First conceived by R.J.Rudd in the late nineteenth century, the idea of a 'free pendulum' was developed by William Hamilton Shortt in the early 1920s. In all clocks, the natural period of the pendulum is disturbed by the impulse needed to keep it swinging. With this system, the pendulum swings almost entirely without outside influence, sustained by a single impulse given every thirty seconds from a pair of remote 'slave' clocks. By suspending the pendulum in a partial vacuum, the usual effects caused by changes in atmospheric pressure and temperature are greatly reduced. The Shortt system was capable of keeping time to within a few seconds a year.