In a cylindrical container of sufficiently large height, two easily moving pistons enclose certain amount of same ideal gas in two chambers as shown in figure.
The upper piston is at a height 20 cm from the bottom and lower piston is at a height 8 cm from the bottom. The mass of each piston is m kg and cross sectional area of each piston is ${\mathrm{Am}}^2,\mathrm{where}\frac{\mathrm{mg}}{\mathrm{A}}={\mathrm{P}}_0\mathrm{and}{\mathrm{P}}_0$  is the atmospheric pressure  $=1\times {10}^5\mathrm{N}/{\mathrm{m}}^2.$
  The cylindrical container and pistons are made of conducting material. Initially the temperature of gas is ${27}^0\mathrm{C}$  and whole system is in equilibrium. Now if the upper piston is slowly lifted by 16 cm and held in the position with the help of some external free. As a result, the lower piston rises slowly by l cm.