Barkhausa

Barkhausa, a stone-and-iron golem, cups a crackling handful of sparks: the sound of iron being magnetised.

The Physics

Magnetise a piece of iron slowly and smoothly and its magnetisation does not change smoothly. It grows in many tiny, sudden jumps as the walls between magnetic domains snag on defects and then snap free. Heinrich Barkhausen heard this in 1919 by winding a coil around the iron and amplifying the induced pulses into a telephone receiver: a crackle (“Zwei mit Hilfe der neuen Verstärker entdeckte Erscheinungen”, Physikalische Zeitschrift 20, 401, 1919). Each jump induces a small voltage pulse in the coil.

V = −N dΦ/dt

• V: voltage induced in the pick-up coil (V)
• N: number of turns of the coil
• Φ: magnetic flux through the coil (Wb)
• t: time (s)

Zoom in on a B–H hysteresis curve and it is not a smooth line but a staircase of small steps. Barkhausen noise is used today to check steel for stress and heat damage without cutting it.

Honest caveat: The jumps come from domain walls moving, not from single atomic magnets flipping one at a time, as the early picture had it. Each step is small; you only hear them after amplification.

On the sticker

Barkhausa holds out a hand full of crackling orange sparks; the B–H steps themselves are not drawn. Printed text: BARKHAUSA with the line CRACKLE. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.