A thermometer is an instrument that can measure the temperature of any hot body when brought in contact with the body. There are three different scales to measure the hotness of a body:

(1) Celsius Scale
(2) Fahrenheit scale
(3) Kelvin scale.

1. Celsius Scale: The standard Temperatures are measured between two points of the thermometer. i.e., between 0°C and 100°C which are nothing but the melting and boiling points of water. The Readings in between are divided in to 100 equal parts called Degrees. Hence this type of calibrated thermometer is known as centigrade thermometer (or) Celsius thermometer. Each division on this scale is termed as degree centigrade and is written as °C.
2. Fahrenheit Scale: On this scale 32° F represents the melting point of Ice and 212°F. The Length in between these two standard points is divided in to 180 equal parts. Each division on this scale is called 1°F. This scale is widely used for Meteorological and Clinical purposes.
3. Kelvin Scale: This scale is calibrated in terms of heat energy itself. The number 0 is assigned to the lowest possible temperature called the absolute zero, at which a substance has absolutely no kinetic energy to give up. Absolute zero corresponds to −273°C on the Celsius scale. The temperature of melting ice is +273 Kelvin and the boiling point of water is 373 K. There are no negative numbers on the Kelvin scale. K = C + 273, where C is the temperature measured in Centigrade scale.

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