<P> Kater built a reversible pendulum (shown at right) consisting of a brass bar with two opposing pivots made of short triangular "knife" blades (a) near either end . It could be swung from either pivot, with the knife blades supported on agate plates . Rather than make one pivot adjustable, he attached the pivots a meter apart and instead adjusted the periods with a moveable weight on the pendulum rod (b, c). In operation, the pendulum is hung in front of a precision clock, and the period timed, then turned upside down and the period timed again . The weight is adjusted with the adjustment screw until the periods are equal . Then putting this period and the distance between the pivots into equation (1) gives the gravitational acceleration g very accurately . </P> <P> 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 . </P> <P> Reversible pendulums (known technically as "convertible" pendulums) employing Kater's principle were used for absolute gravity measurements into the 1930s . </P> <P> The increased accuracy made possible by Kater's pendulum helped make gravimetry a standard part of geodesy . Since the exact location (latitude and longitude) of the' station' where the gravity measurement was made was necessary, gravity measurements became part of surveying, and pendulums were taken on the great geodetic surveys of the 18th century, particularly the Great Trigonometric Survey of India . </P>

What is the period of a simple pendulum 77 cm long on the earth