The fixed points used in temperature scale are:
(i) Lower fixed point or ice point or freezing point and
(ii) Upper fixed point or steam point or boiling point.
(i) Lower fixed point: The temperature at which the pure ice melts or pure water freezes at normal pressure is called lower fixed point or ice point. The value of ice point in Celsius scale is 0°C.
(ii) Upper fixed point: The temperature at which the pure water boils, at normal pressure is called steam point or upper fixed point and it is experimentally found as 100°C.
The difference between the two fixed points in any scale is called the “fundamental interval”. This interval is divided into a certain number of equals divisions to form a temperature scale which can be used for measuring temperature. Each small divisions is called a degree in each scale.