Barometer A device for measuring

Barometer a device for measuring atmospheric pressure. (5.1) Base a substance that produces hydroxide ions in aqueous solution, a proton acceptor. (7.2)... 

Barometer A device for measuring atmospheric pressure. See Figures 12-1 and 12-2. The liquid is usually mercury. 

Band theory molecular orbital theory of metals. (13 J) Barometer a device for measuring the pressure of the atmosphere. (5.1)... 

Figure 1.2 A barometer is a device for measuring pressures. A vacuum-filled glass tube (sealed at one end) is placed in a trough of mercury with its open end beneath the surface of the liquid metal. When the tube is erected, the pressure of the external air presses on the surface and forces mercury up the tube. The height of the mercury column li is directly proportional to the external pressure p...

During the Renaissance, Europeans developed instruments that could refine these ancient Greek theories. The Italian scientist Galileo, for example, used a closed glass container with a system of gauges that showed how air expands and contracts at different temperatures (the principle of the thermometer). The French philosopher Blaise Pascal developed what became the barometer, a device to measure surrounding levels of atmospheric pressure. 

Pressure is defined as force per unit area—for example, pounds per square inch (Ib/in. ), commonly known as psi. Pressure may be expressed in many different units, as we shall see. The mercury barometer is a simple device for measuring atmospheric pressures. Figure 12-la illustrates the heart of the mercury barometer. A glass tube (about 800 mm long) is sealed at one end, filled with mercury, and then carefully inverted into a dish of mercury without air being allowed to enter. The mercury in the tube falls to the level at which the pressure of the air on the sirrface of the mercury in the dish equals the gravitational pull downward on the mercury in the tube. The air pressure is measured in terms... 

Figure 12-1 Some laboratory devices for measuring pressure, (a) Schematic diagram of a closed-end barometer. At the level of the lower mercury surface, the pressure both inside and outside the tube must be equal to that of the atmosphere. There is no air inside the tube, so the pressure is exerted only by the mercury column h mm high. Hence, the atmospheric pressure must equal the pressure exerted by h mm Hg, or h torr. (b) The two-arm mercury barometer is called a manometer. In this sample, the pressure of the gas inside the flask is greater than the external atmospheric pressure. At the level of the lower mercury surface, the total pressure on the mercury in the left arm must equal the total pressure on the mercury in the right arm. The pressure exerted by the gas is equal to the external pressure plus the pressure exerted by the mercury column of height h mm, or (in torr) = (in torr) + h torr. (c) When the gas pressure measured by the manometer is less than the external atmospheric pressure, the pressure exerted by the atmosphere is equal to the gas pressure plus the pressure exerted by the mercury column, or...

Because a manometer is a device for measuringj pressure differences, to use one to measure absolute pressure we must measure the difference between the pressure in question and a perfect vacuum. In principle this is impossible, because there is no such thing as a perfect vacuum,[but in practice we may produce vacuums of sufficient quality that the error introduced by calling them perfect is negligible. This idea is used in the mercury barometer shown in Fig. 2.16. This common device is found in most laboratories for measuring the pressure of the atmosphere. The pressure of the atmosphere acts on the mercury in the cup at the bottom and is opposed by the weight of the column of mercury. Calculating this, we find... 

A manometer is a device used to measure the pressure of gases other than the atmosphere. The principle of operation of a manometer is similar to that of a barometer. There are two types of manometers, shown in Figure 5.4. The closed-tube manometer is normally used to measure pressures below atmospheric pressure [Figure 5.4(a)] while the open-tube manometer is better suited for measuring pressures equal to or greater than atmospheric pressure [Figure 5.4(b)]. 

We use various devices to measure the pressures of enclosed gases. Tire gauges, for example, measure the pressure of air in automobile and bicycle tires. In laboratories we sometimes use a manometer, which operates on a principle similar to that of a barometer, as shown in Sample Exercise 10.2. 

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