Working of Callender's Compensated Constant Pressure Thermometer-I

Let us see the working of a Callender's Thermometer.

A Callender's Thermometer looks like as shown above.

It consists of two parts-"Thermometric part" and "Compensated part".

The thermometric part of this thermometer basically consists a bulb say which is made up of silica and used to measure the temperature.

And this bulb is connected to another bulb say of same dimension and volume as bulb with the capillary tube .

The bulb is completely filled with mercury and have an outlet through which mercury can be taken out.

So, we can say the thermometric part consists of bulb, bulb and capillary tube .

Now, the compensated part consists of a bulb which have the same dimension as bulb and bulb.

And this bulb is connected to a capillary tube say, . which is placed parallel and near to .

This structure helps in making the temperature of the air in both of these capillary tubes the same.

Now, the compensated part and the thermometric part are connected to each other with a U-shaped manometer say .

And the manometer is filled with sulfuric acid as shown in the figure using a dark blue colour.

The difference between the levels of acid in the two arms of the manometer will show that there is some pressure difference.

Initially, the pressure in all the vessels is constant, so the level of liquid in the arms of manometer would be equal.

At first, we will place the bulb, bulb and bulb in melting ice.

And we do this to make temperature in each of these bulbs.

Also, initially the capillary tubes and are opened from the sides so that the air completely fill the bulb, bulb and the capillary tubes.

When it is filled with air(after some time), the ends of the capillary tubes are sealed.

So, we have the pressure, temperature and the volume as constant.

Using the avogadro's hypothesis which states that "if pressure, temperature and volume are same then number of moles in the two gases are also same ".

Now, we take a container and fill it with a substance which temperature, we want to find.

Then, bulb is put in the container as shown above.

Now, as the temperature of the substance is higher, so the temperature of the air inside bulb rises.

Now, we will open the outlet and take out some mercury decreasing its level.

We do this to increase the volume of the gas present in bulb .

This is done to reduce the pressure inside the bulb.

Now, with respect to time we will take out mercury continuously till the pressure() of the gas in bulb restores to its original value ( or Atmospheric pressure).

Let's move further to find out the temperature of the substance.

Let us take as the volume of bulb and bulb and The volume of air in each of the capillary tubes and

Then let us have as the volume of Hg taken out from the reservoir i.e. the volume of air in the reservoir .

Suppose as the temperature of air enclosed in the bulb and as temperature of air enclosed in the capillary tubes.

Also, let us have as the temperature of air enclosed in the bulb band the reservoir

As we know, pressure and no. of grams mole of air and in both the parts of the apparatus respectively.

Applying gas equation to thermometric part let us assume equation as,

And applying gas equation to the compensating part let us assume equation .

After comparing both the equations and simplify the equation as,

So, the temperature of the substance or air present in the bulb is.


The callender's thermometer is used to measure the temperature of a specific substance.

The callender's thermometer consist of two parts.

The diagram of callender's thermometer looks like

The temperature of the substance or air present in the bulb is.

The end