  Transport critical current (Ic) was measured for Fe-sheathed MgB2 round
wires. A critical current density of 5.3 x 10^4 A/cm^2 was obtained at 32K.
Strong magnetic shielding by the iron sheath was observed, resulting in a
decrease in Ic by only 15% in a field of 0.6T at 32K. In addition to shielding,
interaction between the iron sheath and the superconductor resulted in a
constant Ic between 0.2 and 0.6T. This was well beyond the maximum field for
effective shielding of 0.2T. This effect can be used to substantially improve
the field performance of MgB2/Fe wires at fields at least 3 times higher than
the range allowed by mere magnetic shielding by the iron sheath. The dependence
of Ic on the angle between field and current showed that the transport current
does not flow straight across the wire, but meanders between the grains.

Superconductivity