The global atmospheric electrical circuit is the continuous movement of atmospheric electricity between the ionosphere and the Earth. Through the balance of thunderstorms and fair weather, the atmosphere is subject to a continual and substantial electrical current. Principally, thunderstorms throughout the world carry negative charges to the earth, which is then discharged gradually through the air in fair weather. This atmospheric circuit is central to the study of atmospheric physics and meteorology. It is used in the understanding of atmospheric electricity across the planet. In the past it has been suggested as a source of available energy, or communications platform. The global electrical circuit is also relevant to the study of human health and air pollution, due to the interaction of negative ions and aerosols. The effect of global warming, and temperature-sensitivity of the Earth's electrical circuit is unknown. History The history of the global atmospheric electrical circuit is intertwined with the history of atmospheric electricity. For example, in the 18th century, scientists began understanding the link between lightning and electricity. In addition to the iconic kite experiments of Benjamin Franklin and Thomas-François Dalibard, some early studies of charge in a "cloudless atmosphere" (i.e. fair weather) were carried out by Giambatista Beccaria, John Canton, Louis-Guillaume Le Monnier and John Read. Fair weather measurements from the late 18th century onwards often found consistent diurnal variations. During the 19th century, several long series of observations were made. Measurements near cities were (and still are) heavily influenced by smoke pollution. In the early 20th century, balloon ascents provided information about the electric field well above the surface. Important work was done by the research vessel Carnegie, which produced standardised measurements around the world's oceans (where the air is relatively clean). C. T. R. Wilson was the first to present the concept of a global circuit in 1920. Mechanism Lightning Lightning strikes the earth 40,000 times per day, and can be thought to charge the earth like a battery. Thunderstorms generate an electrical potential difference between the earth's surface and the ionosphere, mainly by means of lightning returning current to ground. Because of this, the ionosphere is positively charged relative to the earth. Consequently, there is always a small current of approximately 2pA per square metre transporting charged particles in the form of atmospheric ions between the ionosphere and the surface. Fair weather This current is carried by ions present in the atmosphere (generated