  We show that tubes of melt cast Bi-2212 used as current leads for LTS magnets
can also act as efficient magnetic shields. The magnetic screening properties
under an axial DC magnetic field are characterized at several temperatures
below the liquid nitrogen temperature (77 K). Two main shielding properties are
studied and compared with those of Bi-2223, a material that has been considered
in the past for bulk magnetic shields. The first property is related to the
maximum magnetic flux density that can be screened, Blim; it is defined as the
applied magnetic flux density below which the field attenuation measured at the
centre of the shield exceeds 1000. For a cylinder of Bi-2212 with a wall
thickness of 5 mm and a large ratio of length over radius, Blim is evaluated to
1 T at T = 10 K. This value largely exceeds the Blim value measured at the same
temperature on similar tubes of Bi-2223. The second shielding property that is
characterized is the dependence of Blim with respect to variations of the sweep
rate of the applied field, dBapp/dt. This dependence is interpreted in terms of
the power law E = Ec(J/Jc)^n and allows us to determine the exponent n of this
E(J) characteristics for Bi-2212. The characterization of the magnetic field
relaxation involves very small values of the electric field. This gives us the
opportunity to experimentally determine the E(J) law in an unexplored region of
small electric fields. Combining these results with transport and AC shielding
measurements, we construct a piecewise E(J) law that spans over 8 orders of
magnitude of the electric field.
