  Several consecutive experiments with specifically built set-up are described.
Performing of the consecutive experimental tasks enables possibility to
determine Boltzmann's constant $k_\mathrm{_B}$. The fluctuations of the voltage
$U(t)$ of series of capacitors connected in parallel with a constant resistance
are measured. The voltage is amplified 1~million times $Y=10^6$. The amplified
voltage $YU(t)$ is applied to a device, which give the voltage mean quared in
time $U_2=\left<(Y U(t))^2\right>/U_0$. This voltage $U_2$ is measured with a
multimeter. A series of measurements gives the possibility to determine the
Boltzmann's constant from the equipartition theorem $C\left<U^2
\right>=k_\mathrm{_B}T$. In order to determine the set-up constant $U_0$ a
series of problems connected with Ohm's law are given that are addressed to the
senior students. For the junior high school students, the basic problem is to
analyse the analog mean squaring. The students works are graded in four age
groups S, M, L, XL. The last age group contains problems that are for
university students (XL category) and include theoretical research of the
set-up as an engineering device. This problem is given at the Fifth
Experimental Physics Olympiad "Day of the Electron", on December 2017 in Sofia,
organized by the Sofia Branch of the Union of Physicists in Bulgaria with the
cooperation of the Physics Faculty of Sofia University and the Society of
Physicists of the Republic of Macedonia, Strumica.
