This paper presents an experimental investigation concerning the peak amplitudes of oscillation of a square prism due to Vortex-Induced-Vibrations (VIV) as a function of the mass damping parameter $m^*{zeta}$(the so called Griffin--plot); $m^*$ and ${zeta}$ being, respectively, the non-dimensional mass and the mechanical (structural) damping ratio. With this purpose in mind, an electromagnetic actuator has been employed to provide controlled damping. During the experiments the mass--damping parameter was in the range 0.15 < $m^*{zeta}$ < 2.4. Experiments show that there is a value of $m^*{zeta}$ below which VIV appears combined with galloping and the prism oscillation increases monotonically with the incoming flow velocity. For $m^*{zeta}$ >0.3 the present experiments show a well-defined VIV phenomenon and, consequently, a Griffin-plot can be defined.