We want to install an electric bell. The ringing location is 30 meters away from the bell. The internal resistance of the bell is 2 ohms. We intend to use 2 series-connected Leclanché cells as the power source, each with an electromotive force of 1.5 Volts and an internal resistance of 1 ohm. What diameter copper wire should we use, if the bell only works reliably when at least 0.4 amperes flow through it. The resistance of 1 meter of copper wire with a cross-sectional area of $1 \mathrm{~mm}^{2}$ is $\frac{1}{55} \mathrm{ohm}$.
According to Ohm's law, $I=\frac{E}{R}$, where $I$ is the current intensity, $E$ is the electromotive force, and $R$ is the total resistance. In the present case,

$$
R=\frac{E}{I}=\frac{2 \cdot 1.5}{0.4}=7.5 \Omega
$$

is the maximum allowable resistance. If we denote the diameter of the wire by $x$, then the following relationship holds:

$$
\begin{gathered}
\frac{1}{55} \cdot \frac{2 \cdot 30}{\left(\frac{x}{2}\right)^{2} \cdot \pi}+2+2 \cdot 1=7.5 \\
x=\sqrt{\frac{240}{3.5 \cdot 55 \cdot \pi}} \\
x=0.63 \text{ mm}.
\end{gathered}
$$

(Oszkár Szilas, Budapest.)