An narrow electron beam is generated inside a glass bulb. Ionization of a rarefied gas inside the bulb makes the beam visible. When an external magnetic field is applied via a pair of Helmholtz coils the electron beam assumes a circular path. The strength of the magnetic field, the electric accelerating potential, and the curvature of the electron beam are sufficient to calculate the ratio of the electric charge to the mass of the electron.

K3

PIRA DCS 5H30.15