## Zeroth Law of Thermodynamics – The Basis for Temperature Measurement

written by: Haresh Khemani • edited by: Swagatam • updated: 5/4/2010

Zeroth Law of thermodynamics is very simple to understand. It says that when two bodies are separately in thermal equilibruim with a third body, then the two bodies are also in thermal equilibrium with each other. This law has been used as the basis for measurement of temperature.

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### What is Zeroth Law of Thermodynamics?

Have you ever wondered why freezing point temperature of water is zero degree Celsius and why its boiling point temperature is 100 degree Celsius? Temperature is considered to be the property which distinguishes thermodynamics from the other sciences. It is by the property of temperature that we can make out the difference between hot and cold body. The Zeroth law of thermodynamics is the basis for measurement of temperature and setting its scale.

In simple word, Zeroth law of thermodynamics says that “When two bodies are separately in thermal equilibrium with the third body, then the two are also in thermal equilibrium with each other." Say for instance body A is in thermal equilibrium with body C, body B is also in thermal equilibrium with body C, then as per Zeroth Law of thermodynamics bodies A and B are also in thermal equilibrium with each other. Though thermal equilibrium is related to equilibrium of heat, we can consider thermal equilibrium as related to temperature since heat content depends on the temperature of the body. In layman’s terms we can say that when two bodies have temperature equal to third body then the two also have temperature equal to each other.

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### Importance of Zeroth Law of Thermodynamics

The law looks very easy to understand and quite logical, which may make one wonder why it is given so much importance as it is just a logical fact. In actuality, based on this law the temperature scale has been set. To measure the temperature of the body some reference body is required and some characteristics of the body that change with temperature. The changes in the characteristics of the body with temperature can be taken as an indication of change in temperature.

The selected characteristic that changes with temperature is called thermometric property and the body which is used as the reference is called as thermometer. You all know the ordinary thermometer that consists of mercury in a capillary tube. In this case mercury is the reference body. As the temperature of mercury changes, its volume also changes, this property i.e. change in volume is used as the thermometric property.

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### How Temperature Scales are marked?

Now, let’s come back to our freezing and boiling point temperatures of water. To mark zero and 100 degree Celsius temperatures and other temperatures between this range the head tip of thermometer that contains mercury is dipped in water. As the temperature of water changes the level of mercury within capillary goes up or down. Initially the temperature of water is reduced so the level of mercury in capillary also goes down. The point at which water gets converted to ice is called freezing point temperature of water and the level of mercury obtained at this temperature is marked as zero degree Celsius.

Then the temperature of water is increased gradually and at some particular level it starts boiling, this temperature is called as boiling point temperature of water and the level of mercury at this temperature is marked as 100 degree Celsius. Thereafter, all the other temperatures are marked at equal distance between 0 and 100 degree Celsius. This is how zero degree Celsius as freezing point, 100 Celsius as boiling point, and other temperatures are set.

Zeroth Law is very simple to understand and one may underestimate its importance, but it is this law that has given us crucial bases for measurement of an important property such as temperature. In fact we can say that thermometer is the gift of Zeroth law of thermodynamics.

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### Zeroth Law as an important part of Thermodynamics

The scientific study of thermodynamics is based on the work of Sadi Carnot. It's various laws include: first law of thermodynamics, second law of thermodynamics, third law of thermodynamics as well as Zeroth law of thermodynamics. Thermodynamics is an important subject area studied under mechanical engineering. Many applications of thermodynamics like refrigerators, air-conditioners, automobiles, airplanes, ships etc are studied in mechanical engineering.