Reciprocity theorems and irreversible processes

A synopsis is given of the connection between symmetry relations for electrical networks and Onsager´s reciprocal relations for irreversible processes. It is pointed out that the frequency spectrum of voltage fluctuations in an RC circuit follows immediately from the equationlimT→∞1/2T ∫-T^TV(t)V(t + τ)dt = V²exp (-|τ|/RC). This expression for the correlation function is therefore well supported by experiment. Only a slight generalization of this principle is involved in the application of Onsager´s method to a four-terminal network. The result is an equation of the type A12(H) = A21(-H), some implications of which are discussed.