Thermometers

Temperature is measured with a thermometer. The basic operating principle behind all thermometers is that there is some quantity, called the thermometric variable that changes in response to changes in temperature. It is the thermometric variable that gets measured. There is no way to measure temperature directly.

Figure 1: The simplest of thermometers is the liquid thermometer. They are a thin glass tube filled with a small amount of alcohol usually coloured red. 

Thermometers measure the temperature due to thermal expansion. An increase in the volume of the substance because of the increase in the temperature is known as thermal expansion. A slight change in the temperature causes changes in the volume of a liquid.  

Figure 2: The alcohol in in the capillary tube expands when heated and contracts when cooled. 

When heated, the molecules of the liquid in the thermometer move faster, causing them to get a little further apart. This results in movement up the thermometer. When cooled, the molecules of the liquid in the thermometer move slower, causing them to get a little closer together.

Figure 3: When the alcohol expands, it increases in size by an amount that’s directly related to the temperature. 

If the temperature increases by 20 degrees, the alcohol expands and moves up the scale by twice as much as if the temperature increase is only 10 degrees.

The Celsius scale is based on the temperatures of ice and boiling water because these are the two fixed points. When we dip the thermometer into ice, we observe that the alcohol level marks 0°C on the scale.  The alcohol rises to the top of the thermometer and reads 100°C if the thermometer is placed in boiling water. 

Thermometers can also be filled with mercury, which behaves in the same way as alcohol in a thermometer.  This is now no longer widespread practice because mercury is toxic.