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 equation
lim
T→∞
1/2T ∫
-T
^T
V(t)V(t + τ)dt = V
²
exp (-|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 A
12
(H) = A
21
(-H), some implications of which are discussed.