The Manometer
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
p-179
2025-11-12
5
is an instrument for measuring the pressure of gases. It is made in two forms, the open and the closed.
- The Open Manometer consists of a bent glass tube held in a vertical position by a frame having a graduated scale between the two arms of the tube (Fig. 1). Mercury or some other suitable liquid is poured into the manometer so that it stands at the same height in both arms. The short arm is connected with the vessel containing the gas, and when the gas is turned on, the pressure is shown by the difference in level of the columns. The length of the column of liquid CD is the measure of the pressure in excess of 1 atmosphere.
- The Closed Manometer (Fig. 2), or pressure gauge, differs from the open manometer in being closed at one end and much shorter. Before the pressure is turned on, the mercury stands at the level BD, with the ordinary atmospheric pressure in each arm. When the stopcock is turned, the pressure of the gas not only maintains the pressure of the mercury column CE, but also compresses the volume of air from AB to AC. Pressure gauges are calibrated to show the pressure in excess of 1 atmosphere.
FIG.2.
In a closed manometer sometimes used to measure small pressures, the closed end of the tube is entirely filled with mercury at atmospheric pressure. As the air is exhausted from the vessel with which the manometer is connected, the mercury sinks in the closed end; the height of the column shows the pressure just as a barometer does.
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