Fluctuations of the measured parameters of the investigated electric and non-electric systems are considered on the thermodynamics basis. We have defined that entropy of the investigated system in relation to environment depends on the correlation between measuring duration Тmeas of the system’s fluctuation parameter and the system’s relaxation time τ. Decrement of the energy spectrum of fluctuations with the decrease of frequency is explained by Тmeas > τ. When Тmeas ≈ Nmaxτ (where Nmax is the number of possible ways to create the balanced state of the system), entropy of investigated system tends to zero in relation to the environment, and the measurement error caused by fluctuations becomes the minimal.