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Ørsted's Needle
Plate 016 Physics Poster
Plate 16 speaks for itself: “The needle turned, and two forces became one.”
The Physics
Ørsted saw a compass needle swing when current ran through a nearby wire: electric currents make magnetic fields. The field circles a long straight wire and falls off as 1/r.
B = μ₀ I / (2π r)
- B: magnetic flux density, T
- μ₀: vacuum permeability ≈ 1.25664×10⁻⁶ N A⁻²
- I: current, A
- r: distance from the wire, m
A current turns a compass. Copenhagen, 21 July 1820.
Sources
- H. C. Ørsted, Experimenta circa effectum conflictus electrici in acum magneticam (21 July 1820).
On the poster
Plate 016, “Ørsted's Needle”: the pixel-art plate with its slogan “The needle turned, and two forces became one.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.
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