Quite some time ago, Melde (experimentally) and Rayleigh (theoretically) investigated one of the earliest examples of a parametric oscillator. They showed that a periodic modulation of a string's length (longitudinal excitation) gives rise to transverse string vibrations. A similar effect is observed when a periodic torsional modulation is applied to a wire; i.e. twisting a wire can parametrically excite transverse vibrations.