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The Zeroth Law of Thermodynamics states that if two systems are in thermal equilibrium with a third system at the same time , they are in thermal equilibrium with each other. In other words this law deals with thermal equilibrium.
For example, If A is in thermal equilibrium with B and C, then B is in thermal equilibrium with C. This means that all three are at the same temperature, and it forms a kind of ground for comparison of temperatures. It is called the Zeroth Law because it precedes the First and Second Laws of Thermodynamics.
The above explanation allows us to create thermometers. 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.
In a constant-volume gas thermometer how can you measure temperature by measuring the pressure of the gas?
Answer: When a gas is heated the pressure increases if it is restricted to a fixed volume. When a gas is cooled the pressure decreases at constant volume. This effect is used to measure temperature by measuring the pressure of the gas when it is placed in thermal contact with the substance whose temperature is being measured. If the pressure of a gas in a constant-volume thermometer is plotted versus temperature a straight line is obtained. If you extend the line down to zero pressure you find the temperature at this point to be -273.15 degrees Celsius.
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