Daily entries from the 17th century London diary
If you would like to write a summary for this topic, email phil [at] gyford [dot] com
Wikipedia
A terrella (meaning "little earth") is a small magnetised model ball representing the Earth, that is 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Terrellas had been used up until the late 20th century to attempt to simulate the Earth's magnetosphere, but have now been replaced by computer simulation.
[edit] 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 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. 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.
[edit] 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.[1].
[edit] 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.
[edit] Footnotes
- ^ Kristian Birkeland., The Norwegian Aurora Polaris Expedition 1902-1903, Section 2. Chapter VI (See page 678) Extract online
- ^ Kristian Birkeland., The Norwegian Aurora Polaris Expedition 1902-1903
[edit] External links
1893 text
Professor Silvanus P. Thompson, F.R.S., has kindly supplied me with the following interesting note on the terrella (or terella): The name given by Dr. William Gilbert, author of the famous treatise, “De Magnete” (Lond. 1600), to a spherical loadstone, on account of its acting as a model, magnetically, of the earth; compass-needles pointing to its. poles, as mariners’ compasses do to the poles of the earth. The term was adopted by other writers who followed Gilbert, as the following passage from Wm. Barlowe’s “Magneticall Advertisements” (Lond. 1616) shows: “Wherefore the round Loadstone is significantly termed by Doct. Gilbert Terrella, that is, a little, or rather a very little Earth: For it representeth in an exceeding small model (as it were) the admirable properties magneticall of the huge Globe of the earth” (op. cit, p. 55). Gilbert set great store by his invention of the terrella, since it led him to propound the true theory of the mariners’ compass. In his portrait of himself which he had painted for the University of Oxford he was represented as holding in his hand a globe inscribed terella. In the Galileo Museum in Florence there is a terrella twenty-seven inches in diameter, of loadstone from Elba, constructed for Cosmo de’ Medici. A smaller one contrived by Sir Christopher Wren was long preserved in the museum of the Royal Society (Grew’s “Rarities belonging to the Royal Society,” p. 364). Evelyn was shown “a pretty terrella described with all ye circles and skewing all y magnetic deviations” (Diary, July 3rd, 1655).
Annotations
-
Terrella in the Wikipedia
http://en.wikipedia.org/wiki/TerrellaWilliam Gilbert’s ‘Terrella’
http://www.acmi.net.au/AIC/TERRELLA.htmlWilliam Gilbert (or Gilberd, as he wrote it…)
http://en.wikipedia.org/wiki/William_GilbertWilliam Gilbert aka William of Colchester “set out to debunk magical notions of magnetism, yet in building an intellectual bridge between natural philosophy and emerging sciences, he did not completely abandon reference to the occult. For example, he believed that an invisible ‘orb of virtue’ [force] surrounds a magnet and extends in all directions around it. Other magnets and pieces of iron react to this orb of virtue and move or rotate in response. Magnets within the orb are attracted whereas those outside are unaffected. The source of the orb remained a mystery. Although his language was that of the natural philosophy of the time, some of his ideas were ahead of his time. His orbs of virtue were a fledgling notion of the idea of fields that would revolutionize physics more than two centuries later.”
http://chem.ch.huji.ac.il/~eugeniik/history/gilbert.html
-
*De Magnete*
http://en.wikipedia.org/wiki/De_Magnete‘De Magnete’ page 155
http://www.new-science-theory.com/william-gilbert-de-magnete.htmlHalley and the ‘Paramour’[and *De Magnete* p. 192]
http://www.nmm.ac.uk/server/show/conWebDoc.389
-
De Magnete by William Gilbert [English trans. excerpts ]
[ concerning loadstones and their magnetic properties ] http://campus.udayton.edu/~hume/Gilbert/gilbert.htm -
Exposure to Sam on his first visit to this august group, but mentioned by John Evelyn.Wednesday 23 January 1660/61
“With [Greatorex] to Gresham Colledge (where I never was before), and saw the manner of the house, and found great company of persons of honour there”
http://www.pepysdiary.com/archive/1661/01/23/#c10903
:[jan 1661 J Evelyn ]23. To Lond, at our Society, where was divers Exp: on the Terrella sent us by his Majestie
-
I think you have an very interesting article of the phenomena Terrella her and as a Norwegian of course I know about Birkeland’s Terella.
I’m a blogger and write about Norway, our cultures, traditions and habits and call it my Terella. My little earth or the world seen trough my eyes if you like.