Terrella
Encyclopedia
Magnet
A 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, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
ised model ball representing the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, that is thought to have been invented by the English physician
Physician
A physician is a health care provider who practices the profession of medicine, which is concerned with promoting, maintaining or restoring human health through the study, diagnosis, and treatment of disease, injury and other physical and mental impairments...
William Gilbert while investigating magnetism, and further developed 300 years later by the Norwegian
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
scientist and explorer Kristian Birkeland
Kristian Birkeland
Kristian 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 magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
, but have now been replaced by computer simulation.
William Gilbert's terrella
Royal College of Physicians
The Royal College of Physicians of London was founded in 1518 as the College of Physicians by royal charter of King Henry VIII in 1518 - the first medical institution in England to receive a royal charter...
to Queen Elizabeth I, devoted much of his time, energy and resources to the study of the Earth's magnetism
Earth's magnetic field
Earth's magnetic field is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun...
. It had been known for centuries that a freely suspended compass
Compass
A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the earth. The frame of reference defines the four cardinal directions – north, south, east, and west. Intermediate directions are also defined...
needle pointed north. Later investigators (including Christopher Columbus
Christopher Columbus
Christopher Columbus was an explorer, colonizer, and navigator, born in the Republic of Genoa, in northwestern Italy. Under the auspices of the Catholic Monarchs of Spain, he completed four voyages across the Atlantic Ocean that led to general European awareness of the American continents in the...
) found that direction deviated somewhat from true north, and Robert Norman
Robert Norman
Robert Norman was a 16th century-English mariner, compass builder, and hydrographer who discovered magnetic inclination, the deviation of the Earth's magnetic field from the vertical.- Work :...
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 lodestone
Lodestone
A lodestone or loadstone is a naturally magnetized piece of the mineral magnetite. They are naturally occurring magnets, that 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 needle
Dip circle
Dip 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 Tellure
De Magnete
De 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 Aaron Dowling...
, published in 1600.
Kristian Birkeland's terrella
Kristian Birkeland
Kristian 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 ray
Cathode ray
Cathode rays are streams of electrons observed in vacuum tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite of the negative electrode is observed to glow, due to electrons emitted from and travelling perpendicular to the cathode Cathode...
s, later identified as electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
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 Størmer
Carl Størmer
Fredrik 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 magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
, the space region controlled by the Earth's magnetism. Birkeland believed the electrons came from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
, since large auroral outbursts were associated with sunspot
Sunspot
Sunspots are temporary phenomena on the photosphere of the Sun that appear visibly as dark spots compared to surrounding regions. They are caused by intense magnetic activity, which inhibits convection by an effect comparable to the eddy current brake, forming areas of reduced surface temperature....
activity.
Birkeland constructed several terrellas. One large terrella experiment was reconstructed in Tromsø
Tromsø
Tromsø is a city and municipality in Troms county, Norway. The administrative centre of the municipality is the city of Tromsø.Tromsø city is the ninth largest urban area in Norway by population, and the seventh largest city in Norway by population...
, Norway
Norway
Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...
.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.Baron Karl von Reichenbach, February 12, 1788, Stuttgart - January 1869. The Baron used an electromagnet, placed within a large hollow iron sphere, and this was examined in the darkroom under varying degrees of electrification. The Baron referred to the iron globe as his "terrella", or, "little earth".
Brunberg and Dattner in Sweden
Sweden
Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....
, around 1950, used a terrella to simulate trajectories of particles in the Earth's field; today such simulations are conducted much more conveniently with computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
s. Podgorny in the Soviet Union
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
, around 1972, built terrellas at which a flow of plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
was directed (simulating the solar wind), and Hafiz-Ur Rahman at the University of California, Riverside
University of California, Riverside
The 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. UCR is consistently ranked as one of the most ethnically and economically diverse universities in the United...
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 simulation
Computer simulation
A 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.