Superconducting gravimeter GWR
This instrument measures the variations of the gravity by means of a sphere levitating in a magnetic field generated by a pair of superconducting coils. It operated in Uccle from 1982 to 2000.
Royal Observatory of Belgium
Tidal forces, induced by the Moon and the Sun, modify the shape of the globe and cause variations in gravity g. At the Uccle station, g amounts 9.81 117 xxxx m/s²; the xxxx decimals vary over time because of the tidal forces. These variations are on the order of 0.000 002 5 m/s². This means that, between the minimum and the maximum of the gravity tide, the weight of a one-ton mass varies at most by 0,25 gram.
Gravimeters are instruments designed to measure the gravity or its variations. Most gravimeters include a coiled spring, with a mass attached to it and moving as gravity varies. In the early 1980s, a new type of gravimeter became available, where a magnetic field, generated by superconducting currents, plays the role of the spring and where the mass is a hollow metal sphere of 2.5 cm in diameter, levitating in the magnetic field. The sensor, which contains the coils and the sphere, is immersed in a liquid helium bath at -269°C or 4.2 K, i.e. close to the absolute zero, which is -273,15 °C
The principle of superconductivity is that, at so low temperatures, the electrical resistance of some materials drops to zero and an electrical current can flow indefinitely through a coil. This ensures the extreme stability of the magnetic field and consequently of the sphere levitation and the gravity measurement, reaching the unparalleled precision of tenth of a billionth of g.
The Royal Observatory of Belgium started operating a superconducting gravimeter in Uccle in 1982, one of the first two in Europe. This gravimeter, illustrated by the greeting card drawn by Peyo at the end of 1982, was decommissioned in 2000. However, in 1995 and 2014, new generation devices were installed at the Membach and Rochefort stations. The quality of their measurements made it possible, and still makes it possible, to provide fundamental contributions to the science of measurement, to the study of gravity tides as well as other variations in gravity, such as caused by fluctuations in groundwater content.
Filling the superconducting gravimeter GWR T#003 of Uccle with liquid helium.
Credit: Royal Observatory of Belgium.
The superconducting gravimeter GWR C#021 of Membach.
Credit: Etienne Coveliers.
Installing the superconducting gravimeter GWR iGrav#019 in Rochefort (2014).
Credit: Royal Observatory of Belgium
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