Question: what is the period of a simple pendulum 77 cm long on the earth

title: Pendulum
text: Gulf gravimeter : One of the last and most accurate pendulum gravimeters was the apparatus developed in 1929 by the Gulf Research and Development Co . It used two pendulums made of fused quartz , each 10.7 inches ( 270 mm ) in length with a period of 0.89 second , swinging on pyrex knife edge pivots , 180 ° out of phase . They were mounted in a permanently sealed temperature and humidity controlled vacuum chamber . Stray electrostatic charges on the quartz pendulums had to be discharged by exposing them to a radioactive salt before use . The period was detected by reflecting a light beam from a mirror at the top of the pendulum , recorded by a chart recorder and compared to a precision crystal oscillator calibrated against the WWV radio time signal . This instrument was accurate to within ( 0.3 -- 0.5 ) × 10 ( 30 -- 50 microgals or 3 -- 5 nm / s ) . It was used into the 1960s .