Editing Terrella (section)

==Kristian Birkeland's terrella==
[[Image:Birkeland-terrella.jpg|thumb|right|Kristian Birkeland and his magnetized terrella experiment, which led him to surmise that charged particles interacting with the Earth's magnetic field were the cause of the aurora.<ref name=NAPE />]]
[[Kristian Birkeland]] was a Norwegian physicist who, around 1895, tried to explain why the lights of the [[Aurora (phenomenon)|polar aurora]] appeared only in regions centered at the magnetic poles.

He simulated the effect by directing [[cathode ray]]s (later identified as [[electron]]s) at a terrella in a vacuum tank, 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 directly above the poles themselves. Birkeland believed the electrons came from the [[Sun]], since large auroral outbursts were associated with [[sunspot]] activity.{{Citation needed|date=December 2012}}

Birkeland constructed several terrellas. One large terrella experiment was reconstructed in [[Tromsø]], [[Norway]].<ref>{{cite journal |author=Terje Brundtland |title=The Birkeland Terrella |journal=Sphæra |issue=7 |url=http://www.mhs.ox.ac.uk/sphaera/index.htm?issue7/articl6 |access-date=24 May 2012}}</ref>