We measured a distance at $t=26^{\circ} \mathrm{C}$ temperature using a copper rod that is exactly $a=3 \mathrm{~m}$ long at $0^{\circ} \mathrm{C}$. In this way, we found $n=7856.728 \mathrm{~m}$. What is the actual distance if the linear expansion coefficient of copper is $\lambda=0.000017$.
The actual length of the measuring rod: $a^{\prime}=a(1+\lambda t)$. Since during the measurement, the given distance was tapped $\frac{n}{a}$ times, the actual distance is

$$
\begin{gathered}
L=\frac{n}{a} \cdot a(1+\lambda t)=n(1+\lambda t)=7856.728\left(1+17 \cdot 10^{-6} \cdot 26\right) \\
L=7860.201 \text{ m}
\end{gathered}
$$

(Róbert Zichy, Szombathely.)