# Learning Lesson: Measure the Pressure II - The "Dry" Barometer

## Overview

Barometers using mercury are heavy and fragile. The idea of "dry" barometer was conceived by Gottfried Wilhelm Leibniz around 1700. The idea was to detect pressure changes using sealed bellows. The first working version of an aneroid (without water) barometer was built in 1843 by French scientist Lucien Vidie.

This made the barometer very portable and it became commonly use meteorological instrument. It was still calibrated to the mercurial barometer with readings in inches of mercury.

Even as late at the 1990s, National Weather Service offices still calibrated and verified the accuracy of the aneroid barometer with the mercurial barometer. Using simple items the student will make a device for indicating air pressure changes, called an aneroid barometer.

TOTAL TIME Construction time 5 minutes. Observation time 5-10 days. Straw, Small metal coffee can, Plastic wrap, Scissors, Index card, Rubber band None None Thunderstorms safety

## Procedure

1. Cover the top of the coffee can tightly with the plastic wrap, using the rubber band to hold it in place. (The cover should be a taut, airtight fit.)
2. Position the straw so that it lays across two thirds of the cover with the remaining length of the straw suspended over air. Tape in place.
3. Fold one short end of the index card, about one inch from that end, at a 90° angle. Tape the folded end of the index card to the can behind the straw in such a way that allows you to make marks on the card every day.
4. Record the level of the straw onto the card.
5. For the next 10 days, at the same time each day, record the level of the straw while paying close attention to how changes in the weather affect the straw's level.

## Discussion

What the students have constructed is similar to an aneroid barometer. It is the most common type of barometer for home use.

The aneroid cell volume is very sensitive to changes in atmospheric pressure as it expands and contracts as air pressure decreases or increases. Attached to the aneroid cell is a lever indicating the air pressure. In this case, the aneroid cell is the coffee can.

In this barometer, high pressure in the atmosphere will weigh more the pressure inside the can at the time the barometer was constructed. That added weight will force the plastic wrap into the can, causing the straw tip to rise, indicating higher pressure.

The opposite will occur when low pressure is in the area. The decrease in weight of air on top of the can will help cause the plastic wrap to rise, therefore lowering the straw tip.

Today, even with sensitive electronic sensors having replaced the metal aneroid cells in most barometers, those electronic sensors still need to be calibrated to ensure their accuracy. For that calibration, we still use mercurial barometers.

## Live Weatherwise

One measure of the severity of a thunderstorm is the wind speed. In addition to the size of hail, the National Weather Service defines a severe thunderstorm as one containing wind speed of 58 mph (50 kts / 93 km/h) or greater.

The force of all of the molecules moving at 58 mph (50 kts / 93 km/h), or more, can create hazardous weather conditions such a blowing down phone and power lines, trees, and make driving hazardous. When the National Weather Service issues a Severe Thunderstorm Warning it means a thunderstorm with wind gusts to 58 mph (50 kts / 93 km/h) or greater and/or hail size of 1" (2.5 cm) or greater is occur or about to occur near you.

Discuss severe thunderstorm safety with your family. Everyone should know what to do in case all family members are not together. Discussing disaster response ahead of time helps reduce fear and lets everyone know what to do should a severe thunderstorm occur.

Postpone outdoor activities if thunderstorms are likely. Many people take shelter from the rain, but most people struck by lightning are not in the rain! Postponing activities is your best way to avoid being caught in a dangerous situation.