Air Thermometers



Air ThermometerActually, both of these devices measure the density of the air in the room for there is no difference between this and the Cape Cod Barometer, often made as a glass swan with a dark liquid in it. The only reason they can be used as an air thermometer is that the measurements are taken over a short time when the atmospheric pressure doesn't significantly change. Otherwise, rely on liquid or solid thermometers. The Cape Cod Barometer is probably worth the investment for, if protected from wandering fingers, it is attention getting and may cause a few students to ask what it is. But, remember, it measures air density, normally about 1.3 kilograms per cubic meter (about 3/4 pounds per cubic yard) rather than air pressure.



A way to show the expansion of air when it is heated and cooled requires a dry two liter bottle and a balloon. The activity needs to be done over a short time, say one period so the atmospheric pressure doesn't change over the time of the activity.

Stretch a small balloon over the neck of the bottle and note its shape and size. Put the bottle in hot water and watch the balloon expand. Place the bottle in ice water or put it in the refrigerator freezer for half an hour or so. The balloon will be pulled into the bottle somewhat and if the bottle is again immersed in hot water, the balloon will expand outward.

A variant of this can be done with a clear soda straw by sealing one end (bending it over and hooked with a paper clip works) and drawing in a bit of soda or other dark liquid. A better way is to use glass capillary tube and some dark liquid to make an air thermometer as in the diagram below. Mark the position of the liquid at room temperature and mark the changes as you put the tube in ice water and as you heat it in a candle flame being careful it doesn't shoot hot liquid on somebody. Those who teach chemistry and physics will recognize the apparatus in the laboratory to teach Charles' Law.glass capillary tube and some dark liquid

If the atmospheric pressure changes significantly over the activity, the readings will also reflect the change.