A simple photocell counter for the determination of the logarithmic decrement of a torsional pendulum

A simple device is described, using four photocells placed along the scale of an optical lever, to count directly the number of swings that a freely oscillating torsional pendulum takes to decay to half amplitude. Using a single pen recorder, a trace is obtained recording the number of swings, with an accuracy to better than one swing. This represents an error of 0.25-1% in the logarithmic decrement range 0.003-0.015.