A terrella (Latin of "little earth") is a small magnetised model ball representing the Earth, thought to have been invented by the English physician William Gilbert while investigating magnetism, and further developed 300 years later by the Norwegian scientist and explorer Kristian Birkeland, while investigating the aurora.
Terrellas simulated the Earth's magnetosphere until the late 20th century, when computer simulations replaced them.
William Gilbert's terrella
William Gilbert, royal physician to Queen Elizabeth I, heavily studied Earth's magnetism. Earlier investigators (among them Christopher Columbus) found that a freely suspended compass needle's direction deviated from true north, and Robert Norman showed the force on the needle was not horizontal but slanted into the Earth.
William Gilbert's explained that the Earth was a giant magnet, and he demonstrated this explanation by creating a scale model of the magnetic Earth, a "terrella", a sphere from a lodestone. Passing a small compass over the terrella, Gilbert demonstrated that a horizontal compass would point towards the magnetic pole, while a dip needle, balanced on a horizontal axis perpendicular to the magnetic one, indicated the proper "magnetic inclination" between the magnetic force and the horizontal direction. Gilbert later reported his findings in De Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure, published in 1600.
Kristian Birkeland's terrella
He simulated the effect with a "terrella," a sphere in a vacuum tank whereto he directed cathode rays, later identified as electrons, and found that they produced a glow in regions around the poles of the terrella. Because of residual gas in the chamber, the glow also outlined the path of the particles. Neither he nor his associate Carl Størmer (who calculated such paths) could understand why the actual aurora avoided the polar area. We now know this relates to the origin of the auroral electrons, which is in the Earth's magnetosphere, the region of space that Earth's magnetism controls. Birkeland believed the electrons came from the Sun, since large auroral outbursts were associated with sunspot activity.
The German Baron Carl Reichenbach (1788–1869) experimented with a terrella. He put an electromagnet in a large hollow iron sphere and in the darkroom examined them under varying electrification. The Baron referred to the iron globe as his "terrella", or "little earth".
Brunberg and Dattner in Sweden, around 1950, used a terrella to simulate trajectories of particles in the Earth's field. Podgorny in the Soviet Union, around 1972, built terrellas at which a flow of plasma was directed, simulating the solar wind. Hafiz-Ur Rahman at the University of California, Riverside conducted more realistic experiments around 1990. These experiments are difficult to interpret and cannot scale all the parameters needed to properly simulate the Earth's magnetosphere; computer simulations therefore replaced these experiments.
- Section 2, Chapter VI, page 678