The process of determining lower fixed point and upper fixed point on a newly constructed thermometer in order to make scale on it is called calibration of the thermometer.

Scale are to be made on a thermometer for its calibration. For making a scale, marking the lower fixed point and the upper fixed point is required. Hence, for calibrating a thermometer upper and lower fixed points are determined first.

Lower Fixed Point (Ice Point):

• Prepare a mixture of ice and water in a container.
• Insert the thermometer bulb into the ice-water mixture, ensuring it does not touch the sides or bottom of the container.
• Allow the thermometer to stabilize for several minutes until the thermometric liquid's level remains constant. Mark this level as lower fixed point.

Upper Fixed Point (Steam Point):

• Boil distilled water in a suitable container (like a flask) and ensure that steam surrounds the thermometer bulb.
• Insert the thermometer into the steam, ensuring it does not dip into the boiling water.
• After a few minutes, when the liquid level stabilizes, mark this level indicating the upper fixed point.

Calibration Scale:

• The distance between these two fixed points is divided into equal intervals, typically 100 equal divisions for Celsius  and Kelvin thermometers and 180 equal divisions for Fahrenheit thermometers.
• This division allows for accurate temperature readings between these two fixed points.