  The magnetic penetration depth $\lambda(T,H,j)$ was measured in the presence
of a slowly relaxing supercurrent, $j$. In single crystal
$\mathrm{Bi_2Sr_2CaCu_2O_8}$ below approximately 25 K, $\lambda(T,H,j)$ is
strongly hysteretic. We propose that the irreversibility arises from a shift of
the vortex position within its pinning well as $j$ changes. The Campbell length
depends upon the ratio $j/j_{c}$ where $j_{c}$ is the critical current defined
through the Labusch parameter. Similar effects were observed in other cuprates
and in an organic superconductor.
