Dehaasa

Dehaasa, a stern bald magister, holds a glowing golden coil wrapped round a hanging iron rod.

The Physics

Magnetise an iron rod and it starts to turn. In 1915 Albert Einstein and Wander de Haas hung an iron cylinder on a fine fibre inside a coil (Verh. Dtsch. Phys. Ges. 17, 152). Reversing the magnetisation flips the tiny magnets inside the iron, and their angular momentum flips with them. Angular momentum is conserved, so the whole rod twists the other way. The ratio of magnetic moment to angular momentum sets the g-factor.

L = 2mμ/(ge)

• L: angular momentum of the carriers (J·s)
• μ: magnetic moment (A·m²)
• m: electron mass (kg)
• e: elementary charge (C)
• g: gyromagnetic g-factor

Einstein and de Haas reported g ≈ 1, as expected for orbiting electrons. That was wrong: careful later measurements gave g′ ≈ 2, the mark of electron spin, which was not yet known.

Honest caveat: In iron the measured g′ is about 1.9 because a little orbital moment remains; the effect is tiny and easily swamped by stray fields, which is how the 1915 result went astray.

On the sticker

Dehaasa holds a glowing golden coil with a thin rod running through it. Printed text: DEHAASA with the line L = 2mμ/(ge). Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.