Scales of Temperature

A reference scale with respect to which the temperatures can be measured is known as 'scale of temperature'. Various scales of temperatures are in use. Important scales of temperature are:

• Celsius scale

• Fahrenheit scale

• Kelvin Scale

Lower and upper fixed point of temperature

To devise a scale of temperature, fixed reference points (temperature) are required, with respect to which all other temperatures are measured. For both Celsius and Fahrenheit Scales of temperatures, the fixed points are as follows:

Lower fixed point:

Melting point of pure ice at normal atmospheric pressure is regarded as the lower fixed point.

Upper fixed point:

Boiling point of pure water at normal atmospheric pressure is regarded as the lower fixed point.

Celsius Scale of Temperature

Celsius scale of temperature was devised by a Swedish astronomer Anders Celsius (1701-1744). In this scale, the lower fixed point (the temperature of melting ice at normal atmospheric pressure) is taken as zero degree Celsius, written as 0⁰C. The upper fixed point (the temperature of pure boiling water at normal pressure of 76 cm of mercury) is considered to be hundred degree Celsius, written as 100^OC. The interval between 0⁰C and 100⁰C is divided into hundred equal parts. Each part represents 1⁰C. This is a convenient scale of temperature, which is widely used.

Celsius Scale of Temperature

Fahrenheit Scale of Temperature

This scale of temperature was devised by Gabriel Fahrenheit (1687-1736). The lower and upper fixed points in this scale are considered as 32⁰F and 212⁰F respectively. The interval of 180⁰ F is divided into 180 equal parts. Each part is known as 1⁰F. This is widely used by doctors.

Fahrenheit Scale of Temperature

Relation between Celsius and Fahrenheit scales of temperature:

100⁰C = 180⁰F

Relation between Celsius and Fahrenheit scales of temperature

In the figure, AO represents temperature C. On Celsius scale and O'B represents the same temperature on Fahrenheit scale. Let us see how we can relate them.

Let AO = C divisions = C⁰ C.

But ice point on Fahrenheit scale corresponds to 32⁰F.

For example:

1) Convert 20⁰C into Fahrenheit scale

(2) Convert 85⁰C into Fahrenheit Scale.

Similarly, to represent temperature on Fahrenheit Scale into Celsius Scale:

Examples:

1. Represent 59⁰ F on Celsius Scale.

59⁰ F=15⁰ C

2. Represent 77⁰ F on Celsius Scale.

77⁰ F=25⁰ C