The walls of a closed cubical box of edge 50cm are mdade of a material of thickness 1mm and thermal conductivity `4xx10^(-4)` cal `s^(-1)cm^(-1).^(@)C^(-1)` the interior of the box maintained at `100^(@)C` above the outside temperature by a heater placed inside the box and connected acros a 400 V d.c. source. Calculate the resistance of the heater.