It's Not Difficult, And Takes Just A Few Minutes Using Easily Found Materials
The Homemade Barometer Part 4 - An "Aneroid" Barometer
In Part 2 of this series on How To Make A Barometer,you'll find how to make a barometer (actually two barometers)in much the same way that Evangelista Torricelli did whenhe invented the barometer in 1643. We'll forget that the original invention came by accident while he was investigatingthe production of a vacuum. But such is often the way of science.
The mercury barometer was the instrument of choice for measuringair pressure for the next two centuries, but it wasn't the onlyinstrument used to forecast weather. The Weather or Storm Glass was developed during this time, and is still around today, although mostly as an ornament. Part 3 in the seriesof how to Make A Barometer described how to make something similar without being a glass blower (althoughthe ability to blow bubbles did come into it).
The first barometers used mercury as the liquid to show changes in air pressure. Mercury is an amazing liquid - it is actually a metal which exists in liquid form at normal temperatures, and is about 13.5 times heavier than water.
This is very helpful, because an effective barometer only needs a tube a bit over 30 inches (about 76 cm) in length to measure air pressure, whereas if water was used the tube would need to be around 35 feet high (about 10.7 metres).
Now while there are ways to make a mercury barometer more compact, they are still fragile and must be kept level and in a stable position. In other words, they are not the ideal instrument for home use, and are not particularly easy to transport.
These problems were overcome in 1843, when the aneroid barometer was invented.
Again the principle is simple - a thin metal capsule from which all air has been removed to form a vacuum will change in thickness as air pressure changes. When air pressure is higher the capsule will be compressed, but it will expand when the pressure drops. You can find outmore about Aneroid Barometers here
This idea was recognized fairly early, but the first reliable aneroid had to wait until technology had developed far enough to allow production of thin metal sheets which could be welded along the join.
This homemade barometer is not strictly speaking an aneroid, although it fits the meaning of the name which means "without liquid".
To make it, you will need:
An open topped glass or metal container. the walls must besolid and airtight.
A sheet of thin rubber - a broken party balloon works well.
A strong rubber band.
Some glue and some adhesive tape.
A white card 3 x 5" is plenty (7.5 x 12.5cm).
Stretch the rubber across the top of the empty container andstretch the rubber band around the rim to keep the rubber in place.
Glue one end of the straw to the middle of the rubber so thatthe rest of the straw lies flat along the rubber and protrudes over the rim of the container.
Fold the narrow end of the card about 1" (2.5 cm) from the end. Glue or tape the narrow end to the container so that the wider end sits just behind the straw. Cut off the free end of the straw a little way back from the end of the card. Mark the card level with the end of the straw. You can add a pin to the end of the straw to make it more effective as a pointer
Nothing too difficult there. But what have we got?
Well, when the air pressure increases it will be greater than the pressure inside the container, and will push the rubber seal down into it. The end of the straw attached to the rubber will also move down, the edge of the container acts as a pivot and the free end of the straw will move up.
The opposite happens when the air pressure drops. The higher pressure in the container will push the rubber outwards and upwards, and the free end of the straw will move down.
This simple barometer is fine for telling you whether air pressure is rising or falling, and you can keep a record by recording the position of the straw every day, preferably at the same time.
Like the other home barometers, this fine construction will be affected by temperature and deterioration of materials, but all of them provide excellent demonstrations of air pressure at work.
And like all homemade barometers it will work best if you make your barometer on a day which is neither too wet and stormy (low pressure) or sunny and still (high pressure).If you pick a day when the air pressure is at about the middle of its range, then the straw will have room to moveup or down as the air pressure changes.
That's the end of the section on home experiments withair pressure and homemade barometers.
If you want to know more about Home Experiments Related
to Weather, just follow the link to an
excellent book, and I know from experience it's not just
for kids. And here are some other top introductory weather
And while most scientific attention was concentrated
on the Torricellian or Mercury Barometer, water based barometers were also in use, and in big numbers. They were known as the Weather Glass or Storm Glass. Follow the link to find
out more about these elegant weather instruments.
You can also find information on
Setting and Calibrating Your Barometer, elsewhere in this
site, together with reviews of Modern Barometers.
This link will take you back to the Top, or, when you're ready, here's how to return to the Home page.
But just before you move on...
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