Fig. 2.
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The 17 vacuum chambers used by professor Birkeland for his experiments related to the aurora borealis and other cosmic phenomena. In all these designs, the sphere was suspended. The electric duct is seen in some of these drawings as a bar crossing the chamber (i.e. Terrella n and m). Birkeland did not draw it every times (i.e. Terrella o, p or q).
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