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Experiments on the quantitative determination of the ion drag force of an ion beam on free-falling dust particles in a thermionic filament discharge are presented. The ion beam component of the plasma is oriented perpendicular to the dominating gravitational force. Therefore, the ion drag force is directly given by the observed particle deflection from the vertical direction. The deflection angle is measured as a function of beam energy. The obtained results are compared with theory. Good agreement is found with the Barnes model of the ion drag force, when the linearized Debye-length λD is used.