A very low-frequency pendulum for measurements of helium density in aerogel

We present a new method to measure helium density in aerogel, based on the measurement of the resonance frequency of a simple pendulum. The pendulum i s excited in the same manner as a vibrating wire; its displacements are mea sured with an optical method. The frequency ranges from 3 Hz in vacuum down to 1.5 Hz in liquid He-I. A relative accuracy of 10(-4) is achieved. Data at the liquid-vapour equilibrium in a 5% silica aerogel are presented. (C) 2000 Elsevier Science B.V. All rights reserved.