25 years of continuous measurements of gravity in Membach
On August 4, 2020, the superconducting (or cryogenic) gravimeter has been measuring for 25 years the variations of gravity with the precision of one hundredth of a billionth (10-11) of g (g = 9.81 m/s²).
Since September 18, 2017, this instrument holds a double world record, in gravimetry and physics:
- Record of the cryogenic gravimeter that has operated the longest at a given location;
- It is also, as far as we know, the longest levitation of a superconducting artefact in a magnetic field. This field is generated by persistent currents, which were injected in 1995 into superconducting coils, where they circulate since then without any resistance and therefore, without ever having been dissipated. Although this record does not contradict what physicists specialising in persistent currents expect – in theory, a superconducting current can flow forever – , it is at least worthy of a place in a ‘cabinet of curiosities’.
In the field of geophysical research at the Observatory, these measurements are important, among other things because they provide insight in long-term variations in gravity, still poorly understood, caused by slow tectonic movements or climatic variations. It is also important for the study of the water cycle whose varying masses influence gravity.
To date, this gravimeter participates in many research projects and the Royal Observatory of Belgium hopes to be able to perform this type of measurement for many years to come.
The superconducting gravimeter is located at the end of a 132 m – long gallery, 48 meters underneath the surface, in Membach (city of Baelen, eastern Belgium). The instrument sensor is immersed in liquid helium in which the temperature is held at -269°C, in other words 4 degrees above absolute zero, allowing superconductivity. Photo credit: E. Coveliers & B. Frederick. Copy authorized with mention of the source.
Exploded view of the sensor of a superconducting gravimeter: the 4-gram hollow sphere levitates in a magnetic field generated by currents flowing through the pair of coils. Photo of the sensor from the old instrument of Uccle, today on display at the Planetarium.
Scientific contact:
Michel Van Camp
Ringlaan 3
BE-1180 Ukkel
Tel: +32 (0)2 373 02 65
mvc AT oma.be
http://www.seismology.be