  The time-reversal symmetry of nonequilibrium fluctuations is experimentally
investigated in two out-of-equilibrium systems namely, a Brownian particle in a
trap moving at constant speed and an electric circuit with an imposed mean
current. The dynamical randomness of their nonequilibrium fluctuations is
characterized in terms of the standard and time-reversed entropies per unit
time of dynamical systems theory. We present experimental results showing that
their difference equals the thermodynamic entropy production in units of
Boltzmann's constant.
