An apparatus is decribed which has been used to determine the shear modulus of cylindrical specimens vibrating in the torsional mode. The specimens are maintained in self‐excited vibration by an eddy‐current driver, an eddy‐current pickup, and a feed back circuit consisting of a phase‐locked‐loop and an amplitude control module. The system has a frequency stability of 1 part in 106for high‐Qmaterials (a few thousand) in the audio range. In addition, it can automatically track frequency changes of a few percent with negligible error in phase or frequency.