How long should you choose a lead wire with a cross-sectional area of 0.22 mm2 so that it has a resistance of 42 ohms.

1. Let’s translate given in the SI system:
Ssection = 0.22 mm² = 2.2 * 10-5 m²
2. Let’s write down the expression to determine the resistivity of the wire:
ρsp = R * Ssec / l, where R is the resistance of the wire, l is the length of the wire, Ssec is the cross-sectional area.
3. Let us express l from here:
l = R * Ssec / ρsp
Specific resistance ρsp of lead is determined according to the reference book 2.0 * 10–7 Ohm * m
4. Substituting the numerical values, we find the length of the wire:
l = R * Ssection / ρsp = 42 * 2.2 * 10-5 / 2.0 * 10-7 = 4620 m

Answer: you need to take 4620 m of lead wire.