The fixed temperature points
The principle underlying the graduation of all types of thermometer is to choose two fixed and easily obtainable temperatures called the upper and lower fixed points and then to divide the interval between them into a number of equal parts or degrees. The upper fixed point is the temperature of steam from water boiling under standard atmospheric pressure of 760 mmHg. The temperature of the boiling water itself is not used as a fixed point for tw reasons. First, local overheating may occur, accompanied by “bumping” as th water boils; secondly, any impurities which may be present will raise the boiling point. The temperature of the steam just above the water will always be constan and depends only on the barometric pressure at the time. The lower fixed point is the temperature of pure melting ice. The ice must be pure, since the presence of impurities will lower the melting-point.