Niobium and Gravimeters
Gravimeters measure changes in the Earth’s gravity. Near the town of Membach, Belgium is an exceptionally accurate superconducting gravimeter that uses a small sphere of niobium metal suspended in a magnetic field at below -263oC (9.2 K), close to absolute zero.
Any fluctuations in the gravitational field pull and tug on the sphere, shifting its position ever so slightly, perturbing the magnetic field and sending electrical signals to nearby sensors.
At 9.2 K niobium reaches its superconducting transition temperature and magnetic field lines flow around, rather than through, the sphere, keeping it suspended in the centre of its little chamber.
Niobium wire coils also kept at this low temperature, offer no resistance to the electrical current that flows through them, producing the perfectly stable magnetic field that levitates the niobium sphere.
Before niobium superconductors were developed older gravimeters used reference weights attached to mechanical springs. Further information is available here.