The earliest thermometer was constructed by Galileo in 1593. Newton suggested the necessity of the fixed points. The temperature of the melting point of ice is taken as the lower fixed point and the temperature of steam at a pressure of 76 cm of Hg (normal pressure) is taken as the upper fixed point.

Celsius, in 1742, suggested the centigrade system of temperature. He marked zero at the lower fixed point and 100 at the upper fixed point. The interval between the two fixed points is divided into 100 equal parts. Each part or degree represents 1°C or 1° Celsius. The scale is also known as the Celsius scale.

## Fahrenheit Scale

Fahrenheit, in 1720, suggested this scale by taking zero as the temperature of the human body. Later the correct temperature of the human body on this scale was found to be 98.4°F. The lower fixed point is marked as 32 and the upper fixed point is marked as 212. The interval is divided into 180 equal parts. Each part or degree represents 1°F.

## Relation Between Centigrade and Fahrenheit Scales

Consider two identical thermometers marked in Centigrade and Fahrenheit scales. Place the two thermometers in a bath at a certain fixed temperature. Mercury in each thermometer stands to the same level in the figure.

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