Q.Why is it necessary to make a physical contact between a thermocouple (or thermometer) and the object for measuring its temperature?

The statement describing mutual thermal equilibrium through a common third system is the zeroth law.

The zeroth law of thermodynamics states that if system A is in thermal equilibrium with system C, and system B is also in thermal equilibrium with system C, then A and B are in thermal equilibrium with each other. This law establishes the existence of temperature as a physical quantity: all systems in mutual thermal equilibrium can …

The zeroth law establishes that thermal equilibrium is a transitive relation, which is exactly what lets us define and measure temperature using a thermometer.

The zeroth law of thermodynamics states: if system A is in thermal equilibrium with system C, and system B is also in thermal equilibrium with system C, then A and B are in thermal equilibrium with each other.

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The zeroth law of thermodynamics defines the thermodynamic variable temperature by establishing thermal equilibrium as a transitive relation between systems.

The zeroth law states: if two systems A and B are separately in thermal equilibrium with a third system C, then A and B are also in thermal equilibrium with each other. This may look like a simple observation, but its importance is that it lets us assign a single number — temperature — to represent the thermal state of a system: since equilibrium is transitive, all systems in mutual thermal equilibrium can be said to have the "same value" of this quantity, which we call temperatu …