THE MERCURY-DIFFUSION AIR PUMP The latest development of the air pump is shown in the accompanying diagram. It is over a million times more effective than an air pump of the mechanical kind invented by Von Guericke. The principle is as follows: The jet of water pouring out through Ji from an ordinary water tap T entrains the air in the chamber C and thus pulls the pressure in C' down to from 10 to 15 millimeters of mercury. Next, the mercury jet J2, produced by boiling violently the mercury above the electric furnace F, entrains the air in the chamber C" and thus lowers the pressure in this chamber to, say, .01 millimeter of mercury. Again, the stream of mercury vapor pouring out of J3, under the influence of the furnace F', carries with it the molecules of air coming out of C"". Finally, the liquid-air trap freezes out the mercury vapor, some of which would otherwise find its way through C" into the high-vacuum chamber. So little air is finally left in this high-vacuum chamber that the pressure there may be as low as a hundred-millionth of a millimeter of mercury. Pumps of this sort are now used for exhausting radio bulbs and high-vacuum rectifiers, which are becoming of very great commercial value. The credit for the invention of this form of pump belongs primarily to a fellow countryman of Von Guericke, Professor Gaede, of Freiburg, Germany. Improvements of his design, however, have been made quite independently and along somewhat different lines by the following Americans: Irving Langmuir of the General Electric Company, Schenectady; O. E. Buckley of the Western Electric Company, New York; and W. W. Crawford of the Victor Electric Company, Chicago. The particular design shown in the diagram is due to Dr. J. E. Shrader of the Westinghouse Research Laboratory, Pittsburgh from 73 to 76.5 centimeters in localities which are not far above sea level, the reason being that disturbances in the atmosphere affect the pressure at the earth's surface in the same way in which eddies and high waves in a tank of water would affect the liquid pressure at the bottom of the tank. The barometer does not directly foretell the weather, but it has been found that a low or rapidly falling pressure is usually accompanied (or soon followed) by stormy conditions. Hence the barometer, although it is not an infallible weather prophet, is nevertheless of considerable assistance in forecasting weather conditions some hours ahead. Further, by comparing at a central station the telegraphic reports of barometer readings made every few hours at stations all over the country, it is possible to determine in what direction the atmospheric eddies responsible for barometer changes and stormy conditions are traveling and hence to forecast the weather even a day or two in advance. 42. The first barometers. Torricelli actually constructed a barometer not essentially different from that shown in Fig. 32 and used it for observing changes in the atmospheric pressure; but perhaps the most interesting of the early barometers was that set up about 1650 by Otto von Guericke, mayor of Magdeburg, Germany (1602-1686) (see opposite page 41). He used for his barometer a water column the top of which passed through the roof of his house. A wooden image which floated on the upper surface of the water appeared above the housetop in fair weather but retired from sight in foul, a circumstance which led his neighbors to charge him with being in league with Satan. 43. The aneroid barometer. Since the mercury barometer is somewhat long and inconvenient to carry, geological and surveying parties commonly use an instrument called the aneroid barometer. It consists essentially of one or more air-tight cylindrical boxes, the top of each one being a metallic diaphragm which bends slightly under the influence of change in the atmospheric pressure. This motion is multiplied by a delicate system of levers and is communicated to a hand which moves over a dial whose readings are made to correspond to the readings of a mercury barometer. These instruments are made so sensitive as to indicate a change in pressure when they are moved no farther than from a table to the floor. In the self-recording aneroid barometer, or barograph, used by the United States Weather Bureau (Fig. 33), several of the airtight boxes are superposed for greater sensitiveness, and the pressures are recorded in ink upon paper wound about a drum. Clockwork inside the drum makes it revolve once a week. A somewhat different form of the instrument is used by aviators to record altitude. SUMMARY. Air has weight, 1 cubic centimeter at 0° C. weighing .001293 gram and 1 cubic foot weighing ounce. Because of its weight the atmosphere exerts a pressure at sea level of 76 centimeters (30 inches) of mercury, the equivalent of 1033.6 grams (about 1 kilogram) per square centimeter, or 14.7 pounds per square inch. Atmospheric pressure diminishes with ascent, a fall of 1 millimeter in the barometer corresponding to an ascent of about 12 meters for relatively small distances above the sea level. Weather forecasts are based only in part upon barometric readings. A rapidly falling barometer is usually accompanied or followed by a storm. QUESTIONS AND PROBLEMS 1. Measure the dimensions of your classroom in feet and calculate the number of pounds of air in the room. 2. The body of the average man has 15 sq. ft. of surface. What is the total force of the atmosphere upon him? Why is he not conscious of this crushing force? 3. If a tumbler is filled with water, or partly filled, and a piece of writing paper is placed over the top, it may be inverted, as in Fig. 34, without spilling the water. Explain. What is the function of the paper? 4. Make a labeled drawing of a simple Torricellian barometer, naming all the parts in the diagram. 5. Would the pressure of the atmosphere hold mercury as high in a tube as large as your wrist as in one having the diameter of your finger? Explain. FIG. 34 6. If the variation of the height of a mercury barometer is 2 in., how far did the image rise and fall in Guericke's water barometer? 7. Give three reasons why mercury is better than water for use in barometers. 8. A balloonist once rose to such a height that his barometer read 18 cm. What was the pressure of the atmosphere? 9. If a barometer fell from 30.5 in. to 28.75 in. during the passing of a severe storm, how much did the pressure change in pounds per square inch? 10. Magdeburg hemispheres (see opposite page 34) are so called because they were invented by Otto von Guericke, who was mayor of Magdeburg. When the lips of the hemispheres are placed in contact and the air exhausted from between them, it is found very difficult to pull them apart. Why? 11. Von Guericke's original hemispheres were 22 in. in interior diameter. If the air was all removed from the interior of the hemispheres, what force in pounds was in fact required to pull them apart? (Find the atmospheric force on a circle with a radius of 11 in.) COMPRESSIBILITY AND EXPANSIBILITY OF AIR 44. Incompressibility of liquids. Thus far we have found very striking resemblances between the conditions which exist at the bottom of a body of liquid and those which exist at the bottom of the great ocean of air in which we live. We now come to a most important difference. It is well known that if 2 liters of water be poured into a tall cylindrical vessel, the water will stand exactly twice as high as if the vessel contained but 1 liter; or if 10 liters be poured in, the water will stand ten times as high as if there were but 1 liter. This means that the lowest liter in the vessel is not measurably compressed by the weight of the water above it. It has been found by carefully devised experiments that compressing forces enormously greater than these may be used without producing a marked effect; for example, when a cubic centimeter of water is subjected to the stupendous pressure of 3000 kilograms, its volume is reduced to but .90 cubic centimeter. This means that at a depth of six miles in the ocean a given volume of water is diminished only about 3 per cent. Hence we say that water, and liquids generally, are practically incompressible. Had it not been for this fact we should not have been justified in taking the pressure at any depth below the surface of the sea as the simple product of the depth by the density at the surface. 45. Compressibility of air. When we study the effects of pressure on air, we find a wholly different behavior from that described above for water. It is very easy to compress a body of air to one half, one fifth, or one tenth its normal volume, as we prove every time we inflate a pneumatic tire or cushion of any sort. Further, the expansibility of air (that is, its tendency to spring back to a larger volume as soon as the pressure is relieved) is proved every time a tennis ball or a football bounds, or the air rushes out from a punctured tire. But this readiness to expand as soon as the pressure is diminished does not belong merely to air which has been |