Superconductivity and carbon nanotubes, are Type I, while almost all impure and compound superconductors are Type II. Conversely, a spinning superconductor generates a magnetic field, precisely aligned with the spin axis. The effect, the London moment, was put to good use in Gravity Probe B. This experiment measured the magnetic fields of four superconducting gyroscopes to determine their spin axes. This was critical to the experiment since it is one of the few ways to accurately determine the spin axis of an otherwise featureless sphere. Superconductivity was discovered on April 8, 1911 by Heike Kamerlingh Onnes, who was studying the resistance
History of superconductivity has a boiling point of 4.2 kelvins at atmospheric pressure. Heike Kamerlingh Onnes and Jacob Clay reinvestigated Dewar's earlier experiments on the reduction of resistance at low temperatures. Onnes began the investigations with platinum and gold, replacing these later with mercury (a more readily refinable material). Onnes's research into the resistivity of solid mercury at cryogenic temperatures was accomplished by using liquid helium as a refrigerant. On April 8, 1911, 16:00 hours Onnes noted "Kwik nagenoeg nul", which translates as "[Resistance of] mercury almost zero." At the temperature of 4.19 K, he observed that the resistivity abruptly disappeared (the measuring