  For finite random systems, it is possible to define two types of variances
(noises). It is demonstrated that their ratio is useful in calculating the
correlation length of an infinite and rather general random system, as a
function of temperature. The numerical method of obtaining those variables is
not relevant. It can be real space numerical renormalization, simulation or any
other method. It does not matter. The correlation length obtained by this novel
technique may then be used to obtain directly the critical correlation
exponent, $\nu$, rather than indirectly, using scaling relations, as is often
done. The method is demonstrated by applying it to the random field Ising
model.
