text: The BVS apparatus consists of a specimen surrounded by Helmholtz coils and isolated from external vibrations by a framework constructed from insulating foam and either lead or brass. The specimen is affixed with both a permanent magnet and a mirror. The orientation of the coils with respect to the magnet when a current is driven through them determines whether the specimen undergoes bending or torsion. Angular displacement of the specimen is measured by an interferometer that detects the spatial movement of a reflected laser. This spatial waveform is converted to an electrical one by a light detector and read out on an oscilloscope. This oscilloscope also displays the torque or force waveform from the capacitor driving the current in the Helmholtz coils. Phase delay is determined by comparing these waveforms.
  Resonance is minimized through the use of short specimens—which have higher resonant frequencies—and by reducing the inertia (magnetic and mass moments) of the magnet. Cubic samarium-cobalt magnets are ideal for high frequency studies. Due to the sample geometry being a short rectangular bar or cylinder, the equation governing the resonance of the BVS specimen geometry has an exact analytic solution, which allows the technique to yield results even for high loss materials. This exact solution provides a relationship between dynamic moduli, angular displacement, and geometric parameters. The inherent lack of drift and friction in the apparatus is responsible for its large range of operating frequencies.