  We have experimentally studied the magnetic shielding properties of a
cylindrical shell of BiPbSrCaCuO subjected to low frequency AC axial magnetic
fields. The magnetic response has been investigated as a function of the
dimensions of the tube, the magnitude of the applied field and the frequency.
These results are explained quantitatively by employing the method of Brandt
(1998 Phys. Rev. B 58 6506) with a Jc(B) law appropriate for a polycrystalline
material. Specifically, we observe that the applied field can sweep into the
central region either through the thickness of the shield or through the
opening ends, the latter mechanism being suppressed for long tubes. For the
first time, we systematically detail the spatial variation of the shielding
factor (the ratio of the applied field over the internal magnetic field) along
the axis of a high-temperature superconducting tube. The shielding factor is
shown to be constant in a region around the centre of the tube, and to decrease
as an exponential in the vicinity of the ends. This spatial dependence comes
from the competition between two mechanisms of field penetration. The frequency
dependence of the shielding factor is also discussed and shown to follow a
power law arising from the finite creep exponent n.
