As we know now that the most materials change their volume on heating. Such property of materials can be used to design thermometers to measure the temperature. There are varieties of thermometers in existence based on the phase (state) of materials being used in the thermometer. They are solid state thermometer, liquid thermometer, and gas thermometer. For example: If a solid substance is used in the thermometers, then the thermometers are called solid state thermometers. Platinum resistance thermometers, the thermometer strip on the wall of aquarium, etc. are solid state thermometers. If liquid is used in a thermometer, then it is called a liquid thermometer. Mercury thermometer, alcohol thermometer, etc. are some examples. If a gas is used, then the thermometer is called a gas thermometer. Hydrogen gas thermometer, helium gas thermometer, nitrogen gas thermometer, etc. are some examples.
The most common type of thermometer is the liquid thermometer. In this thermometer, mercury or alcohol is filled in a narrow glass tube with a metallic vessel at the bottom. However, due to major developments in electronics, digital thermometers are taking over the market to become more common thesedays. Here we are going to study the temperature scale on the basis of the liquid thermometer.
There are many types of thermometers in use, but we concentrate our study on three basic types. They are the Fahrenheit thermometer, the Celsius (or Centigrade) thermometer, and the Kelvin scale. Thermometers need calibration once they are made to provide correct temperature readings. To calibrate the thermometer the choice of unit and two reference points are taken. A centigrade (Celsius) thermometer has 100 divisions between the freezing point (0 degree Celsius) and the boiling point (100 degree Celsius) of water. A temperature of 76 degree Celsius is abbreviated as \(76 ^oC\text{.}\) In the Fahrenheit thermometer, there are 180 divisions between the freezing point (32 degrees Fahrenheit) and the boiling point (212 degrees Fahrenheit) of water. A temperature of 76 degree Fahrenheit is abbreviated as \(76 ^oF\text{.}\) Kelvin scale (usually not called a thermometer) of temperature is the standard metric system of temperature measurement and mostly used by scientists. The Kelvin scale has 100 equal degree increments between the freezing point and the boiling point of water just as the Celsius scale. However, the zero-degree mark on the Kelvin scale is \(273.15 \) units cooler than it is on the Celsius scale. So a temperature of \(0\) Kelvin is equivalent to a temperature of \(-273.15 ^oC\text{.}\) Traditionally, a degree symbol is not assigned to the Kelvin scale. So, a temperature of 300 units above 0 Kelvin is referred to as 300 Kelvin and not 300 degrees Kelvin. Hence 300 Kelvin is abbreviated as \(300 K\text{.}\) Conversions among Celsius, Fahrenheit, and Kelvin scales are summarized below. The figure below shows Fahrenheit, Celsius, and Kelvin scale thermometers.
(a)Fahrenheit
(b)Celsius
(c)Kelvin scale
Figure4.1.1.Thermometers
Consider these thermometers are put into three cups of coffee with the same temperature. Where the Celsius thermometer reads a temperature of \(^oC\text{,}\) the Fahrenheit thermometer reads a temperature of \(^oF\text{,}\) and a Kelvin scale reads a temperature of \(K\text{,}\) then we can find their relationship as follows. Let’s take one unknown thermometer which has lower reference points marked as \(L\) (for freezing point of water) and upper reference point marked as \(U\) (for boiling point of water). When dipped into the identical cup of coffee, it reads the temperature as \(X\text{.}\) Hence we can note the temperature of coffee as \(\frac{X-L}{U-L}\text{.}\) By comparing all the thermometers readings we get the relation as
Kelvin Scale: The scale of thermometers depends upon the physical property (thermal expansion, electrical resistance, etc.) of the substance that is used in the thermometer. The physical property (e.g. thermal expansion) of materials varies differently at the different range of temperatures hence all these thermometers have a limited use. With the idea of constructing a temperature scale which should be independent on the material properties, Scottish physicist William Thomson, Baron Kelvin proposed on the basis of thermodynamics principles that the lowest possible temperature that could be achieved was \(-273 ^oC\text{,}\) in 1848. This minimum temperature is the zero point on the Kelvin scale and is known as absolute zero. Kelvin scale is a hypothetical (theoretical) thermometer scale and is also known as absolute scale. 1
According to III law of thermodynamics, it is impossible to obtain absolute zero by any physical means.
