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Barometers

We plan to produce a large number of videos ilustrating the experiments we are performing once in a while in our laboratories. Here is one of the first: "The Baroscope" demonstrating that air has weight.

 

Towards the middle of 17th century the scientist were ready to explain why mine water could not be sucked from surface to more than 32 ft. Evangelista Torricelli figured out that once the water column pressure become equals the atmospheric pressure the water will stop raising towards the pump. Torricelli realized the water is `not sucked up` but works like a weight on a scale in balance with another weight: the atmospheric air column.

The main concern with the simple mercury barometer, shown at right, is the sturdiness of the glass tube ; if breaks the poisonous heavy metal will go all over and it will be very hard to get rid of it. The large total surface area of the small droplets will permit fast reaching equilibrium for the partial pressure of mercury vapors. Everybody will bread those vapors.

When the atmospheric pressure change, it is impossible to account for change in the level of mercury in the reservoir. At least a ruler has to be attached to anable to record the position of the mercury in the tube and so the atmospheric pressure.

simple mercury barrometer

simple barometer

There is as much as needed sophistication in the mercury barometer shown at left: no bottle at the bottom but an "U" turn. A precise measurement of the changes in mercury column can be made. A ruler is built in. The fragile and dangerous mercury tube is protected by the wooden box. The only thing missing is a thermometer (inside the box) to permit corrections for thermal expansion of the mercury.

 
Fortin barometer
 

 

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