Relative Gravity Measurements Research Articles

Potsdam Correction from the Satellite Determined Geopotential

THE absolute value of the acceleration due to gravity at a point on the Earth's surface has generally been measured using reversible pendulums. Recently, however, the method of free fall determinations of gravity1 with its several variations has become increasingly popular because of the availability of electronics sophisticated enough to measure precisely the very short intervals of time involved. Generally, however, the absolute measurements of gravity are expensive, cumbersome and time-consuming; consequently, they are made only at a few selected points called “bases”. The high accuracy of relative measurements achievable with the modern gravimeters has made it possible to find the absolute gravity at any other point by measuring the gravity difference relative to one of these bases. For the sake of uniformity, it seems desirable to select one reference base. The Pendelsaal of the Geodetic Institute, Potsdam, East Germany, has been accepted as the reference base since the beginning of the twentieth century and the international network of relative gravity measurements is tied to it to form the “Potsdam gravity system” Until recently, the value of absolute gravity adopted at the Potsdam site was g=981,274.00 mgal, as determined by F. Kuhnen and Ph. Furtewangler from reversible pendulum measurements during the period 1898–1904. But modern re-examinations of this value have shown that it is in error by about −10 to −15 mgal. This means that the whole system based on this value has a constant scale error; the fiftieth general assembly of the International Union of Geophysics and Geodesy, held in Moscow in August 1971, has therefore recommended that a correction of −14 mgal (based on some of the most recent determinations) should be applied to the Potsdam gravity value.

A Temperature Control System for the Canadian Pendulum Apparatus

A temperature control system has been developed for use with the Canadian Pendulum Apparatus for relative gravity measurements. This system, which maintains the pendulums at a constant temperature of 40.000C, consists of three major components: a vacuum enclosure in which to house the pendulums; an electronic thermometer to measure their temperature; and an electronic thermostat to maintain a constant temperature. Extensive testing of the system indicates that the temperature of the pendulums may be measured with an accuracy of ±0.01°C and that the electronic thermostat has a stability of about 0.002°C when the vacuum chamber is kept continuously closed. Under normal operating conditions, when the pendulum case is opened regularly to change pendulums, the observed temperature fluctuations amount to a maximum of ±0.025°C. As small corrections may be, applied to the pendulum periods to compensate for these temperature fluctuations, the error in gravity due to temperature effects never exceeds 1/6 mGal.

MAGNETIC PERTURBATIONS OF INVAR PENDULUMS USED FOR RELATIVE GRAVITY MEASUREMENTS

Certain discrepancies in recent relative gravity measurements made with the Cambridge pendulum apparatus may be due to magnetic forces on the invar pendulums. In addition to effects which have been described previously, it is shown that disturbances may arise from a magnetic screen surrounding the pendulums and from the magnetic interaction of two pendulums swinging together. Theoretical estimates of these effects are compared with experiments. The behaviour of the pendulums is qualitatively as predicted but the numerical results are in poor agreement. This may be due in part to the difficulty of making calculations for pendulums with dimensions comparable to their distances from the magnetic screen and each other. The application of the results to gravity measurements is considered.

An absolute determination of the acceleration due to gravity

Recent advances in the precision obtainable in length and time measurements have made it possible to determine the absolute value of the acceleration due to gravity with greater accuracy than has hitherto been possible. An accurate knowledge of the actual value of the acceleration due to gravity is essential, for example, in connexion with the determination of the absolute unit of electric current by means of the current balance. It has been usual to refer relative gravity measurements made in this country to the absolute value determined at Potsdam by Kühnen and Furtwängler (1906) over 30 years ago, and although relative determinations can now be carried out with an accuracy approaching one part in a million, it appears that the basic Potsdam value may be in error by something between one and two parts in 100,000. Very few absolute determinations have been made within the last 50 years, but those which have been carried out at various stations show discrepancies of this order when related to the Potsdam value by relative determinations.