## 2008-06-09

### Thermometers and calibration of Thermometers

The definition of temperature as a physical quantity is based on the principle of thermal equilibrium.

Let say there are Thermometer A, Liquid B and Liquid C.

We put thermometer A into liquid B and then after thermal equilibrium is achieved we record the value.

We put thermometer A again into liquid C and after thermal equilibrium is achieved we record the value of reading in the thermometer.

If the temperature in both cases are the same, then liquid B and liquid C are in thermal equilibrium with one another. Eventhough, the two liquids (B and C) are not in thermal contact, they are in thermal equilibrium because their temperatures are the same.

Therefore Temperature is a physical quantity which determines whether or not two objects are in thermal equilibrium.

We measure temperature using a thermometer.

Thermometers must be calibrated before they can be used to measure temperatures.

The calibration of an instrument refers to the process of marking-up a scale on the instrument to be used as measurement.

To produce a scale on a thermometer, two fixed points must be determined first. Then the two points must be the temperatures which can easily and correctly reproduced in any part of the world.

On the Celsius scale, the two fixed points are the ice point (0°C) and the steam/boiling point (100°C).

The ice point (0°C), or lower fixed point is the melting temperature of pure ice at standard atmospheric pressure (760 mm Hg).

The steam point (100°C), or upper fixed point is the temperature of steam at standard atmospheric pressure (760 mm Hg).

After obtaining, the highest point and the lowest point. We divide the length between them to equal parts / scale.