Apparatus for Direct Determination and Continuous Recording of Internal Friction at Constant Amplitude
Richard A. Butera, Raymond S. Craig · Review of Scientific Instruments · 1966
An apparatus is described in which internal friction at constant amplitude is continuously determined in terms of directly measured electrical and mechanical parameters of the system without recourse to a secondary method of calibration. The specimen, in the form of a wire, serves as the torsion member in a torsion pendulum which is maintained in sustained oscillations by an electromechanical system employing positive feedback. Attached to the inertia member of the torsion pendulum are two coaxial coils which are located in the field of a permanent magnet. The emf generated in one of the coils (pickup coil) by the motion in the field is amplified, converted into a square wave and fed back into the other coil (the driver) through a gain control circuit. The power fed back into the driver coil is controlled by a servoloop in such a way as to maintain the amplitude of oscillation constant. The internal friction is proportional to the setting of the gain control unit in the driver circuit and is displayed continuously on a strip chart recorder. Internal friction values measured on this device agree to within better than 2% with those measured from the logarithmic decrement.