A Treatise on Electricity and Magnetism, Vol. 1 (1881) — part 5 of 28

39.] Let us suppose that by either of these methods we can measure the force between two electrified bodies. We shall suppose the dimensions of the bodies small compared with the distance between them, so that the result may not be much altered by any inequality of distribution of the electrification on either body, and we shall suppose that both bodies are so suspended in air as to be at a considerable distance from other bodies on which they might induce electrification.

It is then found that if the bodies are placed at a fixed distance and charged respectively with e and / of our provisional units of electricity, they will repel each other with a force proportional to the product of e and /. If either e or / is negative, that is, if one of the charges is vitreous and the other resinous, the force will be attractive, but if both e and / are negative the force is again repulsive.

41.] MEASUREMENT OF ELECTRIC FORCES. 43

We may suppose the first body, A, charged with m units of vitreous and n units of resinous electricity, which may be con- ceived separately placed within the body, as in Experiment V.

Let the second body, B, be charged with m units of positive and n units of negative electricity.

Then each of the m positive units in A will repel each of the m' positive units in B with a certain force, sayj^ making a total effect equal to m m'f.

Since the effect of negative electricity is exactly equal and opposite to that of positive electricity, each of the m positive units in A will attract each of the n negative units in B with the same force f, making a total effect equal to mnf.

Similarly the n negative units in A will attract the m positive units in B with a force nm'f, and will repel the ri negative units in B with a force nn'f.

The total repulsion will therefore be (mm' '-f nn}f\ and the total attraction will be (mnf + m'ri)f.

The resultant repulsion will be

(mm' -- nn' — mn' — nm'}f or (m — n) (m — nf)f.

Now m — n = e is the algebraical value of the charge on A, and m'—n'=e' is that of the charge on B, so that the resultant re- pulsion may be written ee'f> the quantities e and / being always understood to be taken with their proper signs.

Variation of the Force with the Distance.

40.] Having established the law of force at a fixed distance, we may measure the force between bodies charged in a constant manner and placed at different distances. It is found by direct measurement that the force, whether of attraction or repulsion, varies inversely as the square of the distance, so that if f is the repulsion between two units at unit distance, the repulsion at dis- tance r will be/>~2, and the general expression for the repulsion between e units and e' units at distance r will be

Definition of the Electrostatic Unit of 'Electricity .

41.] We have hitherto used a wholly arbitrary standard for our

unit of electricity, namely, the electrification of a certain piece of

glass as it happened to be electrified at the commencement of our

experiments. We are now able to select a unit on a definite

44 ELECTROSTATIC PHENOMENA. [42.

principle, and in order that this unit may belong1 to a general system we define it so thatj^may be unity, or in other words —

The electrostatic unit of electricity is that quantity of positive elec- tricity which) when placed at unit of distance from an equal quantity ', repels it with unit of force.

This unit is called the Electrostatic unit to distinguish it from the Electromagnetic unit, to be afterwards defined.

We may now write the general law of electrical action in the simple form jF_^'r-2. or^

The repulsion between two small bodies charged respectively with e and ef units of electricity is numerically eqiial to the product of the charges divided ~by the square of the distance.

Dimensions of the Electrostatic Unit of Quantity.

42.] If [Q] is the concrete electrostatic unit of quantity itself, and e, e' the numerical values of particular quantities; if [Z] is the unit of length, and r the numerical value of the distance ; and if [F] is the unit of force, and F the numerical value of the force, then the equation becomes

whence [Q] = \LF*\

This unit is called the Electrostatic Unit of electricity. Other units may be employed for practical purposes, and in other depart- ments of electrical science, but in the equations of electrostatics quantities of electricity are understood to be estimated in electro- static units, just as in physical astronomy we employ a unit of mass which is founded on the phenomena of gravitation, and which differs from the units of mass in common use.

Proof of the Law of Electrical Force.

43.] The experiments of Coulomb with the torsion-balance may be considered to have established the law of force with a certain approximation to accuracy. Experiments of this kind, however, are rendered difficult, and in some degree uncertain, by several disturbing causes, which must be carefully traced and corrected for.

In the first place, the two electrified bodies must be of sensible dimensions relative to the distance between them, in order to be capable of carrying charges sufficient to produce measurable forces.

44-] LAW OF ELECTRIC FORCE. 45

The action of each body will then produce an effect on the dis- tribution of electricity on the other, so that the charge cannot be considered as evenly distributed over the surface, or collected at the centre of gravity ; but its effect must be calculated by an intricate investigation. This, however, has been done as regards two spheres by Poisson in an extremely able manner, and the investigation has been greatly simplified by Sir W. Thomson in his Theory of Electrical Images. See Arts. 172-175.

Another difficulty arises from the action of the electricity induced on the sides of the case containing the instrument. By making the inner surface of the instrument of metal, this effect can be rendered definite and measurable.

An independent difficulty arises from the imperfect insulation of the bodies, on account of which the charge continually de- creases. Coulomb investigated the law of dissipation, and made corrections for it in his experiments.

The methods of insulating charged conductors, and of measuring electrical effects, have been greatly improved since the time of Coulomb, particularly by Sir W. Thomson ; but the perfect ac- curacy of Coulomb's law of force is established, not by any direct experiments and measurements (which may be used as illustrations of the law), but by a mathematical consideration of the pheno- menon described as Experiment VII, namely, that an electrified conductor .5, if made to touch the inside of a hollow closed con- ductor C and then withdrawn without touching (?, is perfectly dis- charged, in whatever manner the outside of C may be electrified. By means of delicate electroscopes it is easy to shew that no electricity remains on B after the operation, and by the mathe- matical theory given at Art. 74, this can only be the case if the force varies inversely as the square of the distance, for if the law were of any different form B would be electrified.

The Electric Field.

44.] The Electric Field is the portion of space in the neigh- bourhood of electrified bodies, considered with reference to electric phenomena. It may be occupied by air or other bodies, or it may be a so-called vacuum, from which we have withdrawn every sub- stance which we can act upon with the means at our disposal.

If an electrified body be placed at any part of the electric field it will, in general, produce a sensible disturbance in the electri- fication of the other bodies.

46 ELECTROSTATIC PHENOMENA. [45.

But if the body is very small, and its charge also very small, the electrification of the other bodies will not be sensibly disturbed, and we may consider the position of the body as determined by its centre of mass. The force acting- on the body will then be proportional to its charge, and will be reversed when the charge is reversed.

Let e be the charge of the body, and F the force acting on the body in a certain direction, then when e is very small F is propor- tional to e, or F=Re,

where R depends on the distribution of electricity on the other bodies in the field. If the charge e could be made equal to unity without disturbing the electrification of other bodies we should have F — E.

We shall call R the Resultant Electromotive Intensity at the given point of the field. When we wish to express the fact that this quantity is a vector we shall denote it by the German letter (£.

Electromotive Force and Potential.

45.] If the small body carrying the small charge e be moved from one given point, A, to another H, along a given path, it will experience at each point of its course a force Re, where R varies from point to point of the course. Let the whole work done on the body by the electrical force be Ee> then E is called the Total Electromotive Force along the path A B. If the path forms a complete circuit, and if the total electromotive force round the circuit does not vanish, the electricity cannot be in equi- librium but a current will be produced. Hence in Electrostatics the electromotive force round any closed circuit must be zero, so that if A and B are two points on the circuit, the electromotive force from A to B is the same along either of the two paths into which the circuit is broken, and since either of these can be altered independently of the other, the electromotive force from A to B is the same for all paths from A to B.

If B is taken as a point of reference for all other points, then the electromotive force from A to B is called the Potential of A. It depends only on the position of A. In mathematical investi- gations, B is generally taken at an infinite distance from the electrified bodies.

A body charged positively tends to move from places of greater positive potential to places of smaller positive, or of negative,

46.] ELECTRIC POTENTIAL. 47

potential, and a body charged negatively tends to move in the opposite direction.

In a conductor the electrification is free to move relatively to the conductor. If therefore two parts of a conductor have different potentials, positive electricity will move from the part having greater potential to the part having less potential as long as that difference continues. A conductor therefore cannot be in electrical equilibrium unless every point in it has the same potential. This potential is called the Potential of the Conductor.

Equipotential Surfaces.

46.] If a surface described or supposed to be described in the electric field is such that the electric potential is the same at every point of the surface it is called an Equipotential surface.

An electrified particle constrained to rest upon such a surface will have no tendency to move from one part of the surface to another, because the potential is the same at every point. An equipotential surface is therefore a surface of equilibrium or a level surface.

The resultant force at any point of the surface is in the direction of the normal to the surface, and the magnitude of the force is such that the work done on an electrical unit in passing from the surface Fto the surface V is V— V.

No two equipotential surfaces having different potentials can meet one another, because the same point cannot have more than one potential, but one equipotential surface may meet itself, and this takes place at all points and along all lines of equilibrium.

The surface of a conductor in electrical equilibrium is necessarily an equipotential surface. If the electrification of the conductor is positive over the whole surface, then the potential will diminish as we move away from the surface on every side, and the conductor will be surrounded by a series of surfaces of lower potential.

But if (owing to the action of external electrified bodies) some regions of the conductor are charged positively and others ne- gatively, the complete equipotential surface will consist of the surface of the conductor itself together with a system of other surfaces, meeting the surface of the conductor in the lines which divide the positive from the negative regions. These lines will be lines of equilibrium, and an electrified particle placed on one of these lines will experience no force in any direction.

When the surface of a conductor is charged positively in some

4:8 ELECTROSTATIC PHENOMENA. [47.

parts and negatively in others, there must be some other electrified body in the field besides itself. For if we allow a positively electrified particle, starting* from a positively charged part of the surface, to move always in the direction of the resultant force upon it, the potential at the point will continually diminish till the point reaches either a negatively charged surface at a potential less than that of the first conductor, or moves off to an infinite distance. Since the potential at an infinite distance is zero, the latter case can only occur when the potential of the conductor is positive.

In the same way a negatively electrified particle, moving off from a negatively charged part of the surface^ must either reach a positively charged surface, or pass off to infinity, and the latter case can only happen when the potential of the conductor is negative.

Therefore, if both positive and negative charge exist on a conductor, there must be some other body in the field whose potential has the same sign as that of the conductor but a greater numerical value, and if a conductor of any form is alone in the field the charge of every part is of the same sign as the potential of the conductor.

The interior surface of a hollow conducting vessel containing no charged bodies is entirely free from charge. For if any part of the surface were charged positively, a positively electrified particle moving in the direction of the force upon it, must reach a nega- tively charged surface at a lower potential. But the whole in- terior surface has the same potential. Hence it can have no charge.

A conductor placed inside the vessel and communicating with it, may be considered as bounded by the interior surface. Hence such a conductor has no charge.

Lines of Force.

47.] The line described by a point moving always in the direc- tion of the resultant intensity is called a Line of force. It cuts the equipotential surfaces at right angles. The properties of lines of force will be more fully explained afterwards, because Faraday has expressed many of the laws of electrical action in terms of his conception of lines of force drawn in the electric field, and in- dicating both the direction and the intensity at every point.

50.] ELECTRIC TENSION. 49

Electric Tension.

48.] Since the surface of a conductor is an equipotential surface, the resultant force is normal to the surface, and it will be shewn in Art. 78 that it is proportional to the superficial density of the electrification. Hence the electricity on any small area of the surface will be acted on by a force tending from the conductor and proportional to the product of the resultant force and the density, that is, proportional to the square of the resultant force.

This force, which acts outwards as a tension on every part of the conductor, will be called electric Tension. It is measured like ordinary mechanical tension, by the force exerted on unit of area.

The word Tension has been used by electricians in several vague senses, and it has been attempted to adopt it in mathematical language as a synonym for Potential ; but on examining the cases in which the word has been used, I think it will be more con- sistent with usage and with mechanical analogy to understand by tension a pulling force of so many pounds weight per square inch exerted on the surface of a conductor or elsewhere. We shall find that the conception of Faraday, that this electric tension exists not only at the electrified surface but all along the lines of force, leads to a theory of electric action as a phenomenon of stress in a medium.

Electromotive Force.

49.] When two conductors at different potentials are connected by a thin conducting wire, the tendency of electricity to flow along the wire is measured by the difference of the potentials of the two bodies. The difference of potentials between two con- ductors or two points is therefore called the Electromotive force between them.

Electromotive force cannot in all cases be expressed in the form of a difference of potentials. These cases, however, are not treated of in Electrostatics. We shall consider them when we come to heterogeneous circuits, chemical actions, motions of mag- nets, inequalities of temperature, &c.

Capacity of a Conductor.

50.] If one conductor is insulated while all the surrounding con- ductors are kept at the zero potential by being put in commu- nication with the earth, and if the conductor, when charged with

VOL. I. E

50 ELECTROSTATIC PHENOMENA. [51.

a quantity E of electricity, has a potential F", the ratio of 2? to F" is called the Capacity of the conductor. If the conductor is com- pletely enclosed within a conducting* vessel without touching- it, then the charge on the inner conductor will be equal and op- posite to the charge on the inner surface of the outer conductor, and will be equal to the capacity of the inner conductor multiplied by the difference of the potentials of the two conductors.

Electric Accumulators.

A system consisting of two conductors whose opposed surfaces are separated from each other by a thin stratum of an insulating medium is called an electric Accumulator. The two conductors are called the Electrodes and the insulating medium is called the Dielectric. The capacity of the accumulator is directly propor- tional to the area of the opposed surfaces and inversely proportional to the thickness of the stratum between them. A Leyden jar is an accumulator in which glass is the insulating medium. Accumu- lators are sometimes called Condensers, but I prefer to restrict the term ' condenser ' to an instrument which is used not to hold electricity but to increase its superficial density.

PROPERTIES OP BODIES IN RELATION TO STATICAL ELECTRICITY.

Resistance to the Passage of Electricity through a Body.

51.] When a charge of electricity is communicated to any part of a mass of metal the electricity is rapidly transferred from places of high to places of low potential till the potential of the whole mass becomes the same. In the case of pieces of metal used in ordinary experiments this process is completed in a time too short to be observed, but in the case of very long and thin wires, such as those used in telegraphs, the potential does not become uniform till after a sensible time, on account of the resistance of the wire to the passage of electricity through it.

The resistance to the passage of electricity is exceedingly dif- ferent in different substances, as may be seen from the tables at Arts. 362, 366, and 369, which will be explained in treating of Electric Currents.

All the metals are good conductors, though the resistance of lead is 12 times that of copper or silver, that of iron 6 times, and that of mercury 60 times that of copper. The resistance of all metals increases as their temperature rises.

51.] ELECTRIC RESISTANCE. 51

Many liquids conduct electricity by electrolysis. This mode of conduction will be considered in Part II. For the present, we may regard all liquids containing water and all damp bodies as con- ductors, far inferior to the metals, but incapable of insulating a charge of electricity for a sufficient time to be observed. The re- sistance of electrolytes diminishes as the temperature rises.

On the other hand, the gases at the atmospheric pressure, whether dry or moist, are insulators so nearly perfect when the electric tension is small that we have as yet obtained no evidence of electricity passing through them by ordinary conduction. The gradual loss of charge by electrified bodies may in every case be traced to imperfect insulation in the supports, the electricity either passing through the substance of the support or creeping over its surface. Hence, when two charged bodies are hung up near each other, they will preserve their charges longer if they are electrified in opposite ways, than if they are electrified in the same way. For though the electromotive force tending to make the electricity pass through the air between them is much greater when they are oppositely electrified, no per- ceptible loss occurs in this way. The actual loss takes place through the supports, and the electromotive force through the supports is greatest when the bodies are electrified in the same way. The result appears anomalous only when we expect the loss to occur by the passage of electricity through the air between the bodies. The passage of electricity through gases takes place, in general, by dis- ruptive discharge, and does not begin till the electromotive force has reached a certain value. The value of the electromotive force which can exist in a dielectric without a discharge taking place is called the Electric Strength of the dielectric. The electric strength of air diminishes as the pressure is reduced from the atmo- spheric pressure to that of about three millimetres of mercury. When the pressure is still further reduced, the electric strength rapidly increases ; and when the exhaustion is carried to the highest degree hitherto attained, the electromotive force required to produce a spark of a quarter of an inch is greater than that which will give a spark of eight inches in air at the ordinary pressure.

A vacuum, that is to say, that which remains in a vessel after we have removed everything which we can remove from it, is there- fore an insulator of very great electric strength.

The electric strength of hydrogen is much less than that of air.

Certain kinds of glass when cold are marvellously perfect in- sulators, and Sir W. Thomson has preserved charges of electricity

E 2

52 ELECTROSTATIC PHENOMENA. [52.

for years in bulbs hermetically sealed. The same glass, however, becomes a conductor at a temperature below that of boiling water.

Gutta-percha, caoutchouc, vulcanite, paraffin, and resins are good insulators, the resistance of gutta-percha at 75° F. being about 6 x 1 019 times that of copper.

Ice, crystals, and solidified electrolytes, are also insulators.

Certain liquids, such as naphtha, turpentine, and some oils, are insulators, but inferior to the best solid insulators.

DIELECTRICS.

Specific Inductive Capacity.

52.] All bodies whose insulating power is such that when they are placed between two conductors at different potentials the elec- tromotive force acting on them does not immediately distribute their electricity so as to reduce the potential to a constant value, are called by Faraday Dielectrics.

It appears from the hitherto unpublished researches of Cavendish that he had, before 1773, measured the capacity of plates of glass, rosin, beeswax, and shellac, and had determined the ratio in which their capacity exceeded that of plates of air of the same dimensions.

Faraday, to whom these researches were unknown, discovered that the capacity of an accumulator depends on the nature of the insulating medium between the two conductors, as well as on the dimensions and relative position of the conductors themselves. By substituting other insulating media for air as the dielectric of the accumulator, without altering it in any other respect, he found that when air and other gases were employed as the insulating medium the capacity of the accumulator remained sensibly the same, but that when shellac, sulphur, glass, &c. were substituted for air, the capacity was increased in a ratio which was different for each substance.

By a more delicate method of measurement Boltzmann succeeded in observing the variation of the inductive capacity of gases at different pressures.

This property of dielectrics, which Faraday called Specific In- ductive Capacity, is also called the Dielectric Constant of the sub- stance. It is defined as the ratio of the capacity of an accumulator when its dielectric is the given substance, to its capacity when the dielectric is a vacuum.

If the dielectric is not a good insulator, it is difficult to measure

53-] ELECTRIC ABSORPTION. 53

its inductive capacity, because the accumulator will not hold a charge for a sufficient time to allow it to be measured ; but it is certain that inductive capacity is a property not confined to good insulators, and it is probable that it exists in all bodies.

Absorption of Electricity.

53.] It is found that when an accumulator is formed of certain dielectrics, the following phenomena occur.

When the accumulator has been for some time electrified and is then suddenly discharged and again insulated, it becomes recharged in the same sense as at first, but to a smaller degree, so that it may be discharged again several times in succession, these discharges always diminishing. This phenomenon is called that of the Re- sidual Discharge.

The instantaneous discharge appears always to be proportional to the difference of potentials at the instant of discharge, and the ratio of these quantities is the true capacity of the accumulator; but if the contact of the discharger is prolonged so as to include some of the residual discharge, the apparent capacity of the accu- mulator, calculated from such a discharge, will be too great.

The accumulator if charged and left insulated appears to lose its charge by conduction, but it is found that the proportionate rate of loss is much greater at first than it is afterwards, so that the measure of conductivity, if deduced from what takes place at first, would be too great. Thus, when the insulation of a submarine cable is tested, the insulation appears to improve as the electrifi- cation continues.

Thermal phenomena of a kind at first sight analogous take place in the case of the conduction of heat when the opposite sides of a body are kept at different temperatures. In the case of heat we know that they depend on the heat taken in and given out by the body itself. Hence, in the case of the electrical phenomena, it has been supposed that electricity is absorbed and emitted by the parts of the body. We shall see, however, in Art. 329, that the phenomena can be explained without the hypothesis of absorp- tion of electricity, by supposing the dielectric in some degree heterogeneous.

That the phenomenon called Electric Absorption is not an actual absorption of electricity by the substance may be shewn by charging the substance in any manner with electricity while it is surrounded by a closed metallic insulated vessel. If, when the

54: ELECTEOSTATIC PHENOMENA. [54.

substance is charged and insulated, the vessel be instantaneously discharged and then left insulated, no charge is ever communicated to the vessel by the gradual dissipation of the electrification of the charged substance within it.

54.] This fact is expressed by the statement of Faraday that it is impossible to charge matter with an absolute and independent charge of one kind of electricity *.

In fact it appears from the result of every experiment which has been tried that in whatever way electrical actions may take place among a system of bodies surrounded by a metallic vessel, the charge on the outside of that vessel is not altered.

Now if any portion of electricity could be forced into a body so as to be absorbed in it, or to become latent, or in any way to exist in it, without being connected with an equal portion of the opposite electricity by lines of induction, or if, after having being absorbed, it could gradually emerge and return to its ordi- nary mode of action, we should find some change of electrification in the surrounding vessel.

As this is never found to be the case, Faraday concluded that it is impossible to communicate an absolute charge to matter, and that no portion of matter can by any change of state evolve or render latent one kind of electricity or the other. He therefore regarded induction as * the essential function both in the first development and the consequent phenomena of electricity.' His 'induction' is (1298) a polarized state of the particles of the dielectric, each particle being positive on one side and negative on the other, the positive and the negative electrification of each particle being always exactly equal.

Disruptive Discharge f.

55.] If the electromotive intensity at any point of a dielectric is gradually increased, a limit is at length reached at which there is a sudden electrical discharge through the dielectric, generally accompanied with light and sound, and with a temporary or per- manent rupture of the dielectric.

The intensity of the electromotive force when this takes place is a measure of what we may call the electric^strength of the di- electric. It depends on the nature of the dielectric, and is greater in dense air than in rare air, and greater in glass than in air, but

• Exp. Res., vol. i. series xi. *[[ ii. 'On the Absolute Charge of Matter,' and (1244). t See Faraday, Exp. Res., vol. i., series xii. and xiii.

55-] ELECTRIC GLOW. 55

in every case, if the electromotive force be made great enough, the dielectric gives way and its insulating power is destroyed, so that a current of electricity takes place through it. It is for this reason that distributions of electricity for which the electromotive intensity becomes anywhere infinite cannot exist.

The Electric Glow.

Thus, when a conductor having a sharp point is electrified, the theory, based on the hypothesis that it retains its charge, leads to the conclusion that as we approach the point the superficial density of the electricity increases without limit, so that at the point itself the surface-density, and therefore the resultant electrical force, would be infinite. If the air, or other surrounding dielectric, had an invincible insulating power, this result would actually occur ; but the fact is, that as soon as the resultant force in the neigh- bourhood of the point has reached a certain limit, the insulating power of the air gives way, so that the air close to the point becomes a conductor. At a certain distance from the point the resultant force is not sufficient to break through the insulation of the air, so that the electric current is checked, and the electricity accumulates in the air round the point.

The point is thus surrounded by particles of air charged with electricity of the same kind with its own. The effect of this charged air round the point is to relieve the air at the point itself from part of the enormous electromotive force which it would have ex- perienced if the conductor alone had been electrified. In fact the surface of the electrified body is no longer pointed, because the point is enveloped by a rounded mass of charged air, the surface of which, rather than that of the solid conductor, may be regarded as the outer electrified surface.

If this portion of charged air could be kept still, the electrified body would retain its charge, if not on itself at least in its neighbourhood, but the charged particles of air being free to move under the action of electrical force, tend to move away from the electrified body because it is charged with the same kind of elec- tricity. The charged particles of air therefore tend to move off in the direction of the lines of force and to approach those sur- rounding bodies which are oppositely electrified. When they are gone, other uncharged particles take their place round the point, and since these cannot shield those next the point itself from the excessive electric tension, a new discharge takes place, after which

56 ELECTROSTATIC PHENOMENA. [55.

the newly charged particles move off, and so on as long as the body remains electrified.

In this way the following phenomena are produced : — At and close to the point there is a steady glow, arising from the con- stant discharges which are taking place between the point and the air very near it.

The charged particles of air tend to move off in the same general direction, and thus produce a current of air from the point, con- sisting of the charged particles, and probably of others carried along by them. By artificially aiding this current we may increase the glow, and by checking the formation of the current we may pre- vent the continuance of the glow *.

The electric wind in the neighbourhood of the point is sometimes very rapid, but it soon loses its velocity, and the air with its charged particles is carried about with the general motions of the atmo- sphere, and constitutes an invisible electric cloud. When the charged particles come near to any conducting surface, such as a wall, they induce on that surface a charge opposite to their own, and are then attracted towards the wall, but since the electro- motive force is small they may remain for a long time near the wall without being drawn up to the surface and discharged. They thus form an electrified atmosphere clinging to conductors, the presence of which may sometimes be detected by the electrometer. The electrical forces, however, acting between large masses of charged air and other bodies are exceedingly feeble compared with the ordinary forces which produce winds, and which depend on inequalities of density due to differences of temperature, so that it is very improbable that any observable part of the motion of ordinary thunder clouds arises from electrical causes.

The passage of electricity from one place to another by the motion of charged particles is called Electrical Convection or Con- vective Discharge.

The electrical glow is therefore produced by the constant passage of electricity through a small portion of air in which the tension is very high, so as to charge the surrounding particles of air which are continually swept off by the electric wind, which is an essential part of the phenomenon.