Consider the experiment of Ingen Hausz. Here when the wax starts melting, we need to find

the relation between the thermal conductivity of a metal rod K, and the length of the rod. As we know, according to Ingen Hauz's experiment, the relation between the thermal conductivity of a metal rod is K and the length of the rod is given by,

That means, the thermal conductivity of the rod is directly proportional to the square of the length of the given rod. This can also be written in the form of,

Therefore, the relation between the thermal conductivity of a metal rod is and the length of the rod is constant.