Barnetta

Barnetta, a hooded spinner, whirls a glowing iron rod upright in his hands.

The Physics

Spin an unmagnetised iron rod and it becomes magnetised. Samuel Barnett showed this in 1915 (Phys. Rev. 6, 239): rotation acts on the electrons' angular momentum like a magnetic field, so their spins tend to line up. It is the reverse of the Einstein–de Haas effect, and gave g ≈ 2 for iron.

M ∝ Ω

• M: magnetisation (A/m)
• Ω: angular speed of rotation (rad/s)

The equivalent field is B = Ω/γ, with γ the gyromagnetic ratio; it is tiny at practical speeds.

Honest caveat: The proportionality holds for small Ω, and the effect is easily masked by Earth's field.

On the sticker

Barnetta grips a spinning rod wrapped in glowing cyan rings. Printed text: BARNETTA with the line M ∝ Ω. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.