In a vacuum tube, two identical and parallel Ag-O-Cs surfaces, A and B, with a work function of 0.8eV, ceaselessly emit thermal electrons at room temperature. The thermal electrons are controlled by a static uniform magnetic field (a magnetic demon), and the number of electrons migrate from A to B exceeds the one from B to A, (or vice versa). The net migration from A to B quickly results in a charge distribution: A charged positively and B negatively. A potential difference between A and B emerges, and the tube outputs ceaselessly an electric current and a power to a resistance (a load) and cools itself slightly. The ambient air is a single heat reservoir in the experiment, and all the heat extracted by the tube from the air is converted into electric energy without producing other effect. We believe the experiment is in contradiction to the Kelvin statement of the second law.