<Li> Repsold - Bessel pendulum: It was time - consuming and error - prone to repeatedly swing the Kater's pendulum and adjust the weights until the periods were equal . Friedrich Bessel showed in 1835 that this was unnecessary . As long as the periods were close together, the gravity could be calculated from the two periods and the center of gravity of the pendulum . So the reversible pendulum didn't need to be adjustable, it could just be a bar with two pivots . Bessel also showed that if the pendulum was made symmetrical in form about its center, but was weighted internally at one end, the errors due to air drag would cancel out . Further, another error due to the finite diameter of the knife edges could be made to cancel out if they were interchanged between measurements . Bessel didn't construct such a pendulum, but in 1864 Adolf Repsold, under contract by the Swiss Geodetic Commission made a pendulum along these lines . The Repsold pendulum was about 56 cm long and had a period of about ​ ⁄ second . It was used extensively by European geodetic agencies, and with the Kater pendulum in the Survey of India . Similar pendulums of this type were designed by Charles Pierce and C. Defforges . </Li> <Ul> <Li> Von Sterneck and Mendenhall gravimeters: In 1887 Austro - Hungarian scientist Robert von Sterneck developed a small gravimeter pendulum mounted in a temperature - controlled vacuum tank to eliminate the effects of temperature and air pressure . It used a "half - second pendulum," having a period close to one second, about 25 cm long . The pendulum was nonreversible, so the instrument was used for relative gravity measurements, but their small size made them small and portable . The period of the pendulum was picked off by reflecting the image of an electric spark created by a precision chronometer off a mirror mounted at the top of the pendulum rod . The Von Sterneck instrument, and a similar instrument developed by Thomas C. Mendenhall of the US Coast and Geodetic Survey in 1890, were used extensively for surveys into the 1920s . </Li> </Ul> <Li> Von Sterneck and Mendenhall gravimeters: In 1887 Austro - Hungarian scientist Robert von Sterneck developed a small gravimeter pendulum mounted in a temperature - controlled vacuum tank to eliminate the effects of temperature and air pressure . It used a "half - second pendulum," having a period close to one second, about 25 cm long . The pendulum was nonreversible, so the instrument was used for relative gravity measurements, but their small size made them small and portable . The period of the pendulum was picked off by reflecting the image of an electric spark created by a precision chronometer off a mirror mounted at the top of the pendulum rod . The Von Sterneck instrument, and a similar instrument developed by Thomas C. Mendenhall of the US Coast and Geodetic Survey in 1890, were used extensively for surveys into the 1920s . </Li> <Dl> <Dd> The Mendenhall pendulum was actually a more accurate timekeeper than the highest precision clocks of the time, and as the' world's best clock' it was used by Albert A. Michelson in his 1924 measurements of the speed of light on Mt . Wilson, California . </Dd> </Dl>

The time period of a pendulum depends on its length