  The response of $nm$-size spherical superconducting clusters to a magnetic
field is studied for the canonical ensemble of electrons in a single degenerate
shell. For temperatures close to zero, the discreteness of the electronic
states causes a step like destruction of the pair correlations with increasing
field strength, which shows up as peaks in the susceptibility and heat
capacity.
  At higher temperatures the transition becomes smoothed out and extends to
field strengths where the pair correlations are destroyed at zero temperature.
