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THE MERCURY BAROMETER (I-5)
The problem. It is important to know the air pressure when ascending mountains or making flights in balloons or airplanes, when working under water or operating submarines, and in many manufacturing operations. We found it convenient to use a gauge for measuring air pressure in Problem 4. The task of devising a convenient instrument for measuring air pressure was solved long ago by the invention of the barometer. How is this instrument constructed and how does it operate ?
What to use. Ring stand, one ring, clamp, short glass tube, meter stick, two. pounds of mercury, beaker, small funnel made from a four-inch test tube, glass tube three sixteenths of an inch in diameter and forty inches long and sealed at one end, two-hole rubber stopper, widemouth bottle, beeswax or paraffin, and rubber tubing.
What to do. 1. Place the rubber stopper on the glass tube as in figure 5. Fill the glass tube with mercury. The mer
Fig. 5 cury should be poured slowly through the small funnel until the tube is filled. A bottle which has a layer of beeswax or paraffin one-half inch thick on the bottom should be inverted over the tube and pressed down firmly. Invert the tube and immediately pour some mercury into the bottle. Lift the glass tube slightly and then make secure with the stopper. Assemble the apparatus as in figure 5.
2. Measure the vertical distance from the mercury level in the basin to the level in the tube. Tilt the tube and measure the vertical distance again.
3. Remove some air from the bottle with the mouth. Blow into the bottle.
4. Answer the questions in the following paragraph.
Questions. What is the height of the mercury column? How does tilting the tube affect the vertical height? What holds the mercury column in place ? What happens when the air is removed ? Why? What occurs when air is blown into the bottle ? Why? What pressure change can you cause by suction ? by blowing ? Summarize the conclusions from this experiment.
Suggestions for report. After discussing the experiment write correct answers to the questions given above.
Reference work. Read sections 7 to 11. Explain the operation of a pocket aneroid barometer. How is a standard barometer made? a barograph ? Look up the history of the invention of the barometer and prepare a report for the class.
Optional problems. With a thistle tube and a sheet of rubber show that the air pressure is the same in all directions at any one point.
EFFECTS PRODUCED BY CHANGING THE TEMPERATURE
OF AIR (I-6)
The problem. Many familiar occurrences depend upon the changing temperature of the air. It is commonly supposed that the impure air in a room is heavy and that it sinks to the floor. Also, it is commonly though wrongly supposed that damp air is heavier than dry air. It is often noted that corn or other crops are frosted first in the valley, indicating that the temperature is lower than on the hillside. The operation of the hot-air furnace also depends upon air temperatures. It is found that milk kept in a refrigerator at 60° F. will develop about fifteen times as many bacteria in one day as milk at a temperature of 50° F. Hence it is extremely important to know where to find the coldest part of the food chamber. How does air behave when
Fig. 6 cooled or heated ?
What to use. Wide-mouth bottle, test tube, one-hole rubber stopper, glass tube one foot long, Bunsen burner with rubber tube, matches, and water.
What to do. 1. Assemble the apparatus as shown in figure 6.
2. Place the glass tube about one-fourth of an inch under water, and warm the test tube with both hands.
3. The Bunsen burner, which is to be used as a source of heat in the laboratory, consists of four parts: a mixing chamber, an air regulator, a base, and a gas tube. Carefully unscrew the tube and examine the burner. Note the use of the parts mentioned