A
terrella (meaning "little earth") is a small
magnetisedA magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials and attracts or repels other magnets.A permanent magnet is an object made from a...
model ball representing the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
, that is thought to have been invented by
EnglishmanEngland is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west and the North Sea to the east, with the English Channel to the south separating it from continental...
physicianA physician — also known as medical practitioner, doctor of medicine, medical doctor, or simply doctor — practices the ancient profession of medicine, which is concerned with maintaining or restoring human health through the study, diagnosis, and treatment of disease or injury...
William GilbertWilliam Gilbert, also known as Gilbard, was an English physician and natural philosopher. He was an early Copernican, and passionately rejected both the prevailing Aristotelian philosophy and the Scholastic method of university teaching...
while investigating magnetism, and further developed 300 years later by the
NorwegianNorway , officially the Kingdom of Norway, is a country in Northern Europe occupying the western portion of the Scandinavian Peninsula, as well as Jan Mayen and the Arctic archipelago of Svalbard under the Spitsbergen Treaty...
scientist and explorer
Kristian BirkelandKristian Olaf Birkeland was a Norwegian scientist. He is best remembered as the person who first elucidated the nature of the Aurora borealis. In order to fund his research on the aurorae, he invented the electromagnetic cannon and the Birkeland-Eyde process of fixing nitrogen from the air...
, while investigating the aurora.
Terrellas had been used up until the late 20th century to attempt to simulate the Earth's
magnetosphereA magnetosphere is a highly magnetized region around and possessed by an astronomical object. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede is magnetized, but too weak to trap solar wind plasma. Mars has...
, but have now been replaced by computer simulation.
William GilbertWilliam Gilbert, also known as Gilbard, was an English physician and natural philosopher. He was an early Copernican, and passionately rejected both the prevailing Aristotelian philosophy and the Scholastic method of university teaching...
, the
royal physicianThe Royal College of Physicians of London was the first medical institution in England to receive a Royal Charter. It was founded in 1518 and is one of the most active of all medical professional organisations...
to Queen Elizabeth I, devoted much of his time, energy and resources to the study of the
Earth's magnetismEarth's magnetic field is approximately a magnetic dipole, with the magnetic field S pole near the Earth's geographic north pole and the other magnetic field N pole near the Earth's geographic south pole...
.
A
terrella (meaning "little earth") is a small
magnetisedA magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials and attracts or repels other magnets.A permanent magnet is an object made from a...
model ball representing the
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
, that is thought to have been invented by
EnglishmanEngland is a country that is part of the United Kingdom. It shares land borders with Scotland to the north and Wales to the west; the Irish Sea is to the north west, the Celtic Sea to the south west and the North Sea to the east, with the English Channel to the south separating it from continental...
physicianA physician — also known as medical practitioner, doctor of medicine, medical doctor, or simply doctor — practices the ancient profession of medicine, which is concerned with maintaining or restoring human health through the study, diagnosis, and treatment of disease or injury...
William GilbertWilliam Gilbert, also known as Gilbard, was an English physician and natural philosopher. He was an early Copernican, and passionately rejected both the prevailing Aristotelian philosophy and the Scholastic method of university teaching...
while investigating magnetism, and further developed 300 years later by the
NorwegianNorway , officially the Kingdom of Norway, is a country in Northern Europe occupying the western portion of the Scandinavian Peninsula, as well as Jan Mayen and the Arctic archipelago of Svalbard under the Spitsbergen Treaty...
scientist and explorer
Kristian BirkelandKristian Olaf Birkeland was a Norwegian scientist. He is best remembered as the person who first elucidated the nature of the Aurora borealis. In order to fund his research on the aurorae, he invented the electromagnetic cannon and the Birkeland-Eyde process of fixing nitrogen from the air...
, while investigating the aurora.
Terrellas had been used up until the late 20th century to attempt to simulate the Earth's
magnetosphereA magnetosphere is a highly magnetized region around and possessed by an astronomical object. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede is magnetized, but too weak to trap solar wind plasma. Mars has...
, but have now been replaced by computer simulation.
William Gilbert's terrella
William GilbertWilliam Gilbert, also known as Gilbard, was an English physician and natural philosopher. He was an early Copernican, and passionately rejected both the prevailing Aristotelian philosophy and the Scholastic method of university teaching...
, the
royal physicianThe Royal College of Physicians of London was the first medical institution in England to receive a Royal Charter. It was founded in 1518 and is one of the most active of all medical professional organisations...
to Queen Elizabeth I, devoted much of his time, energy and resources to the study of the
Earth's magnetismEarth's magnetic field is approximately a magnetic dipole, with the magnetic field S pole near the Earth's geographic north pole and the other magnetic field N pole near the Earth's geographic south pole...
. It had been known for centuries that a freely suspended
compassA compass is a navigational instrument for determining direction relative to the Earth's magnetic poles. It consists of a magnetized pointer free to align itself with Earth's magnetic field. The compass greatly improved the safety and efficiency of travel, especially ocean travel...
needle pointed north. Later investigators (including
Christopher ColumbusChristopher Columbus was a navigator, colonizer and explorer whose voyages across the Atlantic Ocean led to general European awareness of the American continents in the Western Hemisphere...
) found that direction deviated somewhat from true north, and
Robert NormanRobert Norman was a 16th century-English mariner, compass builder, and hydrographer.- Work :Robert Norman is noted for The Newe Attractive, a pamphlet published in 1581 describing the lodestone and practical aspects of navigation. More importantly, it included Norman's discovery of magnetic dip,...
showed the force on the needle was not 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
lodestoneA lodestone or loadstone is a naturally magnetized piece of the mineral magnetite. They are naturally occurring magnets, and attract pieces of iron. Ancient people first discovered the property of magnetism in lodestone...
. Passing a small compass over the terrella, Gilbert demonstrated that a horizontal compass would point towards the magnetic pole, while a
dip needleDip circles are used to measure the angle between the horizon and the Earth's magnetic field . They were used in surveying, mining and prospecting as well as for the demonstration and study of magnetism....
, 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 TellureDe Magnete, Magneticisque Corporibus, et de Magno Magnete Tellure is a scientific work published in 1600 by the English physician and scientist William Gilbert and his partner Christopher Clews...
, published in 1600.
Kristian Birkeland's terrella
Kristian BirkelandKristian Olaf Birkeland was a Norwegian scientist. He is best remembered as the person who first elucidated the nature of the Aurora borealis. In order to fund his research on the aurorae, he invented the electromagnetic cannon and the Birkeland-Eyde process of fixing nitrogen from the air...
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 raysCathode rays are streams of electrons observed in vacuum tubes, i.e. evacuated glass tubes that are equipped with at least two metal electrodes to which a voltage is applied, a cathode or negative electrode and an anode or positive electrode...
, later identified as
electronAn electron is a subatomic particle that carries a negative electric charge. It has no known substructure and is believed to be a point particle. An electron has a mass that is approximately 1836 times less than that of the proton. The intrinsic angular momentum of the electron is a half integer...
s, 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 StørmerFredrik Carl Mülertz Størmer was a Norwegian mathematician and physicist, known both for his work in number theory and for studying the movement of charged particles in the magnetosphere and the formation of aurorae....
(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
magnetosphereA magnetosphere is a highly magnetized region around and possessed by an astronomical object. Earth is surrounded by a magnetosphere, as are the magnetized planets Mercury, Jupiter, Saturn, Uranus, and Neptune. Jupiter's moon Ganymede is magnetized, but too weak to trap solar wind plasma. Mars has...
, the space region controlled by the Earth's magnetism. Birkeland believed the electrons came from the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
, since large auroral outbursts were associated with
sunspotA sunspot is an area on the Sun's surface that is marked by intense magnetic activity, which inhibits convection, forming areas of reduced surface temperature. They can be visible from Earth without the aid of a telescope...
activity.
Birkeland constructed several terrellas. One large terrella experiment was reconstructed in
Tromsøis a city and municipality in Troms county, Norway. The administrative centre of the municipality is the city of Tromsø....
,
NorwayNorway , officially the Kingdom of Norway, is a country in Northern Europe occupying the western portion of the Scandinavian Peninsula, as well as Jan Mayen and the Arctic archipelago of Svalbard under the Spitsbergen Treaty...
.
http://www.mhs.ox.ac.uk/sphaera/index.htm?issue7/articl6.
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
SwedenSweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe...
, around 1950, used a terrella to simulate trajectories of particles in the Earth's field; today such simulations are conducted much more conveniently with
computerA computer is a machine that manipulates data according to a set of instructions.Although mechanical examples of computers have existed through much of recorded human history, the first electronic computers were developed in the mid-20th century . These were the size of a large room, consuming as...
s. Podgorny in the
Soviet UnionThe Union of Soviet Socialist Republics was a constitutionally socialist state that existed in Eurasia from 1922 to 1991. The name is a translation of the , tr. Soyuz Sovetskikh Sotsialisticheskikh Respublik, abbreviated СССР, SSSR. The common short name is Soviet Union, from , Sovetskiy Soyuz...
, around 1972, built terrellas at which a flow of
plasmaIn physics and chemistry, plasma is a partially ionized gas, in which a certain proportion of electrons are free rather than being bound to an atom or molecule. The ability of the positive and negative charges to move somewhat independently makes the plasma electrically conductive so that it...
was directed (simulating the solar wind), and Hafiz-Ur Rahman at the
University of California, RiversideThe University of California, Riverside, commonly known as UCR or UC Riverside, is a public research university and one of the ten general campuses of the University of California system. The main campus sits on in a suburban district of , United States, with a branch campus of in Palm Desert...
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 simulationA computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system...
s.
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