AbstractPolarization measurements of the Balmer α line of a beam emission are performed using a newly developed four-polarizer optical system on the JIPP T-IIU tokamak. Poloidal magnetic field profile in a plasma is obtained successfully in current ramp-up experiments. Electron density dependence of the relative emissivity of σ and π components in the motional Stark spectra is also investigated experimentally. The results suggest that the relative emissivity is independent of the electron density in the range of 2 × 1019-9 × 1019 m−3.