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English: A Shortt-Synchronome free pendulum clock belonging to the US National Institute of Standards and Technology (NIST), in the NIST Museum, Gaithersburg, Maryland, USA. Invented in 1921 by William Hamilton Shortt, it was the most accurate mechanical clock ever manufactured, achieving an accuracy of around one second per year. This example was purchased by NIST in 1929 and used by physicist Paul R. Heyl in his second determination of the gravitational constant. It consists of a master invar pendulum (left) in a copper vacuum tank and a precision pendulum clock (right). The units are connected by electric wires that operate electromagnets to keep the slave pendulum in the clock synchronized to the master pendulum. The pendulum in the clock is attached to the clock's mechanism and performs the function of moving the clock's gears, leaving the master pendulum to swing virtually free of outside influence. The pendulums make one swing per second, and every 30 swings the slave clock gives the master pendulum a push to keep it swinging. Alterations to image: brightened slightly, changed from GIF to JPEG.