But the lower atmosphere is an insulator that separates the two. This makes it possible to maintain the great potential difference between them. In fact, this system forms a giant electric condenser, in which the earth is negative and the electrosphere is positive. The potential across the atmosphere averages about 300,000 volts. It varies considerably from this figure from hour to hour during the day, and from month to month during the year.
Nothing is a perfect insulator.
With sufficiently sensitive instruments, a tiny current can be detected even in the lower atmosphere. It is slightly conductive because of the few cosmic rays that penetrate to the ground. The earth has a surplus of electrons, and these are constantly leaking away from a multitude of points on the surface. Such point discharges occur at the ends of leaves on trees, at the tips of blades of grass, and even from sharp corners on grains of sand. Man-made structures, which stand higher in the air, compress the electric field around their peaks and roof corners, and the discharge of electrons is concentrated at such points.