power amined by the instrument.
is pointed out by 14 grains. It may be readily comprehended that, when the If an electric power of 25 grains be required, the beamn AB has moved, A must pass over twice the space ring s must be removed to 27, and the weight of 17 tbat B does ; and that in the beam CD, the case is the grains be put into the shell H, in order to produce a same in regard to C and D. If AB be therefore con- preponderance of four grains in regard to s. These 17 nected with the external, and CD with the internal side grains are added to the required power of 25 grains, of a battery, but in such a manner that the instrument and the ring Q is pushed to 42, &c. In this manner is at a sufficient distance beyond the electric atmosphere; the repulsive power always acts before the attractive and if the battery be charged, the repulsive effect of power can. the electric power will oblige the ball B to separate It may be readily perceived that the faults and infrom the ball E; the shiell H must therefore naturally convenie nces common to all the electrometers bitherto sink down with double velocity, so that when the employed, and which have been already mentioned, ball B rises a line, the shell H must sink two: when it cannot take place bere ; because the discharging is perreaches this depth it will touch the shell C, and the lat- formed by immediate connection between the positive ter, by the power excited in it, will be obliged to sink, and negative electricity in the instrument itself, without by which D must naturally again ascend in a double any external means being employed. proportion to the sinking of C; so that when C has One of the most essential advantages of this instrufallen two lines, D must have ascended four, and D ment is, the certainty with which the same result may that moment touches the ring by which the two sides be expected when the experiment is repeated. From of the battery are connected with each other, and dis- the same degree of electric power, whatever be the charges the battery.
temperature of the atmosphere, it will always be necesBut as the attractive electric power between unlike sary to commence the separation of the two balls B and atmospheres, under like circumstances, is at least as E from each other, the quantity of coated glass and the strong as its repulsive power between like atmospheres, distance of the ring Q from the axis L being the it would thence follow, that the electric power, instead
same. of repelling the ball B from the ball E, would rather Another no less important advantage of this instru. attract D, and by its contact with G, promote the dis- ment is, that in an experiment where the same electric 4
Electrome- power, often repeated, is pecessary to ascertain the re. one-fourth or one-half of an inch in diameter, m
sult with accuracy ; such, for example, as the charging smooth, and gilded. It is balanced by a verti
cated by a graduated circle xoq, drawn on
la the same scale, with a like quantity of coated glass, must inclosing a dry silk thread. This stalk is gras
a exactly correspond with each other, but above all, that clamp of cleft deal, or any similar contrivance the certainty and accuracy by which experiments may is made to lie firm on the glass cover. When be made with it, and by these means be accurately de- is let down through the hole m, it stands so as
scribed, are advantages which will not be found united the ball a on the arm, when that ball is oppos * Phil in any of the electrometers hitherto iuvented.” * on the graduated circle. Magaz. We shall close this account of electrometers with In order to electrify the ball t, we are to
describing the construction and use of M. Coulomb's the insulating handle, fig. 14. which is a slend
electrometer, or, as he calls it, Electrical Balance. of sealing-wax or lac, holding a metal wire that Fig. 11. ABDC (fig. 11.) represents a glass cylinder, twelve a small polished metallic ball. This is to be
inches in diameter and the same in height, covered by a with some electrified body, such as the prime ce glass plate fitted to it by a projecting fillet on the under of a machine, the knob of a jar, &c. This el surface. This cover is pierced with two round holes ball is to be introduced cautiously into the hole one inch and three-fourths in diameter. One of them the ball t is to be touched with it. The ball a f is in the centre, and receives the lower end of the mediately repelled to a distance, twisting the sus glass tube f h, of twenty-four inches height, which is wire, till the force of twist exerted by the fixed in the liole with a cement made of sealing-wax, lances the mutual repulsion of the balls t and a.
or other electric substance. The top of this tube re- This is the process for examining the law of Fig. 12,
ceives the brass collar H, (fig. 12. N° 3.) bored truly action. When it is desired to examine the a cylindrical with a small shoulder, which rests on the different bodies in different states, another ap top of the tube. This collar is fastened with cement, is wanted. This is represented by the piece ce and receives the bollow cylinder o (fig. 12. N° 2.), 15.) consisting of a plug of sealing-wax A,
to which is joined the circular plate a b, divided on the tightly into the hole m, and pierced by the wi edge into 360 degrees. It is also pierced with a round hooked at c, to receive a wire to connect it occa bole G in the centre, which receives the cylindrical with an electrified body, and baving below a pin i (fig. 12. N° 1.) having a milled head 1, and fur- metal ball d. nished with an index i o, whose point is bent down so The instrument is fitted for observation in the as to mark the divisions on the circle a b. This pin ing manner: The milled button b is turned at turns stiflly in the hole G, and the cylinder p moves the twist index io is at the mark o of the twist steadily in the collar H. To the lower end of the cen. Then the whole is turned in the collar H, till t tre pin is fastened a little pincer, 9, formed like the a stand opposite to the mark o of the paper circle end of a port-crayon, and tightened by the ring 9, so and at the same time the ball t or d is touched as to hold fast the suspension wire, the lower end of observation is thus made. The ball t is first ele which is grasped by a similar pincer, .Po (fig. 13.) as just described, and thus a is repelled, and I tightened by the ring . The lower end pois cylio- twists the wire, settling, after a few oscillations, drical, and is of such a weight, ab to draw the wire a distance as is proportional to the repulsion. The perfectly straight, but without any risk of breaking it. index is now turned so as to force a nearer to t. It may be made equal to half of the weight that will repulsion thus produced is estimated by adding t just break it.
tion of the index to the angle at which the bal This pincer is enlarged at C, and pierced with a stopped. Giving the index another turn, we have a hole, wbich tightly receives the arm g C q of the elec- repulsion, which is estimated in a similar way, ar trometer. This arm is eight inches long; and consists we obtain as many measures as required.. of a dry silk thread, or a slender straw completely It is not necessary to make this iņstrument o dried, and dipped in melted lac or fine sealing-wax, large dimensions; one 14 inches high, and five i and held perpendicularly before a clear fire, till it be meter, of which the arm a g should occupy two come a slender cylinder of about one-tenth of an inch and a half, will be sufficiently large for most pui in diameter. This occupies six of the eight inches, The diameter of the glass cylinder must alwa from g toq: the remaining two inches consist of a fine double the length of the arm a g, that the posit thread of the lac or sealing-wax, as it draius off in this may not be disturbed by the action of the gl: forming the arm.. At a, is a ball of pith or fine cork, Dr Robison considered this electrometer as
we make $1=6,180
Electrome. the most valuable instruments that have been made, as ed by the different states of humidity of the air. In the Elee ter. it is not only extremely delicate, but gives absolute scale of Saussures hygrometer, the relation to the quan.
measures with the greatest accuracy. For all purposes tity of water which a cubic foot of air is capable of in which only repulsions were to be measured, he pre- holding in solution is distinctly marked; the relation ferred it to his own instrument described in ELECTRI. of this solution to the dissipation of electricity in Cou. CITY, N° 206.
lomb's experiments may hence be seen in the following He, however, suggested several improvements in it, table, the first column of which marks the degrees of which are deserving of attention,
Saussure's hygrometer, the second how many grains of
7,295 ent to have the suspension wire strained at both ends ;
Hence it follows, that the dissipation is very nearly
7,197 turning the lower button till the ball a point exactly at
; m will be= 2,764. If we o on the paper circle.
The instrument will be greatly improved, if, in place make 5 = ; m will be = 2,76; and if we of the apparatus with the ball t, we substitute the piece
; m will be = 3,61; or at a methe smallest glass tube that will admit of being varnish
The immediate object that M. Coulomb bad in view The outside of the tube must also be varnished, and in his experiments, was to ascertain the diminution of a brass ball d fixed at its lower end, and a slender wire repulsion. He found that this, in a given state of the surmounted by a ball, is to be inserted into the tube, air, was a certain proportion of the whole repulsion so as to touch the ball below. The position of the ball taken at the moment of diminution, which is double d will not be liable to alteration, when the hole m is the proportion of the density of the Auid; for the reonce stopped with the plug. In making delicate ex- pulsions by which we judge of the dissipation are reciperiments, the upper ball c must be touched with the procal, being exerted by every particle of fluid in the charger, represented at fig. 14. by which means the ball ball t of the electrometer, on every particle of fluid in d is electrified. Then drawing out C by means of the the ball a. The diminution of repulsion is therefore forceps, the ball d is left completely insulated. In ex- proportional to the density of the electric fluid in each amining the electricity of the atmosphere, to which ball; and, as during the whole dissipation, the densities purpose this instrument is well adapted, the wire must continue to have their original proportioi, and as the be allowed to remain in the tube.
diminution of repulsion is directly proportional to the It was by means of this incomparable instrument, diminution of the products of the densities, it is con that M. Coulomb made the valuable experiments, tó sequently directly proportional to the square of eiwhich we alluded in the article ELECTRICITY, when ther. If we put d for the density, the mutual repul. treating of the law of action of the electric fluid. By sion will be represented by do, and its momentary dimeans of this electrometer, he also made his experi- minution by the fluxion of do, or 2 d d=2 dxd. But ments on the dissipation of electricity into the air, and 2dxd: d=2 d: d. The diminution of the repulsion along imperfect conductors. He ascertained the law of observed by experiment will be to the whole repul. dissipation into the air from bodies in contact, and the sion, in double the proportion that the diminution of relation which this bore to the original repulsion, by density, or the dissipation of fluid will have to the first observing the gradual approach of the ball a tn- whole quantity of fluid at the moment of observation. wards t, in proportion as the electricity dissipated from Let us, for instance, suppose the observed diminution of both, and then slackening the twist index till the ball repulsion to be a'5; we may conclude, that the quantia resumed its original situation.
ty of fluid lost by dissipation is gs. M. Coulomb did The following was the general result of Mr Cou- not examine the proportion of the dissipations from bolomb's experiments.
dies of various sizes. But we know, that if two spheres That the momentary dissipation of moderate degrees communicate by a very long canal, their superficial of electricity is proportional to the degree of electricity densities, and the tendencies of fluid to escape from at the moment. He found that the dissipation is not them, are inversely as the diameters of the spheres. sensibly affected by the state of the barometer or ther. Now, in a body that has twice the diameter of another mometer; nor is there any sensible difference of bodies body, the surface of the former is quadruple of that of of different sizes or different substances, or even dif- the latter; and though the tendency of fluid to escape ferent figures, provided that the electricity is very from the former is only the half of its tendency to esweak.
cape from the latter, get the greater surface of the But he found that the dissipation was greatly affect- former may so far make up for its smaller density, that
Electrome- the dissipation of fluid from a large sphere may in fact particle is just sufficient to clear the coercive interval. Electron:e
be greater than that from a small one in the same given Some fluid will come over; and the repulsion of this,
acting now in the opposite direction, will prevent any
val, is equal in every part of the canal.
16. suspended by a silken thread AB; and let us de- Fig. 16.
and let the densities at the different points of the canal The method in which M. Coulomb examined the be denoted by AD, P d, &c. ordinates to some curve dissipation along imperfect conductors, by means of this DdB, cutting the axis in B, the point where the thread instrument, was, by completely insulating the ball t, AB begins to insulate completely. Let Pp be an element and then after observing the loss sustained by a body in of the axis ; draw the ordinate pf, a tangent to the contact with it from the air, sliding a metallic rod curve df F, the normal d E, and draw fe perpendicudown the insulating stalk, till the dissipation began to lar to P d. Suppose AC=r, AP=x, and P d=y.
Then we shall bave Pp=x, and de=-;. It was
Therefore 99 is =R, the coercive power of
(r+x) Water is found to adhere to the surface of all bodies
Pdx de from which it is prevented by adhesion from escaping
the thread, which is supposed to be constant,
Pp when the bodies are electrified, and is thus rendered is therefore equal to some constant line R. But P ? capable of receiving a greater degree of electric power.
(or fe):de=Pd: PE. The subnormal, PE, is
therefore a constant line. But as this is the property
Cor. 1.–The densities at different points of an in-
COR. 2.-The lengths of canal requisite for insulating
Cor. 3.-The length of canal requisite for insulation