What is the Temperature?

Temperature is measured by a device called a thermometer. Thermometer word came from Greek words ‘thermos’ ( means – hot) and ‘metron’ ( means – measure)

In a thermometer, mercury is used. We know that most materials expand with rising temperatures. And mercury is a metal that is in the liquid state at atmospheric temperature and also expands linearly with rising in temperature.

Fig. 2: Mercury-based thermometer

Mercury is stored in the glass bulb of the thermometer. When this glass bulb is exposed to the heat, the mercury metal expands and rises in the capillary. The level of the mercury then shows the corresponding reading of the temperature.
In the initial stages instead of mercury, alcohol was used. But mercury-based thermometers are the most common and accurate. 

In 1714, the Polish-born Dutch physicist, inventor, and scientific instrument maker Daniel Gabriel Fahrenheit (1686–1736) introduced a mercury-based thermometer.

Apart from conventional thermometers, some digital ( Electronic) equipment is used, like a thermistor, thermocouple, etc. We know that as the temperature of the metal increases the resistance of the metal increases means its conductivity decreases. And this principle of electricity is used in these devices.
The emission of infrared is also used to design a thermometer. An infrared thermometer is used to measure the temperature of a person or an object at a distance (This type of device you may have seen in a covid pandemic). Nowadays these types of equipment are widely used for accuracy and in instrumentation.

What are the units of the Temperature?

There are mainly three units of the temperature –

• Centigrade or Celsius scale
• Fahrenheit scale
• Kelvin scale

Some other common temperature units are Rankine, Newton, Rømer, Réaumur, and Delisle.

1. Fahrenheit scale: Daniel Gabriel Fahrenheit (1686–1736) introduced this scale in 1724. According to this scale at atmospheric pressure water freezes at 32°F and boils at 212 °F. And the difference between these two values is divided into equal 180 units. Each unit is called a degree Fahrenheit or °F. The normal human body temperature is 98.6 °F.

2. Centigrade or Celsius scale: This scale was introduced by Anders Celsius (1701–1744) in 1742. Centigrade means 100, it is derived from the Latin word ‘centi’ (means – hundred) and ‘grades’ ( means – degrees). In the year 1948, the centigrade was renamed celsius in honor of Anders Celsius. As per this scale at atmospheric pressure water freezes at 0 °C and boils at 100 °C. And the difference between these two values is divided into 100 equal parts. Each part is called degree centigrade or degree celsius(°C).  This scale is widely used by engineers in India. The normal human body temperature is 37 °C.

3. Kelvin scale: Lord William Kelvin (1824–1907) introduced Kelvin (K) scale in 1854. The freezing point of the water at atmospheric pressure is 273.15 K and the boiling point is 373.15 K. And the difference is divided into 100 equal parts. Each division in the scale is called Kelvin. Neither the term degree nor the symbol (°) is used. The normal human body temperature is 310.15 K.
The Kelvin scale is based on absolute zero temperature.

What is Absolute zero temperature?

The absolute zero temperature is the theoretical minimum temperature we can achieve in the universe.

Fig. 3: Comparison between different units of temperature

We know that temperature is a measure of the average kinetic energy of the molecules. More temperature means greater molecular motion. When we lower the temperature of the material the motion of the molecules decreases and if we keep lowering the temperature the condition comes where the molecular motion becomes zero, the temperature at this point is called absolute zero temperature.

At absolute zero temperature, metal becomes a superconductor.

The absolute zero temperature is 0 K. If you have noticed, you come to know that there is no negative scale in Kelvin because 0 K is the minimum temperature, therefore this scale is widely used to measure extremely low temperatures in scientific research.  

Absolute zero temperature in °C = -273.15 °C
Absolute zero temperature in °F = -459.58 °F

How to convert °C to °F  OR  °C to K  OR  °F to K and vice versa?

The different units of temperature can be converted by using the following formulas,

°F = (°C X 1.8) + 32

K = °C+ 273.15

°F = (K – 273.15) 9/5 + 32

SI unit of Temperature is – Kelvin (K)