A supersonic plasma flow through the diverging lines of magnetic force is investigated experimentally. The Hall parameter of electrons is greater than unity, but that of ions is much smaller than unity. Measurements are made of the floating potential, electron temperature, ion density and ion velocity. The expansion of the plasma flow is suppressed by the external magnetic field. When the stream lines and the lines of magnetic force intersect, the electron temperature is higher at the downstream; when they are parallel to each other, a large potential difference occurs along the streamline, and equi-potential lines are crossed by the streamline. An extraordinary speed of ions is caused by the electrothermal acceleration. The strong electric field intensity along the streamlines agrees with the values estimated by the generalized Ohm's law.