Fedchenko Clock

a highly accurate pendulum clock with an isochronous suspension and an electromagnetically driven pendulum.

The pendulum of a Fedchenko clock consists of a copper bob mounted on an Invar rod and is enclosed in a low-pressure vacuum chamber. Isochronism is ensured by the suspension, which is designed in such a way that the length of the pendulum decreases as the amplitude of the pendulum’s swing increases. The pendulum is kept in motion by short electromagnetic pulses that are imparted to it at the moment of equilibrium. The shape of the copper bob and the point where the bob is supported are chosen so that temperature-induced changes in the size of the bob do not affect the period of the pendulum. The temperature-compensating elements of the pendulum are on the suspension and below the bob; such an arrangement ensures temperature compensation when the temperature gradient is variable.

Fedchenko clocks have the highest accuracy and reliability of any pendulum clock; the clock rate is 0.2–0.3 millisecond per day. The clocks are used as timekeepers and to study variations in the acceleration of gravity. Fedchenko clocks, which are named after their inventor, F. M. Fedchenko, came into use in the late 1950’s and early 1960’s.