THE DISCOVERY OF ATMOSPHERIC PRESSURE

or

The Earth Sucks Not

(Theses demos takes a full period)

Demonstrate a piston pump (tire pump or glass demo pump), and tell how the ancients knew why it worked-- Aristotle had told them... "NATURE ABHORS A VACUUM".

The story of the Duke of Tuscany's pump (17 th cent). The Duke had his pump moved from the water source up the hill into his castle (so foul foe couldn't mess with it). Well, it wouldn't cause water to rise higher than 10 meters above the source. Nature had pooped out at 10 meters of water. He called in Torricelli (a student of Galileo) to solve the problem. Torri experimented with inverted tubes, closed at the top, filled with water and mercury. The water always ran down to 10 meters, and the mercury to about 760 mm.

Set up this device by filling a glass tube, with one end sealed whose length is at least 78 cm, with mercury (use a dropper). Place the "finger of Torricelli" over the open end and invert the tube into a beaker containing mercury (be careful not to allow any air to enter). When the finger and tube end are beneath the mercury in the beaker, remove the former. Clamp the inverted tube of mercury to a ringstand, and place a meter stick next to it to measure the millimeters from the mercury in the beaker to the top of the column of mercury.

On closer examination, Torri found the mercury level varied from day to day, dropping with rainy weather and rising with fair weather. Ah, Yaz, it must be a function of the atmosphere. Hence he deduced that the atmosphere must exert pressure, and therefore must have weight! (Aristotle said, "Air is Levity", so Torricelli has shot down two of the ancient wrongs, and invented the Barometer).

On the same ringstand, attach an 80 cm glass tube (with a 90 deg bend at the top is nice) with one end in the same mercury reservoir with the Tube of Torricelli (Ooohh, the Barometer), and connect the top of this tube to an aspirator (or other vacuum pump) with a piece of heavy walled rubber tubing. Place a safety beaker under the aspirator to catch any mercury that accidentally comes through the system.

Start the aspirator and watch the mercury zip up the tube. Gasp, it stops in the nick of time (we hope), and guess where? Ah, interesting, about the same level as the barometer.

Well, if Torri is right, air must have weight (not levity), so let's weigh air. Use the aspirator to remove air from a hollow metal ball (obtainable from the catalogs) or a separatory funnel, close the cock thereon, and weigh it "empty". Then, let air in (nice hiss), and reweigh it. There is quite an increase in weight. You can even calculate the density of air by dividing the mass of air in the ball by the volume of the ball.

MORE DEMOS ON ATMOSPHERIC PRESSURE:

Give these equivalents of atmospheric pressure:

One Atmosphere = 760 mm of mercury (sea level average)

= 10 m of water (sea level average)

= 1 kg of mass (9.8n) per square cm of area

= 101 kpa (of that kind of pressure)

= 1013 mb (for weather)

Point out that a vacuum is nothing, and nothing cannot do anything, which includes "suction". Vacuums do not suck, the atmosphere pushes! One does not "pull a vacuum" or "put a vacuum in" something. One creates a lower pressure outside, say by expanding the volume in a pump, and molecular motion (kinetic theory) causes the air to move into the lower pressure region.

Evacuate the Magdeburg Hemispheres (obtainable from catalogs) and have two burly guys try to pull them apart. Tell the story of Otto von Guerrike, Mayor of Magdeburg in the 17 th century, setting eight horses to the task. His hemispheres were much larger, and the horses failed to do it.

Now, take back the hemispheres from the burlys, and, while your back is turned, let the air sneak in. Face the class and tell them that only the strongest can succeed. Tear them apart with considerable moaning, etc.

Calculate the number of kilograms (or n) needed to really separate them by measuring the radius in cm, using pi r squared for the area in centimeters squared, and that the atmosphere exerts 1 kg per centimeter squared. It take about 75 kg, which may be about the mass of the instructor. Hence, Spider Man is in trouble with these things as suction cups.

To illustrate this, moisten one hemisphere and place it on the smooth surface of the demo table, remove the air, and get on the table, and pull (watch your back), and you can do it (or a burly can).

Point out, again, that there is no mysterious "force of vacuum", but merely the force of the atmospheric pressure, which is not all that strong. Also, that it is the surface area and the pressure that determine how many horses are needed. But it can be done!

Crush a ditto juice can (or a plastic jug) by removing the air from it with rubber stopper, glass bend, and pump. (Great fun).

Then, wrap a 4-liter glass wine jug in a safety towel, place it in the sink (as a safety precaution) and remove the air from it as with the above can. It doesn't crush! This shows, again, that there is no mysterious irresistible force of "vacuum". The strong construction of the jug enables it to withstand the great total force of a large number of centimeters of surface area on which there is about 1 kg of atmospheric action on each. WOW!

Now for the mysterious Syphon. Take two large beakers and a transparent plastic tube, and fill one beaker with water. Immerse most of the tube into the beaker of water allowing it to fill to the water level. Then block off the upper end of the tube with a finger. Pull the blocked end down until the water level in it is below the water level in the first beaker, and holding it into the empty beaker, open the blocked end. Ah, the water flows. This is the proper way to start a siphon, not with the mouth (unless it's wine being siphoned). Perhaps the action is caused by the heavier "water piston" in the longer tube lowering the pressure so that the atmosphere will push the water into the shorter end which has the lighter "water piston".