A micrometer with a zero error of +0.01 mm gives five raw diameter readings for a nichrome wire: 0.28, 0.29, 0.28, 0.28, 0.29 mm. The gradient of R against L is 18.6 Ω m⁻¹. Find the resistivity.
Mean raw diameter = 0.284 mm. Correct for the zero error: d=0.284−0.01=0.274 mm.
A=π(d/2)2=π(0.137×10−3)2=5.90×
ρ=gradient×A=18.6×5.90×10−
Why use a graph? The crocodile clips add a constant contact resistance, which shifts the line up but leaves the gradient unchanged.