Zeroth law of thermodynamics

Zeroth law of thermodynamics is the basis of temperature measurement. The property which distinguishes thermodynamics from other sciences is temperature. 

One might say that temperature bears as important relation to thermodynamics as the force does to statics or velocity does to dynamics. 

When two bodies maintain at different temperatures are brought into contact, after some time they attain a common temperature and are then said to exist in thermal equilibrium.

Zeroth law of thermodynamics :

When a body A is in thermal equilibrium with a body B, and also separately with a body C, then B and C will be in thermal equilibrium with each other. 

Notes :

In order to obtain a quantitative measure of temperature, a reference body is used, and a certain physical characteristic of this body which changes with temperature is selected. 
The change in the selected characteristics may be taken as an indication of the change in temperature. The selected characteristics are called thermometric property, and the reference body which is used in the determination of temperature is called thermometer. 

There are five different kinds of thermometer used are following below :
  1. Constant volume gas thermometer
  2. Constant pressure gas thermometer 
  3. Electrical resistance thermometer
  4. Thermocouple 
  5. Mercury in glass thermometer