Barometers

Barometers

1. Simple Barometers

A Simple Barometer can be constructed by using a clean dry glass tube of length about 1m. One end of the tube is opened and the other end closed with nearly filled mercury. The open end is cover with the thumb while wearing hand gloves.  The tube is then rapidly inverted several times to remove any bubbles of air which might be trapped in the tube. The tube is then completely filled with mercury. The open end is covered with the thumb and the tube is inverted with the open end placed below mercury in a crucible.

When the thumb is removed the level of mercury in the tube falls leaving a vacuum or empty space at the top. The height of the mercury column is found to be approximately 0.76m. This is the height of mercury that can be supported by Atmosphere.

The height of the mercury represent the Atmospheric  Pressure.

Atmospheric pressure = hρg= 0.76 x 13600 x 10   where ρ is the density and g is the gravitational constant.

Atmospheric pressure =103360Pa

If water is used instead of mercury, then

 hρg=103360

h x 1000 x 10 =103360

h = 10.3m

This means when A.p(atmospheric pressure) and g are constant , then

The density of water is lower than that of mercury and because of this, a large height of 10.3m is displaced . Hence, it would not be convenient to provide such a length of pipe for the experiment.

Therefore, low density liquids  such as water  are not used in construction of a barometer.

Defect of a simple barometer

The Simple Mercury Barometer is not an ideal barometer because of the followings;

1. The space above the mercury is not a perfect vacuum. It is difficult to make the space above the mercury a perfect vacuum. The air or water vapour present in the space will make the barometer read less than the true atmospheric pressure.
2. The barometer can not be used for small daily variation of the atmospereric pressure. This is because there is no provision for an accurate scale for reading the height . only a rough estimate can be made by using a meter rule.

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