The results of the poloidal field (PF) coil efficiency measurements and the values for stray field compensation during premagnetization are presented. The results have been verified by field calculations and compared with plasma breakdown experiments. Determination of the vertical field in the plasma center produced by PF coils is essential for the definition of breakdown conditions and for the control of the horizontal plasma position and of the plasma shape in tokamaks. The electron beam technique has been chosen for the measurements, providing sufficient precision and visibility. Magnetic field lines became visible due to the effect of electron movement in a magnetic field and light emission in a gas. Vertical fields were determined from the measured toroidal field and vertical electron beam displacements. Precise determination of the values for the stray field compensation was achieved by detection of very low stray fields from the deflection of the electron beam.