# What is a thermometer? How does the thermometer work?

You might have heard about the temperature.

You have also heard that the outside temperature is very cold tonight or something like that.

You also need to know the temperature of our body when we have a fever.

How are all these measurements possible?

Obviously we need some device to measure the temperature. This device is known as a thermometer.

The simple definition of thermometer: The device which is used to measure the temperature is known as thermometer.

Generally thermometers are made up of hollow glass tube in which mercury is filled in it.

Mercury is a metal which is in a liquid state.

Working of thermometer:

The working principle of thermometer is very simple.

Thermometer works on the principle of zeroth law of thermodynamics.

If you do not know about the zeroth law, then it is nothing but the concept of thermal equilibrium.

Now let’s get back to the working of thermometer.

As I mentioned earlier in this article, the thermometer is made up of a glass tube in which mercury metal is filled.

There is a sensing bulb at the bottom of the thermometer which is also in the contact of mercury.

Mercury is metal and it expands when its temperature rises.

So based on this property, when the sensing bulb is allowed to come in contact with the hot body, the mercury expands and so its level rises in the hollow tube.

This rise in the mercury column indicates that the object is hotter than the room temperature.

The column of the thermometer is calibrated with the temperature scales. (Celsius scale or fahrenheit scale, or some thermometers have both the scales)

By knowing the coinciding mercury level on the calibrated scale, we can come to know about the temperature of the body.

Now what if we have to measure the temperature of the body which is colder than the room temperature.

In this case, instead of expansion, the mercury metal will contract.

This will result in the drop of mercury column and finally the reading are measured from the calibrated scale.

This is how the mercury thermometer works.

## What is temperature actually measuring?

Short answer: Average kinetic energy of molecules.

Explanation:

Yes, temperature is nothing but the measurement of degree of hotness or coldness of any object.

But what does the hotness or coldness of the body indicate?

You might know that if a metal bob is allowed to pass from the ring, then it will pass easily.

But if we heat that metal bob then it will not pass from the ring. (You might be knowing this example from your school textbooks.)

Well, let me give you another example. You know why there is a gap between the joining of two rails ?

You will get this answer soon. Just in a minute.

When the metal is heated by some external means, its molecules start vibrating with a high velocity.

The molecules and atoms moving with a high velocity possess kinetic energy.

Due to this vibration, the heat is liberated and this indicates the rise in temperature of that body.

Thus the increase in temperature of the body indicates the average kinetic energy of the molecules.

Now, when the rails get heated due to the sun rays, the average kinetic energy of the molecules increases and as a result the rails expand.

Due to this reason the gaps are kept between the joints of the rails.

## What is absolute zero temperature?

Absolute zero temperature: The lowest possible temperature is known as absolute zero temperature. It is 0 kelvin (0 K).

But the question is, how could you achieve this?

Well, it is practically not possible to achieve this lowest temperature, but it is just a concept given by the scientist and they use it to study the properties and behaviour of the gases at this lowest possible temperature.

The entire concept of absolute zero temperature is based on the third law of thermodynamics.

It says that, at the absolute zero temperature, the entropy of the pure crystalline substance will be zero.

In other words, if the substance is pure crystalline, then its entropy will be zero at absolute zero temperature.

Detailed explanation:

Let me tell you what happens at absolute zero temperature.

Assume that we have a gas sample at room temperature. So all the molecules of gases will be randomly moving with some velocity.

This indicates that they possess some kinetic energy.

Now when we reduce the temperature of this gas sample, the molecular movements slow down and the kinetic energy of the molecules reduces.

If we further reduce the temperature of this sample gas, then a point will come when there will be no movement of the gas molecules. This temperature is the absolute zero temperature.

At this temperature, the kinetic energy of the molecules will be zero and so its entropy will also be zero.