The first experimental study of the Schwinger interaction of polarized neutrons with an electric field of a noncentrosymmetric perfect crystal (α-quartz) was made for two wave dynamical diffraction. Phase shifts of Pendellösung fringes for two different spin to crystal field orientations were measured. The theory of the effect is given. The calculated value of SiO₂ (1120) interplanar electr field, seen by the diffracted neutron, is 2.1 × 10⁸ V/cm. This is in good agreement with the experimental result: (1.8 ± 0.3) × 10⁸ V/cm.