Galvana

Galvana, a sea captain of the cable age, reads faint telegraph signals by the swing of a beam of light.

The Physics

William Thomson, later Lord Kelvin, developed the mirror galvanometer around the 1858 transatlantic telegraph cable, whose signals were too faint for ordinary needles. A tiny mirror carrying small magnets hangs inside a coil; a current twists it slightly, and a beam of light reflected from it sweeps across a scale. By the law of reflection, turning a mirror by an angle θ turns the reflected beam by twice that angle.

beam deflection = 2θ

• θ: rotation angle of the mirror (rad)
• 2θ: rotation angle of the reflected beam (rad)

A long light beam acts as a weightless pointer: on a scale a metre away, even a tiny twist of the mirror moves the spot visibly.

Honest caveat: The doubling holds for rotation about an axis in the mirror's plane and perpendicular to the beam's plane. The sensitivity comes from the coil and suspension as well as from the optical lever.

On the sticker

Galvana, in a captain's cap, watches a bright beam of light; the mirror and the 2θ angle are not drawn. Printed text: GALVANA with the line BEAM TURNS 2θ. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.