Fery’s black body consists of a double walled hollow sphere having a small opening O on one side and a conical projection P just opposite to it (Figure). Its inner surface is coated with lamp black. Any radiation entering the body through the opening suffers multiple reflections at its innerwall and about 97% of it is absorbed by lamp black at each reflection.
Therefore, after a few reflections almost entire radiation is absorbed. The projection helps in avoiding any direct reflections which even otherwise is not possible because of the small opening O. When this body is placed in a bath at fixed temperature, the heat radiations come out of the hole. The opening O thus acts as a black body radiator.