[" A point source of heat of power "P" is placed at the centre of "],[" a spherical shell of mean radius "R" .The material of the shell "],[" has thermal conductivity "K" .If the temperature difference "],[" between the outer and inner surface of the shell in not to "],[" between the outer and inner surface of the shell in not to "],[(1991-1" Mark) "],[" exceed "T" ,the thickness of the shell should not be less than "],[qquad (1991-1" Mark) "],[" the stance of mass M kg requires a power input of when the "]