Ørsted's Needle

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.