Passage - I: An instrument known as triple point cell ( at a temperature of 273.16 K and pressure of 4.6mm of mercury) in which triple point is attained for water. It consists of a U-tube containing ice-cold water to evacuate air by means of an exhaust pump and sealed. It is then lowered into a Dewar's flask containing ice-water mixture. The space corresponding to the inner portion of the vessel is filled with a freezing mixture and this space is referred as "thermometric well". As a result, the layer of the water freezes into ice. Thus the space in the U-tube contains ice, water. water-vapor all the three in equilibrium and the corresponding coordinates of pressure and temperature the "triple point of water" 1. Evacuated Cell 2. Inner U-tube 3. Ice-Water Mixture 4. Freezing Mixture 5. Water-frozen into ice 6. Water Vapour 7. Dewar's Tube Triple Point cell is an experimental arrangement