Effects of Temperature and Pressure Changes on Cape-Cod Type Barometer

Cape-Cod barometers come in a great variety of shapes, but all are based on the same principle: A volume of air is separated from the air in the environment by a liquid; as the pressure on the outside increases, the volume of air inside the barometer is compressed, and the level of the liquid falls. When the pressure falls, the air inside the barometer expands and pushes the liquid up. It is easily demonstrated that such a primitive barometer responds also to changes in temperature.

Make a small Cape-Cod barometer using a small pill bottle of glass or plastic, about 2-1/2" long and 3/4" diameter, a small glass tube and a one hole stopper. Put some water, dyed with red or blue ink, into the small glass tube of the barometer so that the water fills the pill bottle to the middle, above the tip of the glass tube as shown in the diagram. Make a scale on a strip of white paper and tape to the portion of the tube that sticks out of the pill bottle. The detail is show in the other diagram.

Tie a piece of wire around the barometer and lower it into a bottle; you may use the same gallon jug used for the fog bottle in another activity. Observe what happens to the water level in the barometer when you withdraw air from the bottle and when you blow air into the bottle.

Now retrieve the barometer from the bottle and immerse it to the neck of the pill bottle first into cold water, then into hot water.

• How does the barometer respond to pressure changes?
• How does the barometer respond to temperature changes?

Material: One gallon or half gallon jug

One small pill bottle which will fit easily through the neck of the jug

A one hole stopper and glass tube which fits into the pill bottle

One foot of wire

Hot and cold water