An instrument that measures the temperature of a body is called a Thermometer. From Zeroth law, we see that, to determine the thermal equilibrium between A and B. C the third body can be used as a reference body. In this case, C acts as a Thermometer.
Principle of Thermometer:
The main principle of a thermometer is that solids and liquids expand on heating. As the temperature rises, mercury expands causing it to move upwards and depict the temperature. According to Medically, a maximum thermometer is often used, which indicates the maximum temperature reached even after it is removed from the body. As the temperature of the bulb rises, the liquid expands up the tube through the constriction or kink which prevents the mercury from falling back.
A scale of temperature is needed to measure the temperature of a body accurately. To draw up a scale we follow the norm that a hot body is at a higher temperature while a cold body is at a lower temperature. It implies that when a hot and a cold body are in contact, the temperature of the hot body will decrease and that of the cold body will increase. When the temperatures of both bodies become equal, the bodies attain thermal equilibrium.
Fixed point: To set up a temperature scale, one or more conveniently reproducible, well-established temperatures are chosen as standard temperatures. These fixed temperatures are called fixed points.
There are several kinds of thermometers for practical use. There are liquid thermometers (mercury or alcohol), ideal gas thermometers, platinum-resistance thermometers, etc. Among these, there is a special kind of thermometer which is used to deduce the accurate value of different fixed points. This is known as Primary Thermometer.
All thermometers, other than the ideal gas thermometer are secondary thermometers. They are called secondary because they are calibrated according to the values of temperatures fixed. Points are already determined by an ideal gas thermometer. So, all secondary thermometers actually obey the ideal gas temperature scale. However, it should be noted that secondary thermometers should never be regarded as less reliable or less efficient. Rather, they’re often highly accurate and very easy to use.