The earth's electric charge

Our earth is not a neutral body. It is coated with a layer of negative electricity of such amount that, at the surface, there is an electrical potential gradient of the order of 150 volts per meter. This potential gradient diminishes with altitude until, at an altitude of 10 kilometers, it becomes insignificant compared with its value at the surface. The potential gradient, and so the negative charge on the earth's surface, goes through fairly regular variations throughout the day and throughout the year; variations amounting to 50 per cent or more of its value.