Arago's Rotating Disc

Plate 17 speaks for itself: “Spin the copper and the needle follows, though copper isn't magnetic.”

The Physics

A spinning copper disc drags a magnetic needle hung above it. Copper is not ferromagnetic; motion through the needle's field drives eddy currents in the disc, and their field pulls the needle along.

J = σ (E + v × B)

• J: current density, A m⁻²
• σ: electrical conductivity, S m⁻¹
• E: electric field, V m⁻¹
• v: conductor velocity, m s⁻¹
• B: magnetic flux density, T

Eddy currents drag the needle. Arago, 1824. Faraday explains it, 1831.

Honest caveat: Copper is weakly diamagnetic, but that is not what drives the effect.

Sources

• M. Faraday, Phil. Trans. R. Soc. 122, 125 (1832).

On the poster

Plate 017, “Arago's Rotating Disc”: the pixel-art plate with its slogan “Spin the copper and the needle follows, though copper isn't magnetic.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.