The terrestrial electricity phenomena observed include earth currents and atmospheric electricity.
Terrestrial electricity has to do with those natural electric phenomena which take place within the earth-earth currents- and those which are found in the atmosphere of the earth- atmospheric electricity.
These currents circulate in the crust of the earth and are in general weak, but in times of disturbances they may attain strength sufficient to interfere seriously with telegraphic transmission. These currents and the atmospheric-electric phenomena may be grouped as the electric field of the earth and its atmosphere.
Earth currents represent a world-wide system which differs from place to place, and which is subject to regular and periodic and to irregular variations, both in intensity and direction. The earlier attempts to measure earth currents were often contradictory, because some natural currents are purely local in character, and artificial currents like those due to leakage from power-system are superimposed.
Atmospheric electricity Source of Image
Even though earth currents may be immediately produced by magnetic changes, they should have a reaction that would affect the measured variations of magnetic force with strength of reaction depending on the internal structure of the earth.
Evidence of diurnal variations on clam and disturbed days, show a small definite and distinctive differences in the magnetic variations during periods of disturbed earth currents.
The classical kite experiments on the electrification of clouds and the nature of lightning by Benjamin Franklin in 1752 furnished the foundation for atmospheric electric investigations. The development of the study of the electric phenomena of the atmosphere advanced somewhat after the introduction of the quadrant electrometer by Lord Kelvin and the introduction by J. J. Thomson of the ionic theory of the electrical conduction in gases, and under the leadership of J. Elster and H. Geitel the foundations were laid for present-day investigations of the electric field.
In general, the atmospheric-electric phenomena determined include potential gradient, ionization, and conductivity. On clear days the earth is negatively charged with respect to the air, and in the lower layers of the atmosphere the difference in potential between the earth and a point in the atmosphere increases with its height above the earth.
The conductivity and the potential gradient set up an electric conduction current. For the total earth, this would amount to a total current of 1,800 amps. The observation that provide a measure of the conduction-current density show that negative electricity is supplied to the earth and that positive electricity is supplied to the higher strata of the atmosphere by a process which neither increase nor decreases through many years, but which may fluctuate from year to year. And from season to season, and which varies in a fairly regular manner during the day.