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Rowlanda
Rowland’s Rotating Charged Disc Physics
Rowlanda, a bearded experimenter, spins a glowing charged disc beside a compass.
The Physics
In 1876, in Helmholtz's Berlin laboratory, Henry Rowland charged an ebonite disc and spun it fast. A magnetic needle nearby deflected (Am. J. Sci. 15, 30, 1878). A moving charge makes a magnetic field, just as a current in a wire does: current is charge in motion.
B ∝ q v
- B: magnetic field near the disc (T)
- q: charge on the disc (C)
- v: rim speed (m/s)
It answered whether ‘convection’ of charge counts as a current: it does.
Honest caveat: The deflection was tiny, a fraction of Earth's field; the full Biot–Savart form is needed for real numbers.
On the sticker
Rowlanda spins a glowing cyan disc in one hand, with a brass compass beside it. Printed text: ROWLANDA. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.
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Details
Size3″ × 3″
3″ × 3″ kiss-cut sticker.
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