STATIC ELECTRICITY The fact that amber, jet and perhaps a few other bodies have the power, after being rubbed, of attracting light objects, such as bits of straw or feathers, is said to have been known to Thales of Miletus (600 B.c. ), and was mentioned by Theophrastus (321 B.c.) and by Pliny (A.D. 7o). These attractions were studied by William Gilbert (1544-1603 ), queen Elizabeth's physician, who found that many substances possess the power in question, and he called such attractions electric after riXEKTpOV the Greek word meaning amber. As he wrote in Latin, the actual phrase which he used for the attraction was vis electrica; the word electricity was first used by Walter Charleton in his Ternary of Paradoxes, 165o. Bodies having this power of attracting light objects are said to be electrified or to be charged with electricity. A striking way of illustrating such attractions is to place a very light cellu loid ball on the top of a smooth table. If a piece of hard rubber (ebonite) rod which has been rubbed on woollen cloth is brought near the ball, the latter will roll towards the rod. Instead of a ball a circular cylinder of smooth paper may be used. Glass, rubbed with silk or sheet rubber, will also attract light objects, and many other pairs of bodies have the same property.
In 1729 Stephen Gray discovered that the attracting power may be transferred by contact from one body to another and is transmitted from one part of some bodies to all other parts. Such bodies through which the power is freely transmitted were called conductors by Desaguliers in 1736. Bodies through which the power is not transmitted are called insulators. Amber, sealing wax, hard rubber, paraffin wax, silk and dry glass are good insula tors. All metals are good conductors. A conducting body sup ported on an insulating stand, e.g., a metal can supported on a hard rubber rod, can be given the power of attracting light ob jects by touching it with a hard rubber rod which has been rubbed on woollen cloth. The conductor is then said to be electrified or to have been given a charge of electricity. If the charged con ductor is allowed to touch another insulated but uncharged con ductor the second conductor also becomes electrified. The charge is shared by the two conductors. If the charged conductor is connected to the earth through any conductor the charge im mediately disappears. For example if the conductor is touched for a moment with the finger the charge disappears since the human body is a fairly good conductor.
About 1733 du Fay, superintendent of gardens to the king of France, discovered that there are two kinds of electricity and that unlike kinds attract each other but like kinds repel each other. If two bodies are electrified by contact with the same electrified body then they repel each other. For example if a light pith ball coated with aluminium or gold leaf is suspended by an insulating silk thread and touched with a glass rod, which has been electrified by rubbing with silk, it will then be repelled by the glass rod. If two such pith balls are suspended by silk threads from the same point, so that they hang touching each other, then if they are touched by an electrified body they will repel each other and so hang at some distance apart.
If a rod of hard rubber, sealing wax or bakelite is rubbed with woollen cloth and then brought near the pith ball which was electrified by means of the glass rod it is found to attract the ball. Thus the ball is attracted by the electrified hard rubber but repelled by the electrified glass. In the same way if the ball is electrified by touching the rubber rod it will be attracted by the glass and repelled by the rubber. The electricity on glass rubbed with silk was called vitreous and that on resin rubbed with wool or fur resinous electricity by du Fay. Vitreous electricity is now usually called positive electricity and resinous negative. It is found that two different solid substances, if mounted on insulating handles and then rubbed together or merely allowed to touch, both become electrified, one with vitreous and the other with resinous electricity, so that they attract each other. Suppose we have two such pairs of the same substances A and B. Then it is found that after rubbing together the A's attract the B's but the A's repel each other as do the B's. Two positively electrified bodies or two negatively electrified bodies repel each other but a positively electrified body and a negatively electrified body attract each other. If insulating handles are not used con ductors immediately lose their electricity and so do not appear to be electrified by rubbing with other bodies. All pairs of dif ferent bodies are not equally electrified by contact; in many cases the effect is very slight.
Frictional electrical machines, by means of which more power ful electrical effects could be obtained, were invented about 1700. The first attempt was made by Guericke, who, in his book De Vacuo Spatio, published in 1672, describes experiments carried out with a large sphere of sulphur, which was mounted on an iron shaft and rubbed with the hand. He demonstrated the elec trical attraction, and subsequent repulsion, of light bodies with it, but did little more. A much more finished and efficient frictional machine, consisting of a glass globe which could be set in rapid rotation, was constructed by Hauksbee and described by him in 1709. This was the first machine with which electric sparks were obtained. The operator turned the handle with one hand and held the other hand against the revolving sphere. Improved ma chines on these lines were soon made, in which electricity could be collected, from the side of the sphere remote from the hand, by means of a chain or similar device connected to an insulated conductor. In this way insulated conductors could be strongly charged, and it was seen that, when a conductor connected to the earth, e.g., a man's finger, is brought near enough to a charged conductor, a spark passes between the two conductors accom panied by a sharp crackle and the emission of light. The spark appears to be a bright narrow streak lasting for only a fraction of a second.
In 1745 an important discovery was made independently by von Kleist at Kummin, and by Musschenbrock at Leyden. Mus schenbrock was trying to charge water contained in a glass bottle. A wire attached to an insulated conductor kept charged by an electrical machine was allowed to dip into the water. A friend, Cunaeus, held the bottle in one hand and then touched the charged conductor with the other whereat he received a violent electric shock which he felt in his arms and chest. It was soon found that a dry bottle with the lower part of the inside and outside surfaces coated with tin-foil is better than a bottle of water and such an apparatus was called a Leyden jar. William Watson who lived in London repeated this experiment and it suggested to him that when the jar is discharged something is transferred from the jar to the insulated conductor through the arms and chest of the experimenter. In a paper published in i 746 he suggested a theory of electrical actions which was a distinct advance on previous ideas. Watson supposed that all bodies contain electricity which is a kind of elastic fluid. Uncharged bodies contain the normal or equilibrium amount which produces no observable effects. The process of charging conductors, Watson supposed, consists in tak ing some of the electricity from one body and giving it to another, so that electricity is not generated or created but is merely trans ferred. Thus a vitreously electrified body might be a body with more than its normal amount of electricity, and a resinously electrified body one with less, or vice versa.
When two bodies are rubbed together and then separated the vitreous charge on one is just enough to neutralize the resinous charge on the other. This may be shown by means of a disk of hard rubber mounted on an insulated handle and an equal wooden disk covered with woollen cloth and also mounted on an insulating handle. If the two disks are rubbed together and then separated they are both found to be electrified, the rubber resinously and the wool vitreously, and either will attract a suspended pith ball. But if they are put one against the other in contact all signs of electri fication disappear, showing that the electricity on one disk is just enough to neutralize the effects due to that on the other one. Facts like this are easily explained by Watson and Franklin's theory which came to be known as the one fluid theory of elec tricity. According to this theory electricity is neither created nor destroyed so that the total amount of it remains constant. Frank lin studied the Leyden jar and showed how its action could be explained on his theory. He showed that, when a Leyden jar is charged, one coating receives a vitreous charge and the other an equal resinous charge, and, when the two coatings are con nected, the two charges just neutralize each other anti disappear. He explained this by supposing that the jar could be charged by taking the electric fluid from one coating and giving it to the other. Also, if the inner coating was vitreously electrified while the outer coating was not insulated, the electricity on the inner coating repelled that in the outer coating, so driving some of it away into the earth and leaving the outer coating resinously or negatively electrified. He supposed that the electric fluid cannot pass through insulators like glass, so that, although the excess of fluid on one coating was attracted by the matter with a defect of the fluid in the other, it could not flow through the glass. He observed that a positively and a negatively charged conductor attract each other when a sheet of glass is put between them. Since the fluid itself could not pass through glass while the action of attraction could, it was clear that the fluid itself did not extend into the space around a charged conductor where the attraction occurred. Frank lin therefore gave up the view, held by his predecessors, that the attractions and repulsions were due to the presence of electric effluvia in the space around charged bodies. He supposed the fluid confined to the body and that the forces it exerted on other charged bodies were actions at a distance. Franklin's views were soon adopted by other physicists and the one fluid action at a distance theory was generally adopted.
It is probable that very few physicists really supposed that this action at a distance meant action with no medium of communi cation in between. What was meant was that one body acted on another across the intervening space by means of some unknown process, and that, since the phenomena could be described ade quately in terms of the bodies themselves without reference to the unknown process, it was not necessary to discuss it. The posi tion was similar to that taken with regard to gravitational attrac tion. Newton had shown that the motion of the planets and their satellites could be explained by supposing that they attracted each other with forces proportional to their masses and inversely proportional to the squares of their distances from each other. This was a theory of the observed motions and the force between the bodies was assumed to exist and was not explained by the theory. In the same way in electricity it was found that electrical phenomena, for example the distribution of charges on conductors, could be explained by assuming that particles of electricity ex erted forces on each other across the intervening space. This was a theory of the electrical phenomena and the assumed forces were not explained any more than Newton's gravitational forces were explained. That lightning and the electric spark are identical had been suggested by several scientists before Franklin's time, but he showed that electricity could be drawn from the clouds during a thunderstorm and that it had all the properties of ordi nary electricity. This celebrated experiment was made in 1752. Franklin flew a kite during a thunderstorm, insulated the twine leading up to the kite with a silk ribbon, and attached a key to the end of the twine. When a thunderstorm passed over the kite he was able to get electric sparks from the key and to charge Leyden jars from it. Thus the identity of atmospheric electricity and ordinary electricity was established.
It had been observed by several physicists that when a charged conductor is brought near to an uncharged insulated conductor then the insulated conductor acquires charges which disappear when the charged conductor is removed. The parts of the insu lated conductor nearest to the charged conductor acquire a charge of the opposite sign to that on the charged conductor and the re moter parts a charge of the same sign. The fact that the charges disappear when the charged conductor is removed or discharged shows that the two charges are equal but of opposite sign. This effect was carefully studied by Canton (17 53) and by Wilcke and is called electrification by induction. The charges on the insulated conductor are called induced charges.
The effect may be illustrated by means of the apparatus shown in fig. 1. A is a metal sphere supported by a hard rubber rod, and BC is an elongated conductor sup ported in the same way. If the sphere A is charged positively by means of an electric machine and then brought near one end B of the conductor BC, it is found that BC is negatively charged near B but positively charged near C, as indicated in the figure by the plus and minus signs. This can be shown to be the case by means of a small metal plate P mounted on an insulating handle. If the plate is put in contact with the conductor near B and removed, it is found to be nega tively charged. It will attract a suspended pith ball which has been charged by contact with a rubbed glass rod. If the plate is put on the conductor near C, it acquires a positive charge and will repel the pith ball. About half way between B and C, it acquires no charge. If the conductor A is removed or discharged the induced charges on BC immediately disappear.
Aepinus showed how to explain electrification by induction on the one fluid action at a distance theory. The electric fluid on A repels that in BC so that, since the fluid can move freely in con ductors, the fluid in BC moves towards C leaving the end B nega tively charged. This goes on until the repulsion on the fluid inside BC due to the fluid on A is balanced by the attraction of the mat ter near B left with less than the normal amount of the fluid and the repulsion of the excess fluid around C. The originally uniform distribution of the fluid in the conductor BC is changed by the presence of the charged conductor A in such a way that every particle of the fluid in BC remains at rest in BC under the action of the repulsion of the charge on A and the attractions and repulsions of the charges induced on BC.
If a charged conductor is brought near to a conductor which is not insulated, a charge of opposite sign is induced on the in sulated conductor; e.g., if the conductor BC in fig. 1 is connected to the earth by touching it, some of the electric fluid in BC flows into the earth because of the repulsion of the charge at A and the conductor BC is left with only negative charge on it near B. When the conductor BC is connected to the earth, it and the earth form one large conductor, and the charge on A induces on BC a negative charge on the part nearest to A and a positive charge on the parts remote from A, i.e., on the earth. If the con nection of BC to the earth is broken before A is discharged or re moved, we have a negative charge on BC, and we can remove BC from the vicinity of A and use the charge for any purpose. In this way a conductor can be charged by induction with a charge of opposite sign to the inducing charge. The inducing charge is not used up in this process so that the conductor BC can be charged an indefinite number of times by induction by means of the orig inal charge on A. An example of this is afforded by the charging of the outer uninsulated coating of a Leyden jar when the inner coating is charged. A positive charge given to the inner coating induces a negative charge on the outer coating by repelling some of the fluid in it away into the earth.
That there is no force on a particle inside a uniform hollow spherical shell, the particles of which exert a force on a particle inside varying inversely as the square of the distance, may be shown easily, as follows : In fig. 2, the circle represents the hollow spherical shell. Let the particle inside it be at any point P. Take any very small area a at A on the surface of the shell, and let a line drawn from the boundary of this area to P be produced so as to cut the shell near B. Let this line move round the boundary of a so tracing out a double cone with vertices at P and cutting off a small area /3 on the surface of the :hell at B. Consider the forces on the particle at P due to the areas a and (3 of the shell. The areas a and 13 are equally inclined to the axis of the double cone, so that these areas are proportional to the squares of their distances from P. Hence ca/ (PA) 2 =/3/ (PB) But the force on the particle at P due to the area a of the shell will be proportional to and that due to the area B to so that we see that these two forces on the particle at P are equal but in oppo site directions, and so give no resultant force on the particle. The whole surface of the shell may be divided up into pairs of areas like a and (3, so that it follows that the force on the particle at P due to the whole shell is zero.
Cavendish discussed electrical phenomena very clearly and showed how they could be explained on the one fluid theory. It is now known that he also carried out a series of electrical re searches which he did not publish, in which he anticipated many important discoveries made later by other physicists. His labora tory notes were edited by Clerk Maxwell and published in 1879, more than a century after they were written. Cavendish's papers on gravitation, chemistry, electricity and other subjects estab lished his reputation as one of the greatest experimental and theoretical scientists of all time, and the presumption is that the researches which he did not publish were not, in his opinion, suffi ciently worked out to be suitable for publication according to the extraordinarily high standard which he set for himself.
One of Cavendish's experiments, which he did not publish, may be conveniently considered here. He made a pasteboard sphere I 2in. in diameter, which was coated with tin-foil and mounted on a glass rod along a diameter of the sphere. Another similar sphere 13in. in diameter was made in two halves which were sup ported by glass rods on a folding frame, so that they could be put round the I2in. sphere enclosing it completely and could be quickly taken away without touching it. A wire attached to a silk thread could be put through a small hole in the larger sphere so as to connect the two spheres together. The larger sphere was put round the smaller one and the wire inserted so that the two spheres were connected together. The spheres were then strongly charged with electricity from a Leyden jar. The wire was then pulled out by means of the silk thread. Next the outer sphere was removed without letting it touch the inner one. The inner sphere was then found to be uncharged, when tested with two cork balls hung by threads attached to the end of a glass rod, so that the threads could be put in contact with the sphere without discharg ing it. Cavendish estimated that the charge remaining on the inner sphere could not be more than a. part of that originally given to the spheres when they were connected together by the wire.
This experiment shows that the charge on a conducting sphere is all on its outer surface. There can be no force on the electricity in conductors inside, for if there were it would cause a motion of the electricity and a redistribution which would go on until there was no force. We may therefore conclude that a uniform spheri cal shell of electricity exerts no force on charges inside it and so that the force between charges varies inversely as the square of the distance between them. Cavendish showed that his experiment required the inverse power of the distance to be between 2•02 and 1.98. This experiment was repeated by Clerk Maxwell in 1878 using a sensitive electrometer with which to test the inner sphere. He showed that the inverse power of the distance did not differ from two by more than one part in twenty thousand.
The sphere G can be lifted out and given a charge. It is then put back in position and allowed to touch the suspended sphere with which it shares the charge. The two spheres then repel each other and separate until the repulsion is balanced by the tor sion of the wire. By turning the support at the top, the angle through which the wire is twisted can be varied and so the distance between the spheres altered. The distance between the spheres can be read off on a scale on the case. Coulomb showed that the angle through which the wire was twisted was nearly inversely proportional to the square of the distance between the centres of the two spheres, and so gave a direct proof of the law of inverse squares. He also showed that the force was pro portional to the product of the two charges. For example, if when both spheres were charged the fixed one was removed and allowed to touch an equal uncharged sphere so as to give up half its charge to it, then on putting the sphere back in the apparatus the repulsion was found to be halved.
Coulomb advocated a two fluid theory of electricity. He sup posed that there were two kinds of electric fluid which may be called positive and negative electricity, and that two particles of the same kind repelled while two unlike particles attracted each other. Uncharged bodies he supposed contained both fluids in such proportions that their actions on charges were equal and opposite. Both fluids could move freely in conductors but not in insulators. It is easy to see that the electrical phenomena so far considered can be explained as well on this two fluid theory as on the one fluid theory of Franklin and Aepinus. On the two fluid theory a conductor with a positive charge may have been charged either by adding positive-electricity to it or by removing negative from it or by both processes. On this theory it ought to be possible to go on removing equal amounts of positive and negative elec tricity from a conductor until it is left without either kind and so should not then conduct, but nothing of this sort has ever been observed. The conducting power of a metal like copper is always the same at the same temperature and pressure, and so the copper must always contain the same amount of electricity. We might suppose that both kinds of electric fluid always move equally but in opposite directions in conductors, so that when a conductor receives any amount of positive electricity then it also loses an equal amount of negative. Since the facts could be explained by the one fluid theory the introduction of two fluids was unnecessary and the modern view as we shall see later agrees closely with the one fluid theory.
Coulomb made a series of experiments on the distribution of the charge on charged conductors. He showed that the charge is confined to the surface of the conductor, and that the force on a small charged body when close to the surface of a charged con ductor is proportional to the charge per unit area on the conductor near to the small charged body. Coulomb investigated the distri bution of the charge between two or more conductors when in contact, e.g., two or more spheres of different sizes, and could measure the relative charges by means of his torsion balance. He examined the distribution of the charge over the surface of con ductors of different shapes by means of a proof plane. The proof plane is a small thin metal plate supported by an insulating handle which can be put on the surface of a conductor so as to form part of the surface. When removed it carries away the charge on the part of the surface which it covered. The charge on the proof plane was measured with the torsion balance. In this way Cou lomb showed that the surface density on a conductor is greater where the surface is more convex and less where it is concave. The fact that there is a great density of charge on a sharp point was well known to Franklin.
The Development of the Mathematical Theory of Elec trostatics.—The law of force between electric charges having been established, the time was ripe for the development of a mathematical theory of electrostatics, which is the name now given to that branch of electrical science which deals with the properties of electricity when at rest or in equilibrium. In 1812 Simeon Denis Poisson (1781-1840) published a paper en the mathemat ical theory of electrostatics which forms the basis of the modern theory of this branch of electricity. He pointed out that the equi librium distribution of the charges on conductors must be such that the force on any particle of electricity in the interior of a conductor is zero, since in a conductor electricity can move freely and the existence of a force on the particles will cause a flow of the electricity.
Poisson showed that many of the methods and results of the mathematical theory of gravitation could be made use of in the theory of electrostatics. For example, it had been proved that a hollow shell of gravitating matter, bounded by two similar and similarly situated ellipsoids, exerts no force on a particle in its interior. Poisson showed that the distribution of the electricity on a charged conducting ellipsoid therefore must be such that the charge is on the surface of the ellipsoid, and that the surface density, or charge per unit area, is proportional to the distance from the surface to a similar and similarly situated ellipsoidal surface drawn very close to it. This makes the surface density greater where the surface is more convex as was known to be the case.
Lagrange had used a function of position V, in the theory of gravitation, the value of which at any point was obtained by adding together the mass of each particle of gravitating matter divided by its distance from the point. He had shown that the force on any particle is equal to the space rate of decrease of this function and that it satisfies the differential equation, 2 a 2 + a + a Z = o, where x, y, z are the co-ordinates of the ax ay az point. Poisson pointed out that, since the law of force due to electric charges was the same as that of gravitation, the same function V could be used in the solution of electrostatic problems and that, since inside a conductor the forces are zero when the electricity is equilibrium, it follows that aV/ax, aV/ay and a are zero in the conductor, so that V must be constant throughout the volume of any conductor. The distribution of the charges on any conductor or system of conductors therefore may be obtained 2 2 by finding a solution of the equation, 2 + - V + 2 = o ax ay dz which makes V constant over the surface of each conductor. The surface density of the charge on the conductors, Poisson showed, is proportional to the electrical force on a particle of electricity just outside the conductor, and so can be obtained when the function V is known in the space between the con ductors.
Poisson's mathematical theory of electrostatics was developed with great power by George Green (1793-1841), a self-taught mathematician, in a paper published at Nottingham, England, in 1828. In this paper he proved the theorem connecting volume and surface integrals, since known as Green's theorem, which is of immense value in nearly all branches of physics.
Bennet's Gold Leaf Electroscope.—A useful instrument called a gold leaf electroscope was invented by Bennet in 1787. This instrument consists simply of two narrow strips of gold leaf attached to the lower end of a metal rod.
When electrified the leaves repel each other and s• diverge. Gold leaf is extremely thin and light so that Bennet's instrument is much more sensitive than a pair of cork or pith balls hung on linen threads. The leaf is also a good conductor and so is readily charged and discharged. A modern form is shown in fig. 4. A brass case A with glass windows in front and behind supports a plug B of hard rubber which is fitted into a hole in the top of the case. A brass rod CD passes through the plug and carries a brass disc at its upper end. The lower end is cut away to a horizontal V-shaped knife edge and two equal strips of genuine gold leaf E, F are gummed on, one on each side of the edge as shown. These strips may be about yin. wide and i lin. long. If an electrified body is brought near the disc C the electroscope is charged by induction and the leaves diverge. A positively charged body induces a negative charge on the disc and a positive charge on the leaves. If the disc is touched for a moment and then the charged body removed, the negative charge remains on the electroscope and the leaves remain diverged. If a negatively charged body is then brought near the disc, the leaves diverge further, but a positively charged body makes them col lapse at first and then diverge again as it is brought nearer.
Faraday's Ice-Pail Experiment.--One of Faraday's experi ments known as the "ice-pail" experiment is of fundamental importance. A metal can (Faraday used an ice-pail, hence the name) is insulated and connected to a gold leaf electroscope. The can may be simply put on the disc of the electroscope as in fig. 5. A metal conductor supported by an insulating handle or hung from a silk thread is charged and lowered into the can. It is found that the leaves diverge when this is done and that the divergence does not alter when the conductor is moved about in the can, so long as it is not brought near to the opening at the top. If the charged conduc tor is removed the leaves collapse. If the charged conductor is allowed to touch the inside of the can, near the bottom, the divergence of the leaves does not change at the moment of the contact, but when the conductor is removed the leaves re main diverging to the same extent as when the conductor was first put in, and the conductor is found to be completely discharged.
Starting again with the electroscope dis charged and the conductor charged, if the conductor is put in the can, without touch ing, the leaves diverge as before. If then the can is touched with the finger, while the charged conductor is inside, the leaves col lapse. On removing the conductor the leaves diverge to the same extent as before, and if now the conductor is allowed to touch the can, either inside or outside, the leaves collapse, and both the conductor and the can are found to be completely discharged. These results were explained by Faraday as follows :—When the charged conductor is put in the can a charge of opposite sign is induced on the inside of the can and a charge of the same sign on the outside of the can and on the leaves. If the conductor is removed the leaves collapse because the two opposite charges on the can just neutralize each other. These induced charges are independent of the position of the conductor in the can, showing that, when a conductor surrounds a charged conductor, the in duced charge on the outside conductor is independent of the position of the inside one. When the conductor is allowed to touch the can inside, near the bottom, the deflection of the leaves does not change, showing that the induced charge on the inside of the can is exactly equal and of opposite sign to the charge on the conductor, so that they just neutralize each other when the con tact is made. All the charge on the conductor disappears, showing that there is no charge on the inside surface of a hollow charged conductor. In the second part of the experiment, when the can is touched the induced charge on the outside goes into the earth but the charge on the inside is attracted by the opposite charge on the conductor and remains. When the conductor is removed the charge on the inside goes to the outside and the leaves and produces the same divergence as before, because it is equal though of opposite sign to the induced charge previously on the outside. The charges on the can and on the conductor are now equal and opposite, so that, if the conductor is put inside without touching, the leaves collapse but diverge again when it is taken out. If the conductor is allowed to touch the can, the equal and opposite charges neutral ize each other so that both are completely discharged.
Faraday's Other Experiments and Ideas.—Another important experiment of Faraday's showed clearly that the total charge produced is always zero. That is to say that positive and negative charges are always produced together in such amounts that the positive electricity is just enough to neutralize the negative electricity. To show this he constructed a large box, coated on the outside with tinfoil which was insulated and connected to a sensi tive electroscope. He then went inside the box and generated charges within by means of frictional electric machines and by induction, but found that the electroscope on the outside was quite unaffected. He also performed the converse of this experi ment. He took the electroscope inside the box and then had the whole box strongly charged so that long sparks could be taken from it while he was inside. He found that no electrical effects whatever could be detected inside the box.
Faraday introduced the idea of lines of force in the electric field around charged bodies, a line of force being a line drawn so as to be everywhere in the direction of the force on a charged par ticle. From a study of the distribution of these lines of force, he was led to suggest that the forces between charges are trans mitted through the field by a system of stresses, consisting of a tension along the lines of force and a pressure perpendicular to them. This idea was afterwards elaborated mathematically by Clerk Maxwell, who showed that the observed forces on charged conductors could be represented accurately in this way.