 Much later, in 1955, George Yntema succeeded in constructing a small 0.7-tesla iron-core electromagnet with superconducting niobium wire windings.Then, in 1961, J. E. Kunzler, E. Buehler, F. S. L. Hsu, and J. H. Wernick made the startling discovery that at 4.2 kelvins, a compound consisting of three parts niobium and one part tin was capable of supporting a current density of more than 100,000 amperes per square centimeter in a magnetic field of 8.8 tesla.Despite being brittle and difficult to fabricate, niobium-tin has since proved extremely useful in supermagnets generating magnetic fields as high as 20 teslas.