A Simple Mercury Barometer

What occupies the space above the mercury column in the barometer s glass tube  

What prevents mercury from flowing out of the glass tube into the bowl of mercury  

Suppose the liquid used to make the barometer was water instead of mercury. How would this substitution affect the barometer Explain. 

Suppose a tiny crack formed at the top of the barometer s glass tube. How would this event affect the column of mercury Explain why. 

Copyright Glencoe/McGraw-Hill, a division of the McGraw-Hill Companies, Inc. 

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The air around us is a huge reservoir of gas that exerts pressure on the Earth s surface. This pressure of the atmosphere can be measured with an instmment called a barometer. Figure 5 shows a schematic view of a simple mercury barometer. A long glass tube, closed at one end, is filled with liquid mercury. The filled tube is inverted carefully into a dish that is partially filled with more mercuiy. The force of gravity pulls downward on the mercury in the tube. With no opposing force, the mercury would all ran out of the tube and mix with the mercury in the dish. 

In Figure 1, a simple mercury barometer is made by filling a long glass tube with mercury and then inverting the open end of the tube into a bowl of mercury. Answer the following questions about the simple mercury barometer shown here. 

Describe a simple mercury barometer. How is such a barometer used to measure the pressure of the atmosphere ... 

Figure 6.5 Setting up a simple mercury barometer. Standard atmospheric pressure is 760 mm of mercury.

Atmospheric pressure varies with atmospheric conditions and distance above sea level. The atmospheric pressure decreases with increasing elevation because there is a smaller mass of air above it. Approximately one-half of the matter in the atmosphere is less than 20,000 feet above sea level. Thus, atmospheric pressure is only about one-half as great at 20,000 feet as it is at sea level. Mountain climbers and pilots use portable barometers to determine their altitudes (Figure 12-2). At sea level, at a latitude of 45 , the average atmospheric pressure supports a column of mercury 760 mm high in a simple mercury barometer when the mercury and air are at 0°C. This average sea-level pressure of 760 mm Hg is called one atmosphere of pressure. 

A manometer is similar to a barometer, but in a manometer gases exert pressure on both liquid surfaces. Consequently, a manometer measures the difference in pressures exerted by two gases. A simple manometer, shown in Figure 5 3, is a U-shaped glass tube containing mercuiy. One side of the tube is exposed to the atmosphere and the other to a gas whose pressure we want to measure, hi Figure 5-3. the pressure exerted by the atmosphere is less than the pressure exerted by the gas in the bulb. The difference in heights of mercury (Ah, in mm) between the two sides of the manometer depends on the difference in the pressures. 

Gas pressure inside a container is often measured using an open-end manometer, a simple instrument similar in principle to the mercury barometer. As shown in Figure 9.4, an open-end manometer consists of a U-tube filled with mercury, with one end connected to a gas-filled container and the other end open to the... 

Pressure can be measured with many different types of devices. A common instrument used to measure atmospheric pressure is called the barometer. A simple barometer consists of a tube closed at one end and open at the other. The tube is filled with a high density liquid such as mercury and inverted in a larger reservoir of mercury which is open to the atmosphere. This geometry will result in a vacuum at the top of the glass tube as the mercury inside the tube runs downward, out of the tube and into the reservoir of mercury (Figure 3.2). 

All gases exert pressure. At any point within the gas, the gas exerts an equal pressure in all directions. Gases at rest exert a pressure equal to the pressure exerted on them. An easy method of measuring gas pressure is to use a simple barometer, as shown in Figure 12.1. Fill a long tube with mercury, hold a finger over the open end, and invert the tube into a dish of mercury. When the... 

A simple barometer is made with a long tube closed at one end. Fill the tube completely with mercury. Hold a finger over the open end, invert the tube and put the open end under the surface of mercury in a Petri dish, and hold the tube vertically. The mercury in the tube falls to a height, h, determined by the pressure of the air on the surface of the mercury in the Petri dish. There is essentially no gas pressure in the tube above the mercury. Caution Mercury is toxic use rubber gloves when handling it. 

The physical properties of gases do not depend on the composition of the gas. Pressure is defined as force per nnit area. The pressure of a gas may be measured with a simple barometer, and one of the usual units used for pressure is related to that apparatus. A torr is the pressure required to hold one millimeter of mercury vertically in the barometer, and a standard atmosphere is the pressure required to hold 760 mm Hg vertically in the barometer (Section 12.1). 

Thus, the height of mercury in a simple barometer is independent of the cross-sectional area of the tube. 

The pressure of the vapor present at equilibrium is called the equilibrium vapor pressure, or, more commonly, the vapor pressure of the liquid. A simple barometer can measure the vapor pressure of a liquid, as shown in Fig. L6.46. The liquid is injected at the bottom of the tube of mercury and floats to the surface because the mercury is so dense. A portion of the liquid evaporates at the top of the column, producing a vapor whose pressure pushes some mercury out of the tube. When the system reaches equilibrium, the va-... 

A convenient arrangement of the manometer and the connections between the pump and the receiver is represented in Fig. 12. A number of forms of manometers are used to register the pressure inside the apparatus. A simple form which can be readily made from a supply of mercury, a meter-stick, and a piece of glass tubing is illustrated in the figure. In order to determine the pressure in the apparatus the readings on the scale opposite the levels of the mercury in the manometer are subtracted, and this difference is subtracted from the height of the barometer. 

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... 

At equilibrium, the molecules in the vapor exert a pressure like any other gas. The pressure exerted by a vapor in equilibrium with its liquid is known as the vapor vapor pressure pressure of the liquid. The vapor pressure may be thought of as a measure of the escaping tendency of molecules to go from the liquid to the vapor state. The vapor pressure of a liquid is independent of the amount of liquid and vapor present, but it increases as the temperature rises. We can measure the vapor pressure of a liquid by using a simple barometer as shown in Figure 13.4. Mercury is so dense that if we inject a sample of liquid at the bottom of the Hg tube, the liquid rises to the top of the Hg column. It is than trapped in a closed space where it produces a vapor that pushes down the Hg column. When equilibrium between the liquid and vapor is reached, we can measure the vapor pressure by the change in height of the Hg column. 

Figure 10. 38 I (a) The vapor pressure of a liquid can be measured easily using a simple barometer of the type shown here, (b) The three liquids, water, ethanol (C2H5OH), and diethyl ether [( 21-15)20], have quite different vapor pressures. Ether is by far the most volatile of the three. Note that in each case a little liquid remains (floating on the mercury).

The atmospheric pressure can be measured by a simple device called a barometer. A simple barometer can be made by filling mercury in a tube (longer than 76 cm) closed at one end and inverting it in an open vessel containing mercury. The mercury level in the tube adjusts itself and stands approximately 76 cm above the level of mercury in the open vessel. 

A simple barometer, an instrument used to measure atmospheric pressure, consists of a long glass tube, closed at one end and filled with mercury. The tube is carefully inverted in a container of... 

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