Transport and thermodynamic properties of a sintered pellet of the newly discovered M gB2 superconductor have been measured to determine the characteristic critical magnetic fields and critical current densities. Both resistive transition and magnetization data give similar values of the upper critical field, Hc2, with magnetization data giving dHc2/dT = 0.44 T /K at the transition temperature of Tc = 40.2 K. Close to the transition temperature, magnetization curves are thermodynamically reversible, but at low temperatures the trapped flux can be on the order of one Tesla. The value of dHc/dT at Tc is estimated to be about 12 mT /K, a value similar to classical superconductors like Sn. Hence, the Ginsburg-Landau parameter κ ∼ 26. Estimates of the critical supercurrent density, Jc, using hysteresis loops and the Bean model give critical current densities on the order of 10 5 A/cm 2 . Hence the supercurrent coupling through the grain boundaries is comparable to intermetallics like N b3Sn.74.25. Bt, 74.25.Fy, 74.25.Ha, 74.60.Ge, 74.60.Jg

M gB2 becomes superconducting just below 40 K. Whereas porous polycrystalline samples of M gB2 can be synthesized from boron powders, in this letter we demonstrate that dense wires of M gB2 can be prepared by exposing boron filaments to M g vapor. The resulting wires have a diameter of 160 µm, are better than 80% dense and manifest the full χ = −1/4π shielding in the superconducting state. Temperaturedependent resistivity measurements indicate that M gB2 is a highly conducting metal in the normal state with ρ(40 K) = 0.38 µOhm-cm. Using this value, an electronic mean free path, l ≈ 600Å can be estimated, indicating that M gB2 wires are well within the clean limit. Tc, Hc2(T ), and Jc data indicate that M gB2 manifests comparable or better superconducting properties in dense wire form than it manifests as a sintered pellet.74.70. Ad, 74.25.Fy, 74.25.Ha, 74.60.Jg