Theory: 
              When a steady state is reached, let T₁ and T₂ be the temperatures of metallic disc M and brass disc B. So, the temperature difference between the two ends of bad conductor is (T₁- T₂ ).
The rate of heat conducted through the bad conductor is,
                            ........................  ( 1 )
where, ‘d' is thickness of the bad conductor and 'A' is the area of cross section.
The rate of heat lost by the brass disc to the surrounding under steady state is,
                                 ........................  ( 2 )   
Where m-mass of the brass disc, C is the heat capacity of brass disc and is its rate of cooling at T₂

From equn(1) and (2),
                             ....................   ( 3 )
 By measuring,  and using Lee's  apparatus the thermal conductivity K can be determined.