1. Introduction 
- The resistivity (ρ) of materials used for cable conductors (like copper and aluminium) depends on temperature. 
- Measurements of linear resistance (R) at ambient temperature need correction to be compared with standard specifications (e.g., IEC 60228 at 20°C). 

2. Correction Factor Formula 
The correction factor k_{Tₘ, T₀} is defined as:

kₜₘ,ₜ₀ = (T₀ + Tₖ) / (Tₘ + Tₖ) = (1 + α(T₀ - 20°C)) / (1 + α(Tₘ - 20°C))
Where: 
- Tₘ = Measurement temperature 
- T₀ = Reference temperature 
- α = Temperature coefficient of resistivity 
- Tₖ = Correction temperature 

Tₖ = (1 - α * 20°C) / α
or

α = 1 / (Tₖ + 20°C)
3. Corrected Resistance Formula 
The corrected resistance at reference temperature T₀ is:

Rₜ₀ = kₜₘ,ₜ₀ * Rₜₘ
4. Material Parameters (from IEC 60228) 

Material	α (°C⁻¹)	Tₖ (°C)
Copper	0.00393	234.5
Aluminium	0.00403	228

5. Example Formula for Copper (to 20°C) 
The simplified formula for copper correction to 20°C is:

kₜₘ,Cu = 254.5°C / (Tₘ + 234.5°C) = 1 / (1 + 0.00393(Tₘ - 20°C))
6. Notes 
- Formulas are approximations valid for temperatures between -20°C to 100°C. 
- Correction to 20°C is only valid for the given range and cannot be applied at extreme temperatures.