  Nonlinear effects were observed in a forced vibrating string. The motion of
the string becomes elliptic as the amplitude of the vibration increases. The
fundamental resonance frequency depends on the amplitude of the vibration. At
sufficient high amplitudes the system exhibits hysterisis: the resonance
frequency is different when measured increasing the driving frequency or
decreasing it. The experimental results are well understood in the framework of
a 2-dimensional model assuming that the string tension is not constant due to
the varying length of the string throughout the oscillation.
