Terrella

thumb|Kristian Birkeland's magnetised terrella. In this experiment, he noted two spirals which he considered may be similar to that of spiral nebulae. thumb|An example of an active terrella

A terrella () is a small magnetised model ball representing the Earth, that is thought to have been invented by the English physician William Gilbert while investigating magnetism, and further developed 300 years later by the Norwegian scientist and explorer Kristian Birkeland, while investigating the aurora.