|| This torsional oscillator is one of the few pieces of physics
apparatus that I have seen from the firm of Gurley of Troy, New York, best
known for its surveying instruments. It is in the demonstration collection
at the University of Texas in Austin.
A very similar piece of apparatus is in the 1928 Central Scientific Company of Chicago, and is sitting next to me as I write. The vertical shaft is pivoted, and the masses on the sliding cross-rod can be slid in and out to vary the moment of inertia of the oscillating system. The spiral spring provides both a zero point, and a linear restoring torque in both angular directions.
The torsion constant of the spring is determined by hanging various masses on one end of the string; the other end is wrapped around a pulley of known radius. The period of the system can be measured as a function of the moment of inertia of the system: the square of the period is proportional to the moment of inertia.
The torsional oscillator at the left is by Central Scientific and is in the collection of Hobart and William Smith Colleges in Geneva, New York.