Wiedemann–Franz law states that the ratio of the electronic contribution of the thermal conductivity (κ) to the electrical conductivity (σ) of a metal is proportional to the temperature (T).
The formula; K / σ = L x T
The proportionality constant L, known as the Lorenz number, is
L = K / σ x T = ( π2 / 3 ) x ( kB / e )2
L = 2.44 x 10-8 W Ω K-2