Two flat steel sheets, each of 2.5 mm thickness, are being resistance spot welded using a current of 6000 a and weld time of 0.2 s. The contact resistance at the interface between the two sheets is 200 μW and the specific energy to melt steel is 10 ´ 109 J/m3. A spherical melt pool of diameter 4 mm is formed at the interface due to the current flow. Consider that electrical energy is completely converted to thermal energy. The ratio of the heat used for melting to the total resistive heat generated is _______