Put two objects with different cold and hot together and touch each other. After a period of time, the temperature of the two objects will reach the same degree of cold and hot.
At this time, in scientific terms, the two objects reached thermal equilibrium with each other.
Furthermore, if there are three objects, namely object A, object B, and object C, the degree of coldness and hotness are different,
The object A and the object B are separated from each other, and the object A and the object B are brought into contact with the object C. After a period of time, both object A and object C and object B and object C have reached thermal equilibrium.
At this time, if the object A and the object B are brought into contact again, it will be found that there is no heat transfer between the object A and the object B, and the respective temperatures do not change.
This shows that object A and object B are in thermal equilibrium when they start to contact.
This leads to a conclusion: If both objects A and B are in thermal equilibrium with the third object C, then A and B must also be in thermal equilibrium.
That is, all objects in thermal equilibrium with each other have the same temperature.
This is the basis for measuring the temperature of an object with a thermometer.
