Atmospheric Electricity
AtmosphereWithIonosphere.gif

Atmospheric electricity is the regular diurnal variations of the Earth's atmospheric electromagnetic network (or, more broadly, any planet's electrical system in its layer of gases). The Earth's surface, the ionosphere, and the atmosphere is known as the global atmospheric electrical circuit. Atmospheric electricity is a multidisciplinary topic.

History

Early History

The detonating sparks drawn from electrical machines and from Leyden jars suggested to the early experimenters, Hauksbee, Newton, Wall, Nollet, and Gray, that lightning and thunder were due to electric discharges. In 1708, Dr. William Wall was one of the first to observe that spark discharges resembled miniature lightning, after observing the sparks from a charged piece of amber.

In the middle of the 18th century, Benjamin Franklin's experiments showed that electrical phenomena of the atmosphere were not fundamentally different from those produced in the laboratory. By 1749, Franklin observed lightning to possess almost all the properties observable in electrical machines.

In July 1750, Franklin hypothesized that electricity could be taken from clouds via a tall metal aerial with a sharp point. Before Franklin could carry out his experiment, in 1752 Thomas-François Dalibard erected a 40-foot (12 m) iron rod at Marly-la-Ville, near Paris, drawing sparks from a passing cloud.[6] With ground-insulated aerials, an experimenter could bring a grounded lead with an insulated wax handle close to the aerial, and observe a spark discharge from the aerial to the grounding wire. In May of 1752, Dalibard affirmed that Franklin's theory was correct.

Franklin listed the following similarities between electricity and lightning:

  • producing light of a similar color;
  • rapid motion;
  • being conducted by metals, water and ice;
  • melting metals and igniting inflammable substances;
  • "sulfurous" smell (which is now known to be due to ozone);
  • magnetizing needles;
  • the similarity between St. Elmo's Fire and glow discharge.

Around June of 1752, Franklin reportedly performed his famous kite experiment. The kite experiment was repeated by Romas, who drew from a metallic string sparks 9 feet (2.7 m) long, and by Cavallo, who made many important observations on atmospheric electricity. L. G. Lemonnier (1752) also reproduced Franklin's experiment with an aerial, but substituted the ground wire with some dust particles (testing attraction). He went on to document the fair weather condition, the clear-day electrification of the atmosphere, and the diurnal variation of the atmosphere's electricity. G. Beccaria (1775) confirmed Lemonnier's diurnal variation data and determined that the atmosphere's charge polarity was positive in fair weather.

H. B. Saussure (1779) recorded data relating to a conductor's induced charge in the atmosphere. Saussure's instrument (which contained two small spheres suspended in parallel with two thin wires) was a precursor to the electrometer. Saussure found that the fair weather condition had an annual variation, and found that there was a variation with height, as well. In 1785, Coulomb discovered the electrical conductivity of air. His discovery was contrary to the prevailing thought at the time, that the atmospheric gases were insulators (which they are to some extent, or at least not very good conductors when not ionized). His research was, unfortunately, completely ignored. P. Erman (1804) theorized that the Earth was negatively charged. J. C. A. Peltier (1842) tested and confirmed Erman's idea. Lord Kelvin (1860s) proposed that atmospheric positive charges explained the fair weather condition and, later, recognized the existence of atmospheric electric fields.

Modern History

Over the course of the next century, using the ideas of Alessandro Volta and Francis Ronald, several researchers contributed to the growing body of knowledge about atmospheric electrical phenomena. With the invention of the portable electrometer and Lord Kelvin's 19th century water-dropping condenser, a greater level of precision was introduced into observational results. Towards the end of the 19th century came the discovery by W. Linss (1887) that even the most perfectly insulated conductors lose their charge, as Coulomb before him had found, and that this loss depended on atmospheric conditions. H. H. Hoffert (1888) identified individual lightning downward strokes using early cameras and would report this in "Intermittent Lightning-Flashes". J. Elster and H. F. Geitel, who also worked on thermionic emission, proposed a theory to explain thunderstorms' electrical structure (1885) and, later, discovered atmospheric radioactivity (1899). By then it had become clear that freely charged positive and negative ions were always present in the atmosphere, and that radiant emanations could be collected. F. Pockels (1897) estimated lightning current intensity by analyzing lightning flashes in basalt and studying the left-over magnetic fields (basalt, being a ferromagnetic mineral, becomes magnetically polarised when exposed to a large external field such as those generated in a lightning strike).

Using a Peltier electrometer, Luigi Palmieri researched atmospheric electricity. Nikola Tesla and Hermann Plauson investigated the production of energy and power via atmospheric electricity. Tesla also proposed to use the atmospheric electrical circuit to transmit energy wirelessly over large distances (see his Wardenclyffe Tower and Magnifying Transmitter). The Polish Polar Station, Hornsund, has researched the magnitude of the Earth's electric field and recorded its vertical component. Discoveries about the electrification of the atmosphere via sensitive electrical instruments and ideas on how the Earth’s negative charge is maintained were developed mainly in the 20th century. Whilst a certain amount of observational work has been done in the branches of atmospheric electricity, the science has not developed to a considerable extent. It is thought that any apparatus which might be used to extract useful energy from atmospheric electricity would be prohibitively costly to build and maintain, which is probably why the field has not attracted much interest.

Atmospheric Electricity Origin

The question naturally suggests itself: What is the origin of the electricity of the air and the ground? A very seductive hypothesis is that which ascribes the electricity to the evaporation of water; the vapour being supposed to carry positivo electricity with it, leaving negative electricity in the water and on the ground. Unfortunately none of the experiments made with a view of confirming this hypothesis have given it any conclusive support; on the contrary, the fact that rain is usually negatively electrified appears to contradict it.

The origin of atmospheric electricity had also been sought for in the induction currents which the motion of the earth might develop in the upper regions of the atmosphere, supposing these to be conductors. Let imagine a conducting arc, AB, of any given shape, connecting the pole and the equator, to be either fixed, or at any rate to have a smaller angular velocity than the earth. The magnetic flux, cut by the arc as the earth turns from west to east, develops therein an inductive electromotive force, the direction of which is from the equator to the pole. If the arc were in contact with the globe by sliding contacts at N and E, and the circuit closed, the induction would produce a continuous current; if the supposed conducting arc were insulated, there would be an accumulation of positive electricity at the pole and of negative at the equator, and to this accumulation might be ascribed the polar auroras on the one hand, and on the other the daily thunderstorms in the equatorial regions. This view of confirming such a hypothesis have given without any conclusive support, also.

Potential at a Point in the Air

NearField-Earth-eField.png

Experiment shows that in an open space the potential at a point in the air is always different from that of the earth.

Two potential determination methods

Difference of Potentials between a point in the Air and the Earth

Let a small insulated sphere, of radius r, be placed at the point in question and connected for a moment with the earth by means of a very fine wire. If V is the value of the potential at this point due to external charges, the potential of the earth being as usual taken as zero, the sphere will acquire a charge, Q, of electricity such that the potential of the sphere becomes zero, like that of the earth. The potential at the centre being V+(Q/r), we have V+(Q/r) = 0. If the sphere is then placed in a Faraday cylinder, we may measure its charge Q, and so obtain v= -(Q/r).

A simplier method consists in putting at the place in question a point which forms part of an insulated conductor (antenna). Assuming that the point is "perfect", equilibrium cannot exist so long as the point, and therewith the conductor of which it forms part, is at a different potential from that of the air near the point.

Saussure_pith-ball_electroscope.png

Saussure used a small electroscope provided with a point (Saussure's pith-ball electroscope). If the case is at the potential of the ground, the divergence of the leaves varies with the potential of the air at the end of the point; but the action of the point is too imperfect to allow us to consider that equilibrium is attained.

If an insulated conductor is placed in an electric field, the surface of the conductor is everywhere at the same potential, and becomes continuous with some one equipotential surface of the field. Those parts of the surface of the conductor which lie to one side of this equipotential surface extend into regions where the undisturbed potential of the field would be higher than that of the conductor, and the other parts of the surface extend into regions where the potential would be lower. The electrification assumed by the conductor is such as to keep the potential of the first-mentioned parts down, and to bring that of the remaining parts up to the actual uniform potential possessed by the conductor as a whole. That is to say, the parts of the surface of the conductor which extend into the region of higher potential acquire a negative charge, and those parts which extend into the region of lower potential acquire a positive charge. If now a portion of the surface of the conductor were to become detached, say from the part which is negatively electrified, carrying its electrification with it, the potential of the whole remaining part of the conductor would rise; and if the part that has been detached were in some way to grow again, a conductor of the original form would be reproduced, but it would have a somewhat higher potential than before.

Now, imagine this process of successive separation and reproduction of a portion of the surface of the conductor to go on, over and over again, always at the same part: it is obvious that the potential of the conductor as a whole will go on rising (if the portions that become detached are negatively electrified, or, in the opposite case, that it will go on falling) until the density of the surface-charge of the detached portions becomes zero, when there will be no further change. The whole conductor will then have been brought to the potential of the equipotential surface passing through the part from which the successive portions break away. To apply this principle to the experimental determination of the potential at a point in the air, a vessel of water is placed on an insulating stand, and provided with a long, very narrow tube, from the end of which the water escapes in a fine stream. The whole vessel is thus gradually brought to the potential of the point at which the stream of water breaks into separate drops. To observe this, one pair of quadrants is connected with the watervessel, and the other pair with the earth. The flame of a spiritlamp connected by a fine wire with the electrometer acts in the same way as a water-jet, or a piece of touch paper prepared by soaking paper with a solution of nitrate of lead may be used.

It was found that in fine weather the potential anywhere in the open air is always positive; that its value increases with the height of the point above the ground, and almost in direct proportion; but that at the same place rapid and often large variations occur.

The results vary so much that it is difficult to give numerical statements. Above an open plain, for example, the change of potential with height is often between 10 and 1000 volts per metre, but it is sometimes far more.

If, instead of insulating the point, or the arrangement which acts as a point, one connects it with the earth, statical equilibrium cannot be established, and a continuous flow of electricity traverses the conductor. The flow is manifestly equal to the amount given out by the point. It increases with the difference of potential, but it cannot be used to measure this difference, the quantity given off being always so extremely small.

If there is a small break in the conducting wire, the difference of potential at the break may be great enough to produce a succession of sparks. Sparks are sometimes produced in this way between the needle and the quadrants of the electrometer.

Distribution of Potential

Lightingrodplacement.png
Landscape explanation
(1) Represents Lord Kelvin's "reduced" area of the region; See Sir William Thomson, ''Papers on Electrostatics and Magnetism''.
(2) Surface concentric with the Earth such that the quantities stored over it and under it are equal;
(3) Building on a site of excessive electrostatic charge density; (4) Building on a site of low electrostatic charge density.
(Image via US patent 1266175.)

At a given instant, the surfaces where the potential has constant and equidistant values above an open plain are equidistant horizontal planes. If the surface of the ground is irregular, the nearest equipotential surfaces follow its undulations, and approach each other over the elevated parts, and the more so the higher and more abrupt these are. Around a house, all parts of which may be regarded as connected with the earth, and therefore at zero potential, the equipotential surfaces are vertical near the walls and follow the contours of the roof, being closer to each other over the ridge; on the other hand, they separate widely from each other in a court surrounded by high walls, or in a street. As we rise, the effects of the inequalities disappear, and it may be assumed that beyond a certain height the equipotential surfaces are horizontal planes.

As the equipotential surfaces tend to become more nearly parallel to the surface of the ground the nearer they are to it, the electric force at each point is perpendicular to the surface. And since, in fair weather, the value of the potential increases with the distance, it follows that the electric force is directed towards the earth; its intensity at each point is inversely proportional to the distance between two consecutive equipotential surfaces.

External articles

Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License