A copper sphere is maintained at 500 K temperature by connecting it to a battery of emf `V_(1) = 100` V (see figure). The surrounding temperature is 300 K. When half the surface of the copper sphere is completely blackened (so that the surface behaves almost like a black body), a cell of emf `V_(2)` = 141 V is needed to maintain its temperature at 500 K. Calculate the emissivity of the copper surface.

Answer

`(1)/(3)`