In the course of some work in this laboratory a determination of the radius of gyration of an Atwood's machine pulley was necessary. The radius was found by using a bifilar suspension. Having been unable to find any published record of the method, which is capable of giving fairly accurate results, and as the measurement of moment of inertia or of radius of gyration is a frequently occurring and ill-understood exercise in a physical laboratory, it seemed to me that the following details might be of interest.