  We use 't Hooft loops of maximal size on finite lattices to calculate the
free energy in the sectors of SU(2) Yang-Mills theory with fixed electric flux
as a function of temperature and (spatial) volume. Our results provide evidence
for the mass gap. The confinement of electric fluxes in the low temperature
phase and their condensation in the high temperature phase are demonstrated. In
a surprisingly large scaling window around criticality, the transition is
quantitatively well described by universal exponents and amplitude ratios
relating the properties of the two phases.
