English: Copper low pressure tank containing the master pendulum of a Shortt Synchronome astronomical regulator clock , the most accurate pendulum clock ever manufactured, in the World Museum, Liverpool . The pendulum was electrically linked to a slave pendulum in a separate clock unit standing several feet away (out of shot) . The slave pendulum connected to the clock gears, leaving the master pendulum to swing free of disturbance. This clock was the highest standard of timekeeping, used as a primary time standard from the 1920s until atomic clocks became available in the 1970s. It had an accuracy of better than a second per year, sufficiently precise to measure variations in the rotational speed of the Earth. Also used in WWII to calibrate ASDIC systems. Invented by William Hamilton Shortt and made by the Synchronome Company in 1937.