  Planar, double-torsional oscillators are especially suitable for short-range
macroscopic force search experiments, since they can be operated at the limit
of instrumental thermal noise. As a study of this limit, we report a
measurement of the noise kinetic energy of a polycrystalline tungsten
oscillator in thermal equilibrium at room temperature. The fluctuations of the
oscillator in a high-Q torsional mode with a resonance frequency near 1 kHz are
detected with capacitive transducers coupled to a sensitive differential
amplifier. The electronic processing is calibrated by means of a known
electrostatic force and input from a finite element model. The measured average
kinetic energy is in agreement with the expected value of 1/2 kT.
