We connect two poles of a dynamo with a $1 \mathrm{~km}$ long and $1.2 \mathrm{~mm}$ diameter iron wire and find that a current of 12 amperes flows in this wire. What is the power of the dynamo in watts? What is the voltage at the pole, and how many horsepower engine can drive the dynamo if its efficiency is $0.85.1 \mathrm{~m}$ long and $1 \mathrm{~mm}^{2}$ cross-sectional iron wire has a resistance of $\frac{1}{12}$ ohm?
The resistance of the conductor

$$
R=\frac{1}{12} \cdot \frac{1000}{0.6^{2} \cdot \pi}=73.7 \Omega
$$

The power of the machine

$$
\begin{gathered}
H=I^{2} \cdot R\left\{A^{2} \cdot \Omega\right\}=I^{2} \cdot R\left\{A^{2} \cdot \frac{\mathrm{V}}{\mathrm{A}}\right\}=I^{2} R V=I^{2} R \text { watt } \\
H=12^{2} \cdot 73.7 \mathrm{~W}=10610 \mathrm{~W}
\end{gathered}
$$

The armature voltage

$$
E=I \cdot R=12 \cdot 73.7 \mathrm{~V}=884 \mathrm{~V}
$$

The number of horsepower required

$$
L=\frac{10610}{736 \cdot 0.85}=17 \mathrm{HP}
$$

(Neuhold Özséb, Eger)