A theoretical relation is derived for the variance of a 1/fnoise signal as a function of the sample durationT. With the help of this relation the 1/fcharacter of a noise signal in the microhertz range is verified by measuring the variance as a function ofT, withT≤3×105s. Atf=3×10−6Hz the spectral density of the noise signal was still found to be inversely proportional to the frequency.