Physics. Willis E. Tower
target="_blank" rel="nofollow" href="#fb3_img_img_ab497ef5-e191-5591-b17e-df597d9e10fb.png" alt=""/> Fig. 33.—A standard barometer.
56. The Barometer.—The modern barometer (Fig. 33), consists of a Torricellian tube properly mounted. Reading a barometer consists in accurately reading the height of the mercury column. This height varies from 75 to 76.5 cm. or 29 to 30 in. in localities not far from the sea-level. The atmospheric pressure varies because of disturbances in the atmosphere. It is found that these disturbances of the atmosphere pass across the country from west to east in a somewhat regular manner, hence a series of readings of the barometer may give reliable information of the movement of these disturbances and so assist in forecasting the weather. The weather Bureau has observations taken at the same moment at various stations over the country. These observations form the basis for the daily forecast of the weather.
Another form of barometer in common use is the Aneroid Barometer (Fig. 34). Its essential parts are a cylindrical air-tight box with an elastic corrugated cover. Inside the box is a partial vacuum. This makes the cover very sensitive to slight changes of pressure. The motion of the top of the box is conveyed by a series of levers to an indicating hand which moves over a dial. This barometer can be made so sensitive as to indicate the change of air pressure from a table top to the floor. It is much used by travelers, explorers, surveying parties and aviators, since the mercurial barometer is inconvenient to carry.
Important Topics
1. Weight and Pressure of air in English and metric units. How shown. Evidences.
2. Work of Galileo, Torricelli, and Perrier.
3. Barometer: construction, action, mercurial, aneroid.
Exercises
1. Do you think Archimedes' Principle applies to the air? Does Pascal's Law? Why?
2. Find the downward pressure of the mercury in a barometer tube if the cross-section is 1 sq. cm. and the height 75 cm. at the level of the mercury surface in contact with the air. (The density of mercury is 13.6 grams per cc.)
3. What is the weight of the air in a room if it is 10 × 8 × 4 meters?
4. What weight of air is in a room 10 × 15 × 10 ft.?
5. When smoke rises in a straight line from chimneys, is it an indication of a high or low barometric pressure? Why?
6. Why does a tumbler filled with water and inverted in a dish with its rim under water remain full?
7. If the barometer tube is inclined the mercury remains at the same horizontal level. How can this be explained?
8. When the mercurial barometer stands at 76 cm., how high would a water barometer stand? Explain.
9. Explain why it is possible for one to suck soda water through a tube?
10. Fill a tumbler with water. Place a sheet of paper over the top and invert. The paper clings to the tumbler and prevents the water from escaping. Explain. (See Fig. 35.)
11. Why must a kerosene oil can have two openings in order to allow the oil to flow freely?
12. Explain the action of the modern drinking fountain (Fig. 36).
(2) Compressibility and Expansibility of the air
57. Effect of Pressure on Liquids and Gases.—Both classes of fluids, liquids and gases, have many characteristics in common. Both are composed of molecules that move freely; hence both flow. At any point within a fluid the pressure is the same in all directions. Archimedes' Principle applies, therefore, to both liquids and gases.
We now come to an important difference between liquids and gases. Liquids are practically incompressible. "So much so, that if water is subjected to a pressure of 3000 kg. per sq. cm., its volume is reduced only about one-tenth." Gases show a very different behavior from liquids on being subjected to pressure. They may readily be compressed to a small fraction of their volume as is noticed on inflating a pneumatic tire. A gas has also the ability to spring back to a larger volume as soon as the pressure is released, as when a cork is driven from a pop gun. Not only is compressed air able to expand, but air under ordinary conditions will expand if it is released in a space where the pressure is less.
Hollow bodies, animals and plants, are not crushed by atmospheric pressure, because the air and gases contained within exert as much force outward as the air exerts inward.
58. Boyle's Law.—The relation between the volume and pressure of a gas was first investigated by Robert Boyle in the seventeenth century. The experiment by which he first discovered the law or the relation between the volume and the pressure of a gas is briefly described as follows:
A glass tube is bent in the form of the capital letter J, the short arm being closed. A little mercury is poured in to cover the bend. (See Fig. 37 a.) Since the mercury is at the same level in both arms, the pressure in (A) is the same as in (B). Mercury is now poured into (A) until it stands in the long tube at a height above that in (B) which is equal to the height of the mercury column of the barometer. (See Fig. 37 b.) The air in (BC) is now under a pressure of two atmospheres (one atmosphere is due to the mercury column). On measurement the air in (BC) will be found to have just one-half of its original volume.
Thus doubling the pressure to which a gas is subjected reduces its volume to one-half. Tripling the pressure, reduces the volume to one-third and so on.
Careful experiments reveal the following law: The volume of a given mass of gas at constant temperature is inversely proportional to the pressure to which it is subjected.
This law is often expressed mathematically. P/P´ = V´/V, or PV = P´V´. Since doubling the pressure reduces the volume one-half, it doubles the density. Tripling the pressure triples the density. We therefore have P/P´ = D/D´ or the density of a gas directly proportional to its pressure.
59. Height of the Atmosphere.—From its properties of compression and expansion, the air varies in density and pressure as one ascends in it. At a height of 3 miles the pressure is reduced to about one-half. This is an indication that one-half of the air is below this level. Balloonists have gone to a height of 7 miles, Glaser and Coxwell in England in 1862 and Berson in France in 1901. The atmosphere has been explored to a height of 30,500 meters (18.95 miles) by sending up self-registering barometers in small balloons which burst at great altitudes. A parachute protects the instruments from breakage from too rapid fall. This height of 30,500 meters was reached by a balloon sent up by William R. Blair, at Huron, South Dakota, September 1, 1910.
At a height of 35 miles, the density is estimated at ⅓0,000 of its value at sea-level. (See Fig. 38.) It is believed that some rarefied air exists for a considerable distance above this point, some estimates placing the extent at 100 miles, and others from 200 to 500 miles. Evidences of some air at