- 1). Pour enough cold water into the jar so that it is a little less than half full. Add food coloring if desired to make readings easier.
- 2). Tape the straw or tube to the inside of the jar so that the bottom is under the water but not touching the bottom of the jar. Now the water can move freely into the straw.
- 3). Suck the water about 2/3 of the way up the straw and pinch the top end to trap the water.
- 4). Seal the top with the piece of modeling clay (or a chewed wad of gum). The seal must be airtight.
- 5). Place the rubber band around the jar to mark the water level. When air pressure rises, the water will rise in the straw; when pressure falls, the level of water in the straw falls.
- 6). Observe the change of water level compared to the placement of the rubber band. The water level will be visible below the rubber band when the water rises into the straw, indicating rising pressure. Conversely, it will be visible above the rubber band when the water level in the straw becomes lower, as does the pressure.
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