A light vertical spindle carries a steel rod on top of it. The rod is horizontal and can be clamped to the spindle at any point along its length. A thread wound around the spindle passes over the pulley to a weight. The weight is supported on a lever which actuates a microswitch. Releasing the weight starts a Hunter klockounter, which is stopped when the weight hits a second microswitch after falling through a distance of about 40 cm. In addition to measuring the moment of inertia of the rod about various axes through the rod, the moment of inertia can be determined with a cylindrical weight positioned at any point along the rod.