Step-by-step explanation:
- A flask is half filled with water and heated until the water starts boiling.
- A thermometer is clamped inside the flask without letting it be in contact with water
- The thermometer is left in the steam such that the bulb is heated by the steam at standard atmospheric pressure.
- As the bulb is heated, the mercury thread rises up and ehre it remains steady for sometime is marked with a light scratch as the upper fixed point
(Add diagram )