The Spiral in the Chamber

Plate 51 speaks for itself: “Read from a curve.”

The Physics

A charged particle in a magnetic field curves with a radius set by its momentum. In a bubble or cloud chamber, measuring the curvature gives the momentum; energy loss tightens the track into a spiral.

p ≈ 0.3 B r

• p: momentum, GeV/c
• B: magnetic flux density, T
• r: radius of curvature, m

1 T, r = 1 m → 0.3 GeV/c. p in GeV/c, B in T, r in m (unit charge; 0.3 is 0.29979).

Sources

• Particle Data Group, Review of Particle Physics (2024), kinematics section.

On the poster

Plate 051, “The Spiral in the Chamber”: the pixel-art plate with its slogan “Read from a curve.”, the key equation and the facts panel shown above. Pixelated Physics. Physics. Personified.