  In a closed economic system, money is conserved. Thus, by analogy with
energy, the equilibrium probability distribution of money must follow the
exponential Gibbs law characterized by an effective temperature equal to the
average amount of money per economic agent. We demonstrate how the Gibbs
distribution emerges in computer simulations of economic models. Then we
consider a thermal machine, in which the difference of temperatures allows one
to extract a monetary profit. We also discuss the role of debt, and models with
broken time-reversal symmetry for which the Gibbs law does not hold.
