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To measure the density of dry air A clean, dry round-bottomed flask is fitted with rubber bung through which passes a short glass tube carrying a short length of rubber pressure tubing fitted with a screw clip. With the clip open, the air is evacuated from the flask by connecting it to a good vacuum pump (Fig. 10.16). All […]

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Calculation of atmospheric pressure in various units For ordinary laboratory purposes, atmospheric pressure is measured in mmHg by a Fortin barometer (page 113). The average value of the barometeric height at sea-level over a long period is 760 mmHg, and this has been taken as a unit of pressure and called standard atmospheric pressure or one atmosphere. The unit […]

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Pressure in relation to diving and aviation It should be clear from the previous paragraph that for every 10.3 m (approx. 10 m in sea-water) a diver descends the pressure on his body increases by one atmosphere. The aqualung diving-suit incorporates a rubber helmet fitted with a circular window and supplied with air from compressed-air cylinders carried on […]

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The water barometer Since the density of mercury is 13.6 g/cm ‘, it follows that if water were used as the liquid in a simple barometer the water column would have to be 76 x 13.6 em = 10.3 m long. Such a barometer was constructed in the seventeenth century by von Guericke and fixed on the outside […]

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The aneroid barometer and altimeter Barometers of the aneroid (without liquid) type are commonly used as weather glasses, the idea being that low pressure, or a sudden fall in pressure, generally indicates unsettled weather while a rising barometer or high pressure is associated with fine weather. The essential part of an aneroid barometer is a flat cylindrical metal box […]

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The Fortin barometer Accurate measurements of the barometric height, often required in the laboratory, are made with a Fortin barometer (Fig. 10.13). This precision barometer was designed in the late eighteenth century by Nicolas Fortin, a French instrument maker. The tube containing the mercury is protected by enclosing it in a brass tube, the pper part of which […]

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Pascal’s’ experiments with barometers Torricelli died a few years after the barometer experiment had been performed, an did not live to see his explanation of it, in terms of atmospheric pressure, generally accepted among scientists. After Torricelli’s death Pascal repeated the experiment in France and set up two barometers. The first of these was placed at the foot of […]

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Torricelli’s experiment. Simple barometer About the middle of the seventeenth century an Italian scientist named Torricelli, living at Pisa, suggested an experiment to discount the theory that nature abhorred a vacuum. Torricelli believed that nature’s supposed horror of a vacuum was caused simply by atmospheric pressure. In a famous experiment, first performed in 1643, he set up the first […]

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Care in the use of mercury The usefulness of mercury for scientific purposes has already been mentioned on page 100. It does, however, give ofT a small quantity of odourless vapour which is poisonous if breathed for any length of time in a poorly ventilated room. Great care should be taken not to spill it anywhere it […]

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Measurement of gas pressure by the manometer Earlier in this chapter it was stated that the pressure of the atmosphere was about 100 kNzm? (or kPa). Before explaining how this is measured let us make a study of the manometer, an instrument for measuring the pressure of gas. The manometer consists of a U-tube containing water. When both […]