Pendulum gravimeter

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Pendulum gravimeters , gravitational pendulums and gravity pendulums (or pendulum resonators) are pendulums that can be used as absolute gravimeters to measure gravity in the vertical direction. The period  T of a swinging pendulum depends on the thread length  l and the gravitational acceleration  g . The local acceleration due to gravity can thus be determined by determining the period duration and the thread length. A special form is the double pendulum gravimeter .

Pendulum gravimeters are to be viewed as very simple, primitive gravimeters. Due to their sensitivity to interference and other restrictions, they only have historical significance today. The period of a pendulum results from:

The acceleration due to gravity is thus:

With amateur means, under optimal conditions, measurements with an accuracy of about 0.2% can be achieved. Both the accuracy of the length measurement of the pendulum and the accuracy of the measurement of the period are included in the accuracy of the measurement result. The good length measurement can be done e.g. B. achieve with a cathetometer . The determination of the period duration can be improved by counting and forming the arithmetic mean of many periods and the measurement error in the determination can thus be reduced. The pendulum should be fixed and protected from the influence of the wind. Hand-held pendulum gravimeters are unusable due to the carpenter effect .

More modern gravimeters are based on gravity-field-dependent elongations of springs , such as the LaCoste-Romberg gravimeters.

The acceleration due to gravity on the earth's surface has an average value of 9.806 m / s 2 at sea level (mostly rounded to 9.81 m / s 2 ). Depending on the latitude, values ​​between 9.78306 m / s 2 and 9.83208 m / s 2 also occur, with the acceleration due to gravity being lowest at the equator and highest at the poles. The gravitational anomalies of natural origin are therefore approx. 0.5%. Investigations in geophysics usually reveal relevant differences in the range of a hundred thousandth to one millionth, which must be recorded by a gravimeter. A height difference of one meter leads to a change in the acceleration due to gravity by about 3 millionths of a m / s 2 .

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