Zeroth Law of Thermodynamics

Zeroth law :

Consider three systems A, B and C.

Suppose A and B happened to be in thermal equilibrium.

Also, B and C are in thermal equilibrium.

Then, the above conditions imply that A and C are in thermal equilibrium..!!

If two systems are both in thermal equilibrium with a third then they are in thermal equilibrium with each other.

Why is zeroth law important?

It is because, temperature measuring instruments work based on zeroth law. For eg, when a thermometer is in contact with your body, heat flow occurs until both systems are in thermal equillibrium. At this point, we know, the temperature indicated by the thermometer is same as that of your body..!! As simple as that..!!

(In case of mercury thermometer, change in volume of mercury is the thermometric property)

Zeroth law was introduced later than other laws. But it was much more basic than the other laws, and hence the term "Zeroth".