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Thermometry is the branch of physics concerned with the measurement of temperature and the design and use of thermometers and pyrometers.

Thermometry or temperature measurement is important to a wide range of activities, including manufacturing, scientific research, and medical practice. The accurate measurement of temperature was developed only recently.

What is temperature?

Temperature is a measure of the average kinetic energy of the particles making up the substance being examined.

Temperature must not be confused with heat.

Heat is the amount of vibrational energy contained in a particular mass.

Temperature (like mass, length, time) is a chosen fundamental quantity. Therefore, arbitrarily chosen units (such as degrees Celsius, degrees Fahrenheit or Kelvin) are used to measure temperature. Temperature is measured quantitatively by constructing a thermometer which makes use of a physical property of matter that varies with temperature. A physical property that changes continuously with temperature can be used to measure temperature and is usually referred to as its thermometric property.

Interesting: Galileo invented the first thermometer. In his instrument, the changing temperature of an inverted glass vessel produced the expansion or contraction of the air within it, which in turn changed the level of the liquid with which the vessel’s long, open-mouthed neck was partially filled. This general principle was perfected in succeeding years by experimenting with liquids such as mercury and by providing a scale to measure the expansion and contraction brought about in such liquids by rising and falling temperature.

Fixed Points

For any temperature scale, it is necessary to have two fixed points. These are temperatures at which particular physical properties manifest themselves eg melting, boiling.

Common fixed points:

ice point - temperature when pure water ice is in equilibrium with liquid water at standard atmospheric pressure (76mm Hg).

steam point - temperature when pure liquid water exists in equilibrium with water vapour at standard atmospheric pressure.

triple point* - temperature when pure water ice, pure liquid water and pure water vapour exist in equilibrium.

  • there are triple points for many other substances besides water

Temperature scales are divided into a specific number of degrees between the two fixed points. The gap between the upper and lower fixed points is called the fundamental interval.

Fundamental Interval Illustration

As a matter of historical interest, the Fahrenheit scale was founded on the following fixed points:

lower fixed point 0 deg. (0 degrees F) - the coldest salt mixture temperature attained

higher fixed point 100 deg. (100 degrees F) - horse blood temperature

The Kelvin (Absolute) temperature scale

This scale uses the the triple point of water as the upper fixed point and absolute zero (zero molecular motion) as the lower fixed point. The triple point of water is assigned the temperature 273.16K while the the ice point is 273.15K and the steam point 373.15K.

Kelvin vs Celsius Scale Illustration

By definition one degree Kelvin is: 1 / 273.16 (approximately 0.00366) of the difference between absolute zero and the triple point of water.

The Celsius temperature scale

The Celsius scale was introduced to replace the Centigrade scale. The Centigrade scale had the melting point of ice as its lower fixed point and the boiling point of water as the upper fixed point, both measured at standard atmospheric pressure. However, unlike the triple point of water, the melting point of ice cannot be measured with enough precision to make it an absolute marker. The Celsius scale is closely related to the Kelvin scale. By definition one degree Celsius is: 1 / 273.16 (approximately 0.00366) of the difference between absolute zero and the triple point of water.

This is the same definition as for a degree Kelvin. So a temperature change of 1K equates to a change of 1 degrees C.

The two scales are also defined by the equation:

Theta = T - 273.15

where Theta is in degrees Celsius and T is temperature in Kelvin