Inertix

Inertix, a hooded spinner, brakes a whirling coil glowing cyan at its centre.

The Physics

Spin a coil of wire fast and stop it suddenly: the free charges inside keep going for a moment, like passengers in a braking bus, and a pulse of current flows. Richard Tolman and Thomas Stewart measured it in 1916 (Phys. Rev. 8, 97). From the charge that flowed they got the carriers' charge-to-mass ratio, close to that of the electron, showing that current in metals is carried by electrons.

Q = (m/e) v₀ L / R

• Q: charge that flows after stopping (C)
• m/e: mass-to-charge ratio of the carriers (kg/C)
• v₀: rim speed of the wire before stopping (m/s)
• L: length of the wire (m)
• R: resistance of the coil (Ω)

It is the mechanical twin of the Einstein–de Haas effect: inertia of the carriers shows up as an electrical signal.

Honest caveat: The effect is tiny and needs careful compensation of Earth's field; their m/e was within about 20% of the electron value.

On the sticker

Inertix holds a large spinning coil with a glowing cyan core. Printed text: INERTIX with the line STOP = CURRENT. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.