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Example 12.5
Resistance of a metal wire of length 1 m is 26 Ω at 20°C. If the diameter
of the wire is 0.3 mm, what will be the resistivity of the metal at that
temperature? Using Table 12.2, predict the material of the wire.
Solution
We are given the resistance R of the wire = 26 Ω, the diameter
d = 0.3 mm = 3 × 10^-4 m, and the length l of the wire = 1 m.
Therefore, fromR = ρl/A

The resistivity of the given metallic wire is
ρ = (RA/l) = (Rπd2/4l )
Substitution of values in this gives
ρ = 1.84 × 10^-6 Ω m         .....(Calculation mistake in NCERT)
The resistivity of the metal at 20°C is 1.84 × 10_^-6 Ω m.   ...(Calculation mistake in NCERT)