Three rods, one made of silver (l = 420W/m K), second made of aluminum (k = 210W/m K) and the third made of iron (k = 70W/m K) are coated with a uniform layer of wax all around. The rods are placed vertically in a boiling water bath with 250 mm length of each rod projecting outside. If all the rods are having following dimensions i.e. diameter = 15 mm and length = 300 mm and have identical surface coefficient 12.5W/ m2 K, work out the ratio of lengths up to which wax will melt on each rod

Correct Answer: 2.45:1.732:1
k 1/l 12 = k 2/l 22 = k 3/l 32.