We report measurements of the ratio of the perpendicular velocity to the parallel velocity, &agr;=v⊥ /v∥, of a relativistic electron beam gyrating in a magnetic field by the use of (1) Cerenkov emission from a glass plate, detected by a gated microchannel plate image intensifier camera, and (2) electron‐beam‐induced radiation darkening pattern on the same glass plate. The measurements are based on a direct determination of the Larmor radius of an electron beam of known energy. Experiments were performed on a long‐pulse electron beam accelerator withe‐beam diode parameters:VD= 0.6–0.9 MV, pulse length=0.5–1 &mgr;s,ID= 1–10 kA. The experimental value of &agr; agrees with simulation results from particle trajectory codes as well as theoretical predictions from Busch’s theorem and adiabatic theory.