This paper describes the measured damping characteristics of a cable with perpendicularly attached magnetorheological fluid damper. First, the damping of the free cable is measured for reference. Then, the magnetorheological fluid damper is connected to the cable in order to measure the resulting damping at different constant damper current levels. The experimental data shows clearly that the optimal current level providing maximum additional damping to one targeted mode is in inverse ratio to the mode number. Since the force trajectory at constant current of the MR damper under consideration describes nearly a Coulomb friction, the viscosity of an ideal viscous damper dissipating the same amount of energy is estimated. Also this equivalent viscosity depends in inverse ratio on the mode number. If the control target is maximum damping of several modes, the damper current hardly depends on the control target. Furthermore, the measurements demonstrate that external dampers lead to an increase of the structural resonance frequencies since MR dampers producing large forces at high current levels represent additional, fairly stiff supports. This is in contradiction to enhanced structural damping which evokes decreasing resonance frequencies.