It is required to make a 20 Ohm rheostat from a nickelin wire with a cross-sectional area of 3 mm2.

It is required to make a 20 Ohm rheostat from a nickelin wire with a cross-sectional area of 3 mm2. How long will the wire be required for this?

Initial data: R (required resistance of the rheostat) = 20 Ohm; S (cross-sectional area of nickel wire) = 3 mm2.

Reference data: ρ (specific resistance of nickelin) = 0.4 Ohm * mm2 / m.

The required length of nickel wire for the manufacture of a rheostat can be determined from the formula: R = ρ * l / S, whence l = R * S / ρ.

Let’s make a calculation: l = 20 * 3 / 0.4 = 150 m.

Answer: To make a rheostat, a nickel wire 150 meters long is required.