Temperature - a concept originates with our sense perceptions.

Several properties of materials change with temperature in a repeatable and predictable way, and this forms the basis for accurate temperature.

Examples: volume expansion (mercury), electrical resistance.

Any body with at least one measurable property that changes with temperature can be used as a thermometer. Such a property is called thermometric property. The particular substance that exhibits changes in the thermometric property is known as the thermometric substance.

Devices	Property	Substance
Liquid-in-Glass Thermometers	Volume (Height)	Mercury, Alcohol
Gas Thermometers	Pressure	Helium, Hydrogen
Thermocouples	Electromotive Force (emf)	Copper-Constantan, Platinum-Platinum & Rhodium
RTD-Resistance Temperature Devices	Electric Resistance	Platinum, Nickel Copper
Thermistor	Electric Resistance	Semiconductors
The gas thermometers are used as a standard worldwide by bureaus of standards and research laboratories.

3.2 Zeroth Law of Thermodynamics

Two systems in thermal equilibrium with a third system are in thermal equilibrium with each other.

It may seem silly that such an obvious fact is called one of the basic law of thermodynamics. However, it cannot be concluded from the other law of thermodynamics, and it serves as a basis for the validity of temperature measurement.

By replacing the third body with a thermometer, the zeroth law can be restated as: Two bodies are in thermal equilibrium if both have the same temperature reading (even if they are not in contact).

Zeroth Law of Thermodynamics was first formulated by R. H. Fowler in 1931. Since the so-called First and Second Law of Thermodynamics were formulated much earlier, and since in a logical development of the subject it must be stated before them, it has been agreed to designate it as the Zeroth Law.

Temperature scales enable scientists to use a common basis for temperature measurement. Temperature scales are defined by the numerical values assigned to standard fixed points.

Examples:

Ice point: a mixture of ice and water which is in equilibrium with air saturated with vapor at 1-atm pressure.

Steam point: a mixture of liquid water and water vapor, with no air, in equilibrium at 1-atm pressure.

Triple point: the state of equilibrium among steam, ice and liquid water.

Celsius Scale (Centigrade Scale)		Fahrenheit Scale
Named After Swedish Astronomer A. Celsius		Named After German Instrument Maker G. Fahrenheit
Ice Point	0 C	Ice Point	32 F
Steam Point	100 C	Steam Point	212 F
Triple Point	0.01 C	Triple Point	32.02 F

At the triple point of water, T=273.16K (0.01),

or

[1]

Repeat the measurements several times with less gas in the bulb in each successive attempt. For each trial the ratio is calculated from Eq. (1) and plotted versus the corresponding reference pressure of the gas at the triple point temperature. As pressure decreases, the values for different gases approach one another, and in the limit as pressure tends to zero, the same value for is obtained for each gas.

The temperature scale thus defined is independent of the properties of any one gas.