Katers pendulum

A double pendulum? designed by Captain [Henry Kater]? (1777-1835) in 1817 to measure the acceleration of free fall so that gravity may be calculated without knowledge of the pendulum's radius of gyration. It consists of a metal bar with knife edges (for near frictionless pivot points) attached near the ends and two identical masses that can slide between the knife edges. The bar is pivoted from each knife edge in turn and the positions of the masses can be adjusted so that the period of the pendulum is the same with both. The period is then that of a simple pendulum, which allows gravity to be calculated.