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Kater's pendulum

Kater timed the swing of his pendulum using the " method of coincidences " and measured the distance between the two pivots with a micrometer. After applying corrections for the finite amplitude of swing, the buoyancy of the bob, the barometric pressure and altitude, and temperature, he obtained a value of 39.13929 inches for the seconds pendulum at London, in vacuum, at sea level, at 62 °F. The largest variation from the mean of his 12 observations was 0.00028 in. representing a precision of gravity measurement of 7×10 (7 mGal or 70 μm/s). Kater's measurement was used as Britain's official standard of length (see below) from 1824 to 1855.