Pendulum Apparatus

a device to measure the acceleration of gravity by a relative method. Gravimetric studies using pendulum apparatus are based on the measurement of the difference between the periods of free oscillations of a pendulum of constant length at two points: the point under study and a point having a known acceleration of gravity. In this case, the periods depend on the acceleration of gravity. The main components of the apparatus are two to four pendulums with a period of natural oscillations of about 1 sec.

In modern pendulum apparatus the period is determined with an accuracy of 1 × 10^-8 sec by means of quartz frequency standards. This allows the acceleration of gravity to be measured with an accuracy of up to 10^-2 milligal; a photographic or photoelectric system is used to record the oscillations. The pendulums are thermostated and placed in a vacuum chamber. During the measurements the possible changes in such factors as the oscillation amplitude of the pendulums, the temperature of the pendulums, the density of the residual air in the vacuum chamber, and the rocking of the stand are monitored at the points; appropriate corrections are made when necessary. When measuring the acceleration of gravity, we must take into account its variation as a result of lunar or solar attractions. Specially designed pendulum apparatus are used for measuring the acceleration of gravity at sea. Pendulum apparatus with reversible pendulums have been used for the absolute determinations of the accelerations of gravity at Potsdam (German Democratic Republic), Washington D.C. (United States), Teddington (Great Britain), Leningrad (USSR), and Buenos Aires (Argentina).