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OTTO VON GUERICKE (1602-1686)
German physicist, astronomer, and man of affairs; mayor of Magdeburg; invented the air pump in 1650, and performed many new experiments with liquids and gases; discovered electrostatic repulsion; constructed the famous Magdeburg hemispheres which four teams of horses could not pull apart (see p. 33)
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 J1 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 mm. 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 mm. 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 audion bulbs and highvacuum 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 several Americans: namely, 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
3. If a small quantity of air should get into the space at the top of the mercury column of a barometer, how would it affect the readings? Why?
4. 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.
5. Give three reasons why mercury is better than water for use in barometers.
6. Calculate the number of tons atmospheric force on the roof of an apartment house 50 ft. × 100 ft. Why does the roof not cave in? 7. Measure the dimensions of your classroom in feet and calculate the number of pounds of air in the room.
8. Magdeburg hemispheres (Fig. 32) 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?
9. Von Guericke's original hemispheres are still preserved in the museum at Berlin. Their interior diameter is 22 in. On the cover of the book which describes his experiments is a picture which represents four teams of horses on each side of the hemispheres, trying to separate them. The experiment was actually performed in this way before the German emperor Ferdinand III. 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 of 11 in. radius.)
FIG. 32. Magdeburg hemispheres
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 10 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 weights 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 3,000,000 grams, its volume is reduced to but .90 cubic centimeter. 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.
The depth bomb, so successful in the destruction of submarines, is effective because of the practical incompressibility of water. If the bomb explodes within a hundred feet of the submarine and is far enough down so that the force of the explosion is not lost through expansion at the surface, the effect is likely to be disastrous.
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 of 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 it is not only air which has been crowded into a pneumatic cushion by some sort of pressure pump which is in this state of readiness to expand as soon as the pressure is diminished; the ordinary air of the room will expand in the same way if the pressure to which it is subjected is relieved.
Thus, let a liter beaker with a sheet of rubber dam tied tightly over the top be placed under the receiver of an air pump. As soon as the pump is set into operation the
inside air will expand with sufficient force to burst the rubber or greatly distend it, as shown in Fig. 33.
Again, let two bottles be arranged as in Fig. 34, one being stoppered air-tight, while the other is uncorked. As soon as the two are placed under the receiver of an air pump and the air exhausted, the water in A will pass over into B. When the air is readmitted to the receiver, the water will flow back. Explain.
Illustrations of the expansibility of air
46. Why hollow bodies are not crushed by atmospheric pressure. The preceding experiments show why the walls of hollow bodies are not crushed in by the enormous forces which the weight of the atmosphere exerts against them. For the air inside such bodies presses their walls out with as much force as the outside air presses them in. In the experiment of § 35 the inside air was removed by the escaping steam. When this steam was condensed by the cold water, the inside pressure became very small and the outside pressure then crushed the can. In the experiment shown in Fig. 33 it was the outside pressure which was removed by the air pump, and the pressure of the inside air then burst the rubber.
47. Boyle's law. The first man to investigate the exact relation between the change in the pressure exerted by a confined body of gas and its change in volume was an Irishman, Robert Boyle (1627-1691). We shall repeat a modified form of his experiment much more carefully in the laboratory, but the following will illustrate the method by which he discovered one of the most important laws of physics, a law which is now known by his name.