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ELECTROSTATICS
, the name given to that See also:department of See also:electrical See also:science in which the phenomena of See also:electricity at See also:rest are considered
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Besides their See also:ordinary See also:condition all bodies are capable of being thrown into a See also:physical See also:state in which they are said to be electrified or charged with electricity
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When in this condition they become See also:sources of electric force, and the space See also:round them in which this force is manifested is called an "electric See also: If the See also:knuckle is approached to the electrified tray, a small spark will be seen, and afterwards the tray will be found to be discharged or unelectrified . If the electrified tray is touched with the sealing-wax or ebonite rod, it will not be discharged, but if touched with a metal See also:wire, the hand, or a See also:damp See also:thread, it is discharged at once . This shows that some bodies are conductors and others non-conductors or insulators of electricity, and that bodies can be electrified by See also:friction and impart their electric See also:charge to other bodies . A charged conductor supported on a non-conductor retains its charge . It is then said to be insulated . Experiment II.—Arrange two tea-trays, each on dry tumblers as before . Rub the sheet of ebonite with flannel, lay it See also:face downwards on one tray, touch that tray with the finger for a moment and lift up the ebonite sheet, rub it again, and lay it face downwards on the second tray and leave it there . Then take two suspended gilt pith balls and touch them (a) both against one tray; they will be found to repel each other; (b) touch one against one tray and the other against the other tray, and they will be found to attract each other . This proves the existence of two kinds of electricity, called See also:positive and negative . ' See See also:Lord See also:Kelvin, " See also:Report on Electrometers and Electrostatic Measurements," Brit . Assoc . Report for 1867, or Lord Kelvin's Reprint of Papers on Electrostatics and See also:Magnetism, p . 26o . _Lk 1- The first tea-tray is positively electrified, and the second negatively . If an insulated brass ball is touched against the first tray and then against the knob or See also:plate of the electroscope, the gold leaves will diverge . If the ball is discharged and touched against the other tray, and then afterwards against the previously charged electroscope, the leaves will collapse . This shows that the two electricities neutralize each other's effect when imparted equally to the same conductor . Experiment III.—Let one tray be insulated as before, and the electrified sheet of ebonite held over it, but not allowed to touch the tray . If the ebonite is withdrawn without touching the tray, the latter will be found to be unelectrified . If whilst holding the ebonite sheet over the tray the latter is also touched with an insulated brass ball, then this ball when removed and tested with the electroscope will be found to be negatively electrified . The sign of the electrification imparted to the electroscope when so charged—that is, whether positive or negative—can be determined by rubbing the sealing-wax rod with flannel and the glass rod with silk, and approaching them gently to the electroscope one at a time . The sealing-wax so treated is electrified negatively or resinously, and the glass with positive or vitreous electricity . Hence if the electrified sealing-wax rod makes the leaves collapse, the electroscopic charge is positive, but if the glass rod does the same, the electroscopic charge is negative . Again, if, whilst holding the electrified ebonite over the tray, we touch the latter for a moment and then withdraw the ebonite sheet, the tray will be found to be positively electrified .
The electrified ebonite is said to See also:act by " electrostatic See also:induction " on the tray, and creates on it two induced charges, one of positive and the other of negative electricity
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The last goes to See also:earth when the tray is touched, and the first remains when the tray is insulated and the ebonite withdrawn
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Experiment I V.—Place a tin canister on a warm See also:tumbler and connect it by a wire with the gold-leaf electroscope
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Charge positively a brass ball held on an ebonite See also:stem, and introduce it, without touching, into the canister
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The leaves of the electroscope will diverge with positive electricity
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Withdraw the ball and the leaves will collapse
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Replace the ball again and touch the outside of the canister; the leaves will collapse
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If then the ball be withdrawn, the leaves will diverge a second time with negative electrification
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If, before withdrawing the ball, after touching the outside of the canister for a moment the ball is touched against the inside of the canister, then on withdrawing it the ball and canister are found to be discharged
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This experiment proves that when a charged See also:body acts by induction on an insulated conductor it causes an electrical separation to take place; electricity of opposite sign is See also:drawn to the side nearest the inducing body, and that of like sign is repelled to the remote side, and these quantities are equal in amount
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Seat of the Electric Charge.—So far we have spoken of electric charge as if it resided on the conductors which are electrified
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The See also:work of See also:Benjamin See also:Franklin, See also: Clerk See also:Maxwell demonstrated, however, that all electric charge or electrification of conductors consists simply in the See also:establishment of a physical state in the surrounding insulator or See also:dielectric, which state is variously called electric See also:strain, electric displacement or electric polarization . Under the See also:action of the same or identical electric forces the intensity of this state in various insulators is determined by a quality of them called their dielectric See also:constant, specific inductive capacity or inductivity . In the next place we must See also:notice that electrification is a measurable magnitude and in electrostatics is estimated in terms of a unit called the electrostatic unit of electric quantity . In the See also:absolute C.G.S. See also:system this unit quantity is defined as follows:--If we consider a very small electrified spherical conductor, experiment shows that it exerts a repulsive force upon another similar and similarly electrified body . Cavendish and C . A . See also:Coulomb proved that this mechanical force varies inversely as the square of the distance between the centres of the See also:spheres . The unit of mechanical force in the " centimetre, gramme, second " (C.G.S.) system of See also:units is the dyne, which is approximately equal to r/981 See also:part of the See also:weight of one gramme . Avery small See also:sphere is said then to possess a charge of one electrostatic unit of quantity, when it repels another similar and similarly electrified body with a force of one dyne, the centres being at a distance of one centimetre, provided that the spheres are in vacuo or immersed in some insulator, the dielectric constant of which is taken as unity . If the two small conducting spheres are placed with centres at a distance d centimetres, and immersed in an insulator of dielectric constant K, and carry charges of Q and Q' electrostatic units respectively, measured as above described, then the mechanical force between them is equal to QQ'/Kd2 dynes . For constant charges and distances the mechanical force is inversely as the dielectric constant . Electric Force.—If a small conducting body is charged with Q electrostatic units of electricity, and placed in any electric field at a point where the electric force has a value E, it will be subject to a mechanical force equal to QE dynes, tending to move it in the direction of the resultant electric force . This provides us with a See also:definition of a unit of electric force, for it is the strength of an electric field at that point where a small conductor carrying a unit charge is acted upon by unit mechanical force, assuming the dielectric constant of the surrounding See also:medium to be unity . To avoid unnecessary complications we shall assume this latter condition in all the following discussion, which is See also:equivalent simply to assuming that all our electrical measurements are made in See also:air or in vacuo . Owing to the confusion introduced by the employment of the See also:term force, Maxwell and other writers sometimes use the words electromotive intensity instead of electric force . The reader should, however, notice that what is generally called electric force is the analogue in electricity of the so-called See also:acceleration of gravity in See also:mechanics, whilst electrification or quantity, of electricity is analogous to See also:mass . If a mass of M grammes be placed in the earth's field at a place where the acceleration of gravity has a value g centimetres per second, then the mechanical force acting on it and pulling it downwards is Mg dynes . In the same manner, if an electrified body carries a positive charge Q electrostatic units and is placed in an electric field at a place where the electric force or electromotive intensity has a value E units, it is urged in the direction of the electric force with a mechanical force equal to QE dynes . We must, however, assume that the charge Q is so small that it does not sensibly disturb the See also:original electric field, and that the dielectric constant of the insulator is unity . Faraday introduced the important and useful conception of lines and tubes of electric force . If we consider a very small conductor charged with a unit of positive electricity to be placed in an electric field, it will move or tend to move under the action of the electric force in a certain direction . The path described by it when removed from the action of gravity and all other physical forces is called a See also:line of electric force . We may other-See also:wise define it by saying that a line of electric force is a line so drawn in a field of electric force that its direction coincides at every point with the resultant electric force at that point . Let any line drawn in an electric field be divided up into small elements of length . We can take the sum of all the products of the length of each See also:element by the resolved part of the electric force in its direction . This sum, or integral, is called the "line integral of electric force " or the electromotive force (E.M.F.) along this line . In some cases the value of this electromotive force between two points or conductors is See also:independent of the precise path selected, and it is then called the potential difference (P.D.) of the two points or conductors . We may define the term potential difference otherwise by saying that it is the work done in carrying a small conductor charged with one unit of electricity from one point to the other in a direction opposite to that in which it would move under the electric forces if See also:left to itself . Electric Potential.—Suppose then that we have a conductor charged with electricity;we may imagine its See also:surface to be divided up into small unequal areas, each of which carries a unit charge of electricity . If we consider lines of electric force to be drawn from the boundaries of these areas, they will cut up the space round the conductor into tubular surfaces called tubes of electric force, and each tube will See also:spring from an See also:area of the conductor carrying a unit electric charge . Hence the charge on the conductor can be measured by the number of unit electric tubes springing from it . In the next place we may consider the charged body to be surrounded by a number of closed surfaces, such that the potential difference between any point on one surface and the earth is the same . These surfaces are called "equipotential" or "level surfaces," and we may so locate them that the potential difference between two adjacent surfaces is one unit of potential; that is, it requires one absolute unit of work (I erg) to move a small body charged with one unit of electricity from one surface to the next . These enclosing surfaces, therefore, cut up the space into shells of potential, and See also:divide up the tubes of force into electric cells . The surface of a charged conductor is an equipotential surface, because when the electric charge is in See also:equilibrium there is no tendency for electricity to move from one part to the other . We arbitrarily See also:call the potential of the earth zero, since all potential difference is relative and there is no absolute potential any more than absolute level .
We call the difference of potential between a charged conductor and the earth the potential of the conductor
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Hence when a body is charged positively its potential is raised above that of the earth, and when negatively it is lowered beneath that of the earth
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Potential in a certain sense is to electricity as difference of level is to liquids or difference of temperature to See also:heat
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It must be noted, how-ever, that potential is a See also:mere mathematical concept, and has no See also:objective existence like difference of level, nor is it capable per se of producing physical changes in bodies, such as those which are brought about by rise of temperature, apart from any question of difference of temperature
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There is, however, this similarity between them
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Electricity tends to flow from places of high to places of See also:low potential, water to flow down See also: It follows from this that the electric force at the surface of the conductor has no component along the surface, in other words, the electric force at the bounding surface of the conductor and insulator is everywhere at right angles to it . By the surface See also:density of electrification on a conductor is meant the charge per unit of area, or the number of tubes of electric force which spring from unit area of its surface . Coulomb proved experimentally that the electric force just outside a conductor at any point is proportional to the electric density at that point . It can be shown that the resultant electric force normal to the surface at a point just outside a conductor is ' See Maxwell, Elementary See also:Treatise on Electricity (See also:Oxford, 1881), - . 47.equal to 47ru, where a is the surface density at that point . This is usually called Coulomb's See also:Law.2 (ii) Seat of Charge.—The charge on an electrified conductor is wholly on the surface, and there is no electric force in the interior of a closed electrified conducting surface which does not contain any other electrified bodies . Faraday proved this experimentally (see Experimental Researches, See also:series xi . § 1173) by constructing a large chamber or See also:box of'paper covered with tinfoil or thin metal . This was insulated and highly electrified . In the interior no trace of electric charge could be found when tested by electroscopes or other means . Cavendish proved it by enclosing a metal sphere in two hemispheres of thin metal held on insulating supports . If the sphere is charged and then the jacketing hemispheres fitted on it and removed, the sphere is found to be perfectly discharged.3 Numerous other demonstrations of this fact were given by Faraday . The thinnest possible spherical See also:shell of metal, such as a sphere of insulator coated with gold-leaf, behaves as a conductor for static charge just as if it were a sphere of solid metal . The fact that there is no electric force in the interior of such a closed electrified shell is one of the most certainly ascertained facts in the science of electrostatics, and it enables us to demonstrate at once that particles of electricity attract and repel each other with a force which is inversely as the square of their distance . We may give in the first place an elementary See also:proof of the See also:con-See also:verse proposition by the aid of a simple lemma: Lemma.—If particles of See also:matter attract one another according to the law of the inverse'square the attraction of all sections of a See also:cone for a particle at the vertex is the same . Definition.—The solid See also:angle subtended by any surface at a point is measured by the quotient of its apparent surface by the square of its distance from that point . Hence the See also:total solid angle round any point is 41r . The solid angles subtended by all normal sections of a cone at the vertex are therefore equal, and since the attractions of these sections on a particle at the vertex, are proportional to their distances from the vertex, they are numerically equal to one another and to the solid angle of the cone . Let us then suppose a spherical shell 0 to be electrified . Select any point P in the interior and let a line drawn through it sweep out a small See also:double cone (see fig . 1) . Each cone cuts out an area on the surface equally inclined to the cone See also:axis . The electric density on the sphere being See also:uniform, the quantities of electricity on these areas are proportional to the areas, and if the electric force varies inversely as the square of the distance, the forces exerted by these two surface charges at the point in question are proportional to the solid angle of the little cone . Hence the forces due to the two areas at opposite ends of the chord are equal and opposed . Hence we see that if the whole surface of the sphere is divided into pairs of elements by cones described through any interior point, the resultant force at that point must consist of the sum of pairs of equal and opposite forces, and is therefore zero . For the proof of the converse proposition we must refer the reader to the Electrical Researches of the Hon . Henry Cavendish, p . 419, or to Maxwell's Treatise on Electricity and Magnetism, and ed., vol. i. p . 76, where Maxwell gives an elegant proof that if the force in the interior of a closed conductor is zero, the law of the force must be that of the inverse square of the distance.4 From this fact it follows that we can See also:shield any conductor entirely from See also:external See also:influence by other _charged conductors by enclosing it in a metal case . It is not even necessary that 2 See Maxwell, Treatise on Electricity and Magnetism (3rd ed., Oxford, 1892), vol. i. p . 80 . Maxwell, Ibid. vol. i . § 74a; also Electrical Researches of the Hon . Henry Cavendish, edited by J . Clerk Maxwell (See also:Cambridge, 1879), p . 104 . 4 See also:Laplace (Mee . Cel. vol. i. ch. ii.) gave the first See also:direct demonstration that no See also:function of the distance except the inverse square can satisfy the condition that a uniform spherical shell exerts no force on a particle within it . this envelope should be of solid metal; a cage made of See also:fine metal wire See also:gauze which permits See also:objects in its interior to be seen will yet be a perfect electrical See also:screen for them . Electroscopes and electrometers, therefore, See also:standing in proximity to electrified bodies can be perfectly shielded from influence by enclosing them in cylinders of metal gauze . Even if a charged and insulated conductor, such as an open canister or deep See also:cup, is not perfectly closed, it will be found that a proof-See also:plane consisting of a small disk of gilt paper carried at the end of a rod of See also:gum-See also:lac will not bring away any charge if applied to the deep inside portions . In fact it is curious to See also:note how large an opening may be made in a See also:vessel which yet remains for all electrical purposes " a closed conductor." Maxwell (Elementary Treatise, &c., p . 15) ingeniously applied this fact to the insulation of conductors . If we See also:desire to insulate a metal ball to make it hold a charge of electricity, it is usual to do so by attaching it to a handle or stem of glass or ebonite . In this case the electric charge exists at the point where the stem is attached, and there leakage by creeping takes place . If, however, we employ a hollow sphere and let the stem pass through a hole in the side larger than itself, and attach the end to the interior of the sphere, then leakage cannot take place . Another corollary of the fact that there is no electric force in the interior of a charged conductor is that the potential in the interior is constant and equal to that at the surface . For by the definition of potential it follows that the electric force in any direction at any point is measured by the space See also:rate of See also:change of potential in that direction or E = ± dV/dx . Hence if the force is zero the potential V must be constant . (iii.) Association of Positive and Negative Electricities.—The third leading fact in electrostatics is that positive and negative electricity are always created in equal quantities, and that for every charge, say, of positive electricity on one conductor there must exist on some other bodies an equal total charge of negative electricity . Faraday expressed this fact by saying that no absolute electric charge could be given to matter . If we consider the charge of a conductor to be measured by the number of tubes of electric force which proceed from it, then, since each tube must end on some other conductor, the above statement is equivalent to saying that the charges at each end of a tube of electric force are equal . The facts may, however, best be understood and demonstrated by considering an experiment due to Faraday, commonly called the See also:ice See also:pail experiment, because he employed for it a See also:pewter ice pail (Exp . Res. vol. ii. p . 279, or Phil . 'Wag . 1843, 22) . On the plate of a gold-leaf electroscope place a metal canister having a loose lid . Let a metal ball . be suspended by a silk thread, and the canister lid so fixed to the thread that when the lid isin place the ball hangs in the centre of the canister . Let the ball and lid be removed by the silk, and let a charge, say, of positive electricity (+ Q) be given to the ball . Let the canister be touched with the finger to See also:discharge it perfectly . Then let the ball be lowered into the canister . It will be found that as it does so the gold-leaves of the electroscope diverge, but collapse again if the ball is withdrawn . If the ball is lowered until the lid is in place, the leaves take a steady deflection . Next let the canister be touched with the finger, the leaves collapse, but diverge again when the ball is withdrawn . A test will show that in this last case the canister is left negatively electrified . If before the ball is withdrawn, after touching the outside of the canister with the finger, the ball is tilted over to make it touch the inside of the canister, then on withdrawing it the canister and ball are found to be perfectly discharged . The explanation is as follows: the charge (+ Q) of positive electricity on the ball creates by induction an equal charge (-Q) on the inside of the canister when placed in it, and repels to the exterior surface of the canister an equal charge (+ Q) . On touching the canister this last charge goes to earth . Hence when the ball is touched against the inside of the canister before withdrawing it a second time, the fact that the system is found subsequently to be completely discharged proves that the charge-Q induced on the inside of the canister must be exactly equal to the charge-FQ on the ball, and also that the inducing action of the charge +Q on the ball created equal quantities of electricity of opposite sign, one drawn to the inside and the other repelled to the outside of the canister . Electrical Capacity.—We must next consider the quality of a conductor called. its electrical capacity . The potential of a conductor has already been defined as the mechanical work which must be done to bring up a very small body charged with a unit of positive electricity from the earth's surface or other boundary taken as the place of zero potential to the surface of this conductor in question . The mathematical expression for this potential can in some cases be calculated or predetermined . Thus, consider a sphere uniformly charged with Q units of positive electricity . It is a fundamental theorem in attractions that a thin spherical shell of matter which attracts according to the law of the inverse square acts on all external points as if it were concentrated at its centre . Hence a sphere having a charge Q repels a unit charge placed at a distance x from its centre with a force Q/x2 dynes, and therefore the work W in ergs expended in bringing the unit up to that point from an See also:infinite distance is given by the integral of the wire of width dx; the charge on it is equal to ofa 2lrra/dx units, and the potential V at a point on the axis then rod . at a distance x from the annulus due to this elementary charge is l/2 2a-See also:ea V =2 (See also:r2+x2) dx =4rrra See also:log, (11+./ r2+412) -loge* 0 If, then, r is small compared with 1, we have V =4arra loge l/r . But the charge is Q= area, and therefore the capacity of the thin wire is given by C=I/2 log, l/r (2) . A more difficult case is presented by the See also:ellipsoid.' We have first to determine the mode in which electricity distributes itself on a conducting ellipsoid in free space . It must be such a potential See also:distribution that the potential in the interior will be ofan constant, since the electric force must be zero . It is a ellipsoid. well-known theorem in attractions that if a shell is made of gravitative matter whose inner and See also:outer surfaces are similar ellipsoids, it exercises no attraction on a particle of matter in its interior' Consider then an ellipsoidal shell the axes of whose bounding surfaces are (a, b, c) and (a+da), (b+db), (c+dc), where da(a=db b=do/c=µ . The potential of such a shell at any See also:internal point is constant, and the equi-potential surfaces for external space are ellipsoids confocal with the ellipsoidal shell . Hence if we distribute electricity over an ellipsoid, so that its density is everywhere proportional to the thickness of a shell formed by describing round ' The See also:solution of the problem of determining the distribution on an ellipsoid of a fluid the particles of which repel each other with a force inversely as the nth See also:power of the distance was first given by See also:George See also:Green (see Ferrer's edition of Green's Collected Papers, p . 119, 1871) . 2 See See also:Thomson and See also:Tait, Treatise on Natural See also:Philosophy, § 519 . Potential of a sphere . W=) Qx—'dx =Q/x (I) . Hence the potential at the surface of the sphere, and therefore the potential of the sphere, is Q/R, where R is the See also:radius of the sphere in centimetres . The quantity of electricity which must be given to the sphere to raise it to unit potential is therefore R electrostatic units . The capacity of a conductor is defined to be the charge/ required to raise its potential to unity, all other charged conductors) being at an infinite distance . This capacity is then a function of the geometrical dimensions of the conductor, and can be mathematically determined in certain cases . Since the potential of a small charge of electricity dQ at a distance r is equal to dQ/r, and since the potential of all parts of a conductor is the same in those cases in which the distribution of surface density of electrification is uniform or symmetrical with respect to some point or axis in the conductor, we can calculate the potential by simply summing up terms like crdS/r, where dS is an element of surface, a the surface density of electricity on it, and r the distance from the symmetrical centre . The capacity is then obtained as the quotient of the whole charge by this potential . Thus the distribution of electricity on a sphere in free space must be uniform, and all parts of the charge are at an equal distance R from the centre . Accordingly the potential at the centre is Q/R . But this must be the potential of the Capacity sphere, since all parts are at the same potential V . Since ofa the capacity C is the ratio of charge to potential, the sphere. capacity of the sphere in free space is Q/V = R, or is numerically the same as its radius reckoned in centimetres . We can thus easily calculate the capacity of a See also:long thin wire like a See also:telegraph wire far removed from the earth, as follows: Let 2r be the See also:diameter of the wire, l its length, and a the uniform Capacity surface electric density . Then consider a thin annulus the ellipsoid a similar and slightly larger one, that distribution will be in equilibrium and will produce a constant potential through-out the interior . Thus if a is the surface density, S the thickness of the shell at any point, and p the assumed See also:volume density of the matter of the shell, we have a=See also:ASp . Then the quantity of electricity on any element of surface dS is A times the mass of the corresponding element of the shell; and if Q is the whole quantity of electricity on the ellipsoid, Q =A times the whole mass of the shell . This mass is equal to 4lrabcp,u ; therefore Q = A4rabcp,u and S =up, where p is the length of the perpendicular let fall from the centre of the ellipsoid on the tangent plane . Hence a=Qp/47rabc (3) . Accordingly for a given ellipsoid the surface density of free distribution of electricity on it is everywhere proportional to the let fall from Capacity tehngth t panerpeandicularthat point . Fromtthise we ecan of an determine the capacity of the ellipsoid as follows: Let ellipsoid. p be the length of the perpendicular from the centre of the ellipsoid, whose See also:equation is x2/aa2+y2/b2+z2/c2 = 1 to the tangent plane at x, y, z . Then it can be shown that I /p2 =x2/See also:a4+y2/b4+z2/c4 (see See also:Frost's Solid See also:Geometry, p . 172) . Hence the density a is given by Q I a=47rabc (x2/a4+y2/b4+22/c4) . and the potential at the centre of the ellipsoid, and therefore its potential as a whole is given by the expression, fadS_ Q dS V = J r -42rabc,f rs/ (x2/a°-I-y2/b"+z2/c4) (4)• Accordingly the capacity C of the ellipsoid is given by the equation I I dS (5)• C=4rabcJ (x2+y2+z2) (x2/a4+y2/b4+z2/c4) It has been shown by See also:Professor Chrystal that the above integral may also be presented in the form,' I 1 dX C=2J o 1/{(See also:a2+X)(b2+A)(c2--X)l (6) . The above expressions for the capacity of an ellipsoid of three unequal axes are in See also:general elliptic integrals, but they can be evaluated for the reduced cases when the ellipsoid is one of revolution, and hence in the limit either takes the form of a long rod or of a circular disk . Thus if the ellipsoid is one of revolution, and ds is an element of arc which sweeps out the element of surface dS, we have dS=2?ryds22rydx/ (ds) =22rydx/ (by) =2pb-dx . Hence, since a=Qp/4aab2,odS =Qdx/2a . Accordingly the distribution of electricity is such that equal parallel slices of the ellipsoid of revolution taken normal to the axis of revolution carry equal charges on their curved surface . The capacity C of the ellipsoid of revolution is therefore given by the expression 1 I dx (7) C=2a ~(x2+y2) If the ellipsoid is one of revolution round the See also:major axis a (prolate) and of eccentricity e, then the above See also:formula reduces to 1 1 (I+e) C1= 2ae loge 1– e Whereas if it is an ellipsoid of revolution round the See also:minor axis b (oblate), we have I See also:sin—'ae C2= ae (9) . In each case we have C =a when e=0, and the ellipsoid thus becomes a sphere . In the extreme case when e=1, the prolate ellipsoid becomes a long thin rod, and then the capacity is given by CI = a/log52a/b (io), which is identical with the formula (2) already obtained . In the other extreme case the oblate See also:spheroid becomes a circular disk when e =1, and then the capacity C2 =2a/r . This last result shows that the capacity of a thin disk is 2/,r=1/1.571 of that of a sphere of the same radius . Cavendish (Elec . Res. pp . 137 and 347) deter-See also:mined in 1773 experimentally that the capacity of a sphere was 1.541 times that of a disk of the same radius, a truly remarkable result for that date . Three other cases of See also:practical See also:interest See also:present themselves, viz. the I See See also:article " Electricity," See also:Encyclopaedia Britannica (9th edition), vol. viii. p . 30 . The reader is also referred to an article by Lord Kelvin (Reprint of Papers on Electrostatics and Magnetism, p . 178), entitled " Determination of the Distribution of Electricity on a Circular Segment of a Plane, or Spherical Conducting Surface under any given Influence," where another equivalent expression is given for the capacity of an ellipsoid.capacity of two concentric spheres, of two coaxial cylinders and of two parallel planes . Consider the case of two concentric spheres, a solid one enclosed in a hollow one . Let RI be the radius of the inner sphere, R2 the inside radius of the outer sphere, and R2 the outside Capacity radius of the outer spherical shell . Let a charge +Q be of two given to the inner sphere . Then this produces a charge concentrk —Q on the inside of the enclosing spherical shell, and a spheres. charge +Q on the outside of the shell . Hence the potential V at the centre of the inner sphere is given by V =Q/RI-Q/R2+Q/R3 . If the outer shell is connected to the earth, the charge +Q on it disappears, and we have the capacity the_ of the inner sphere given by C=1/Ri—1/R2=(R2—RI) RiR2 (II) . Such a pair of concentric spheres constitute a See also:condenser (see See also:LEYDEN See also:JAR), and it is obvious that by making R2 nearly equal to RI, we may enormously increase the capacity of the inner sphere . Hence the name condenser . The other case of importance is that of two coaxial cylinders . Let a solid circular sectioned See also:cylinder of radius RI be enclosed in a coaxial tube of inner radius R2 . Then when the inner C;Pt acity cylinder is at potential V, and the outer one kept at o potential V2 the lines of electric force between the cylinders coaxial are radial . Hence the electric force E in the interspace cylinders. varies inversely as the distance from the axis . Accordingly cy the potential V at any point in the interspace is given by E=–dV/dR=A/R or V=–A fR–'dR, (12), where R is the distance of the point in the interspace from the axis, and A is a constant . Hence V2—VI=—A log R2/Rl . If we consider a length 1 of the cylinder, the charge Q on the inner cylinder is Q=2lrRlla, where a is the surface density, and by Coulomb's law a=E,/4,r, where E,=A/RI is the force at the surface of the inner cylinder . Accordingly Q=22rRIlA/4lrRI=Al/2 . If then the outer cylinder be at zero potential the potential V of the inner one is V =A log (R2/R1), and its capacity C =1/2 log R2/RI . This formula is important in connexion with the capacity of electric cables, which consist of a cylindrical conductor (a wire) enclosed in a conducting sheath . If the dielectric or separating insulator has a constant K, then the capacity becomes K times as See also:great .
The capacity of two parallel planes can be calculated at once if we neglect the distribution of the lines of force near the edges of the plates, and assume that the only field is the uniform field Capacity between the plates
.
Let VI and V2 be the potentials of
the plates, and let a charge Q be given to one of them. of two If S is the surface of each plate, and d their distance, then parallel the electric force E in the space between them is E = Planes
.
(V1–V2)/d
.
But if a is the surface density, E=4zra, and a=Q/S
.
Hence we have
(VI—V2) d=47fQ/S or C=Q/(Vi—V2)=S/4rd (13)• In this calculation we neglect altogether the fact that electric force distributed on curved lines exists outside the interspace between the
plates, and these lines in fact extend from the back of one „Ed
plate to that of the other
.
G
.
R
.
See also:Kirchhoff (Gesammelte effect.”
Abhandl. p
.
112) has given a full expression for the capacity e
C of two circular plates of thickness t and radius r placed at any distance d apart in air from which the edge effect can be calculated
.
Kirchhoff's expression is as follows:
C=42nd+ -; r dlog, 16,re~d+t)+tlogd C tt (14)•
In the above formula e is the See also:base of the Napierian logarithms
.
The first term on the right-hand side of the equation is the expression for the capacity, neglecting the curved edge distribution of electric force, and the other terms take into See also:account, not only the uniform field between the plates, but also the non-uniform field round the edges and beyond the plates
.
In practice we can avoid the difficulty due to irregular distribution of electric force at the edges of the plate by the use of a guard plate
as first suggested by Lord Kelvin .2 If a large plate has a See also: The outer larger plate in which the hole is cut is called the " guard plate," and must be kept at the same potential as the smaller inner or " See also:trap-See also:door plate." The same arrangement can be supplied to a pair of coaxial cylinders . By placing metal plates on either side of a larger sheet of dielectric or insulator we can construct a condenser of relatively large capacity . The See also:instrument known as a Leyden jar (q.v.) consists of a glass See also:bottle coated within and without for three parts of the way up with tinfoil . 2 See Maxwell, Electricity and Magnetism, vol. i. pp . 284-305 (3rd ed., 1892) . (8) . If we have a number of such condensers we can combine them in " parallel " or in " series." If all the plates on one side are connected Systems together and also those on the other, the condensers are of con joined in parallel . If CI, C2, C3, &c., are the See also:separate denser-s. capacities, then I(C)=CI+C2+C3+ &c., is the total capacity in parallel . If the condensers are so joined that the inner coating of one is connected to the outer coating of the next, they are said to be in series . Since then they are all charged with the same quantity of electricity, and the total over all potential difference V is the sum of each of the individual potential See also:differences VI, V2, V3, &c., we have Q=CIVI=C2V2=C3V3=&c., and V = Vi +V2 +V2+ &c . The resultant capacity is C=Q/V, and C = 1/(1/CI+I/C2+I/C3+&c) = 1/1(1/C) (15) . These rules provide means for calculating the resultant capacity when any number of condensers are joined up in any way . If one condenser is charged, and then joined in parallel with another uncharged condenser, the charge is divided between them in the ratio of their capacities . For if CI and C2 are the capacities and Qi and Q2 are the charges after contact, then QI/CI and Q2./C2 are the potential differences of the coatings and must be equal . Hence QI/CI=Q2/C2 or QI/Q2=Cl/C2 . It is See also:worth noting that if we have a charged sphere we can perfectly discharge it by introducing it into the interior of another hollow insulated conductor and making contact . The small sphere then becomes part of the interior of the other and loses all charge . Measurement of Capacity.—Numerous methods have been devised for the measurement of the electrical capacity of conductors in those cases in which it cannot be determined by calculation . Such a measurement may be an absolute determination or a relative one . The dimensions of a capacity in electrostatic measure is a length (see UNITS, PHYSICAL) . Thus the capacity of a sphere in electrostatic units (E.S.U.) is the same as the number denoting its radius in centimetres . The unit of electrostatic capacity is therefore that of a sphere of 1 cm. radius.' This unit is too small for practical purposes, and hence a unit of capacity 900,000 greater, called a microfarad, is generally employed . Thus for instance the capacity in free space of a sphere 2 metres in diameter would be 100/900,000= 1/9000 of a microfarad . The electrical capacity of the whole earth considered as a sphere is about 800 microfarads .
An absolute measurement of capacity means, therefore, a determination in E.S. units made directly without reference to any other condenser
.
On the other hand there are numerous methods by which the capacities of condensers may be compared and a relative measurement made in terms of some See also:standard
.
One well-known comparison method is that of C
.
V. de Sauty
.
The two condensers to be compared are connected in the branches
Relative of a See also:Wheatstone's See also:Bridge (q.v.) and the other two arms
deter- completed with variable resistance boxes
.
These arms minations. are then altered until on raising or depressing the See also:battery
See also: These are proportional to the capacities . For the various precautions necessary in conducting the above tests special See also:treatises on electrical testing must be consulted . It is an interesting fact that Cavendish measured capacity in " globular inches," using as his unit the capacity of a metal ball, 1 in. in diameter . Hence multiplication of his values for capacities by 2.54 reduces them to E.S. units in the C.G.S. system . See Elec . Res. p . 347 . 2 For See also:fuller details of these methods of comparison of capacities see J . A . See also:Fleming, A Handbook for the Electrical Laboratory and Testing See also:Room, vol. ii. ch. ii . (See also:London, 1903) . In the absolute determination of capacity we have to measure the ratio of the charge of a condenser to its plate potential difference . 
One of the best methods for doing this is to charge the Absoiate condenser by the known voltage of a battery, and then deter. discharge it through a galvanometer and repeat this minations. See also:process rapidly and successively . If a condenser of capacity C is charged to potential V, and discharged n times per second through a galvanometer, this series of intermittent discharges is equivalent to a current nCV . Hence if the galvanometer is calibrated by a See also:potentiometer (q.v.) we can determine the value of this current in amperes, and knowing the value of n and V thus determine C . Various forms of commutator have been devised for effecting this charge and discharge rapidly by J . J . Thomson, R . T . Glazebrook, J . A . Fleming and W . C . See also:Clinton and others.' One form consists of a tuning-See also:fork electrically maintained in vibration of known See also:period, which closes an elec |