Everything about Terrella totally explained
A
terrella (meaning "little earth") is a small
magnetised model ball representing the
Earth, that's thought to have been invented by
Englishman 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 had been used up until the late 20th century to attempt to simulate the Earth's
magnetosphere, but have now been replaced by computer simulation.
William Gilbert's terrella
William Gilbert, the
royal physician to
Queen Elizabeth I, devoted much of his time, energy and resources to the study of the
Earth's magnetism. It had been known for centuries that a freely suspended
compass needle pointed north. Later investigators (including
Christopher Columbus) found that direction deviated somewhat from true north, and
Robert Norman showed the force on the needle wasn't horizontal but slanted into the Earth.
William Gilbert's explanation was that the Earth itself was a giant magnet, and he demonstrated this by creating a scale model of the magnetic Earth, a "terrella", a sphere formed out of 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
Kristian Birkeland was a Norwegian physicist who, around
1895, tried to explain why the lights of the
polar aurora appeared only in regions centered at the magnetic poles.
He tried to simulate the effect using a "terrella," a sphere in a vacuum tank to which he directed beams of
cathode rays, later identified as
electrons, and found they indeed 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 area around the poles themselves.
We now know this relates to the origin of the auroral electrons, which is actually inside the Earth's
Magnetosphere, the space region controlled by the Earth's magnetism. Birkeland believed the electrons came from the
Sun, since large auroral outbursts were associated with
sunspot activity.
Birkeland constructed several terrellas. One large terrella experiment was reconstructed in
Tromsø,
Norway.
(External Link
).
Other terrellas
Later terella experiments were performed by various scientists, studying various aspects of the interaction of the Earth's magnetic field in space. Brunberg and Dattner in
Sweden, around
1950, used a terrella to simulate trajectories of particles in the Earth's field; today such simulations are conducted much more conveniently with
computers. Podgorny in the
Soviet Union, around
1972, built terrellas at which a flow of
plasma was directed (simulating the solar wind), and Hafiz-Ur Rahman at the
University of California, Riverside conducted more realistic experiments around
1990. All such experiments are difficult to interpret, and are never able to scale all parameters to properly simulate the Earth's magnetosphere, which is why such experiments have now been completely replaced by
computer simulations.
Footnotes
Further Information
Get more info on 'Terrella'.
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