Some Experiments below Resonance with a Mechanical Parametric Amplifier

It is shown that the parametric action produced in a mechanical circuit by a spring with time-variable stiffness can increase the subresonant or long period response of a pendulum. The theory of operation of such a mechanical parametric amplifier is described in terms of Mathieu´s equation. The construction of the device in the form of a seismometer is described. Experimental data are given showing a two order of magnitude increase in low-frequency response.