use of thermometer is based on which law of thermodynamics........law k naam k sath explanation b de dena

Zeroth Law, 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.  The ordinary thermometer consists of mercury in a capillary tube. In this case mercury is the reference body. We can adjust the change in a thermal property, or length of a column of mercury, by putting the thermometer in thermal equilibrium with a known physical system. Metric thermometers, in Celsius, have the reference points fixed at the freezing and boiling point of water.If we then bring the thermometer into thermal equilibrium with any other system for example if you place it under your tongue we can determine the temperature of the other system by noting the change in the thermal property, which is shown in the thermometer. Objects in thermodynamic equilibrium have the same temperature.