Measurement of temperature :
Temperature is measured by an instrument called thermometer.
A simple thermometer has a long, narrow, uniform glass tube at one end of which there is a glass bulb filled with mercury.
Along the length of the glass tube, there are graduations (Scales) made on the basis of two standard temperatures. These are called lower fixed point and upper fixed point.

Lower fixed point is the temperature at which ice melts and upper fixed point is the temperature at which water boils.
There are two commonly used temperature scales
(i) Celsius Scale        (ii) Fahrenheit scale        (iii) Kelvin Scale

(a)     Celsius or Centigrade Scale :
As the name suggests, this scale has 100 divisions between the upper and lower standard points. This scale was introduced by a Swedish astronomer Celsius and is known after his name. Each division on this scale is called one degree centigrade or one          degree Celsius and is written as 0C. More sensitive thermometers have 200 divisions between standard points and each division is equal to ½ 0C. Sometimes these thermometers are called half 0C thermometers.

(b)     Fahrenheit Scale :
This scale was introduced by Fahrenheit. On this scale 320 F represents the melting point of ice and
2120 F the steam point. Zero is marked 320 F below the ice point. The length in between the standard points is divided into 180 equal parts. Each division on this scale is called 10 F. This scale is widely used for meteorological and clinical purposes.

Relation between Celsius and Fahrenheit scales :

Where C and F are the corresponding temperatures shown on the scales of thermometers in Celsius and Fahrenheit units respectively.

(c)     Kelvin Scale :
The scale of measurement of temperature, in which lowest temperature is zero Kelvin (–2730 C) is called
Kelvin scale. This is also called S.I. scale of temperature.

•    Characteristics of Kelvin scale :
(i) There cannot be any temperature below zero Kelvin.
(ii) The temperature is expressed in (K), but no degree symbol is attached to it.
(iii) Rise in temperature in kelvin = Rise in temperature in degree Celsius.

Note :     Conversion of temperature from Kelvin scale to Celsius scale is K = (C + 273)

•    Relation between different temperature scales :