The Fixed Points
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-240
2025-11-23
16
Since the boiling point and the freezing point of pure water are always the same under the same pressure, these points are taken as the fixed points for the thermometer.
The freezing point is determined by placing the bulb and part of the stem in snow or finely crushed ice, contained in a suitable vessel (Fig. 1). The point at which the end of the mercury column comes to rest, when close to the ice, is marked as the freezing point.
The boiling point is fixed by suspending the thermometer in the steam from boiling pure water. The bulb should be at least an inch above the water and the boiler should be tall enough so that the mercury will come only just above the stopper by which it is supported. Whenever the steam is coming briskly from the escape pipe and the mercury has ceased to rise, the end of the column is marked as the boiling point, provided the barometer reads 760 mm. at the time.
The manometer tube m (Fig. 2) shows whether the pressure of the steam is the same as that of the atmosphere. If this is not the case, a correction has to be made since the temperature of steam rises rapidly with an increase of pressure. When the pressure is near 760 mm., an increase of 27 mm. in the pressure produces a change of 1° C. in the boiling point.
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