The difference between lengths of a certain brass rod of a steel rod is claimed to be constant at all temperatures. This implies (${\mathrm{L}}_{\mathrm{B}} \mathrm{\&} {\mathrm{L}}_{\mathrm{S}} :$ length of brass & steel,  ${\mathrm{\alpha }}_{\mathrm{B}} \mathrm{\&} {\mathrm{\alpha }}_{\mathrm{S}}:$ coefficient of linear expansion of brass & steel respectively)