Heat is supplied to a slab of cork, 5cm thick and of `2m^(2)` by a heating coil spread over its surface. When the current in the coil is 1.18 A and the potential difference across its end is 20 V, the steady temperatures of the faces of the slab are `12.5^(@)C` and `0^(@)C`. Assuming that the whole of the heat developed in the coil is conducted through the slab, calculate the thermal conductivity of the cork.