In 1933 Meissner, in collaboration with R. Oschenfeld, discovered what has since been known as the Meissner effect. He was examining the magnetic properties of materials as they became superconductive, a condition met with as the temperature of the element or compound falls below a critical point, T _c. It was found, quite unexpectedly, that if a solid lead sphere is placed in a magnetic field and the temperature allowed to fall below the T _c of lead, the magnetic field is expelled from the lead sphere, which becomes perfectly diamagnetic. The presence of the Meissner effect is now used as a routine test for superconductivity.