  Using the high-sensitive mechanical method of measuring the dissipation
processes in superconductors, the 3D-2D phase transition in magnetic field was
investigated in the vortex lattice of strongly anisotropic high-temperature
superconductors of Bi(2223) system. It was shown that with the increase of
external magnetic field the dissipation connected with a motion of vortexes
relative to the crystal lattice of a sample, first increases, reaching the
maximum and then with a further increase of the magnetic field sharply reduces
as a result of pinning enhancement in connection with the transition of
three-dimensional vortex system in quasi-two-dimensional "pancake" one.
