1 A simple barometer of mercury

A simple barometer of mercury. A barometer is a manometer which measures atmospheric pressure.

Describing how a simple mercury barometer is made in the laboratory.

The description involves the following the following points:-

Filling a 1m long thick walled tube
A 1 m long thick walled tube is filled with mercury

Inverting the filled tube
The above filled is inverted several time with finger over the open end. This is done in order for the large air bubble to
run up and down collecting any air small air bubble in mercury.

Refilling the tube
After inverting several times, the tube is refilled with

Inverting the filled tube into a bowl.
With a finger on the open end, the filled tube is inverted into a bowl of mercury. When the finger is removed, the mercury column falls until it is equal to atmospheric pressure.

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From the above apparatus, when the air above the mercury in the bottle is pumped out, the column falls.

Testing the vacuum
If the vacuum is faulty and contains air or water-vapour, the barometer reads than the true atmospheric pressure.

Testing for the vacuum of a mercury barometer.
This is done by tilting the tube until at a position when mercury was a vacuum.

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When the tube is tilted as in the diagram, the vertical height of column “h” of mercury remains the same but the length of mercury increases.

When a mercury barometer is taken from sea level to the top of a mountain i.e. low altitude to high altitude, the mercury column falls.

This is because the atmospheric pressure decreases at the top of the mountain. The decrease in atmospheric pressure is due to density of air decreasing because air is less compressed above. Deep-sea divers must return slowly to the surface because the sudden decrease in pressure when they return fast from deep water is very painful

Pilots operating at great heights must have protective headgear to prevent nose bleeding because atmospheric pressure at great height is much smaller tan blood pressure.

Calculating the height of the reading of the mercury barometer at high altitude:

This is calculated from;

Pressure change for air=Pressure change for mercury

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Where: 𝐡𝐚 is the height of altitude, 𝛒𝐚 is the density of air, 𝐡𝐦 is the mercury column barometer at that altitude and 𝐇𝐚𝐭𝐦 is atmospheric pressure before rising.

Example; 1
A barometer is taken to the top of a mountain 440cm high. If the atmospheric pressure is 76cm Hg at sea level, the average density of air = 1.2Kg/m3 and mercury is 13600Kg/m3. Calculate the barometer reading.


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Other types of Barometers.

1) Fortin Barometer

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It is constructed like a simple mercury barometer but with a provision for accurate determination of atmospheric pressure.

There is a vernier scale for accurate reading of the mercury level.

2) Aneroid Barometer

It does not use any liquid.

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It consists of a sealed flat box (chamber) with flexible walls.

The box is evacuated but prevented from collapsing by means of a spring.

The box expands and contracts in response to changes in atmospheric pressure.

The movements of the box are magnified by a system of levers and transmitted to a fine chain attached to a pointer, which moves along a suitably calibrated scale.

Applications of atmospheric pressure

1.The Lift Pump:
Lift pumps are used to raise water from deep under ground wells.

It consists of a long cylindrical barrel, inside which is a plunger (piston). It has two valves one at the entry point to the barrel and the other at the plunger.


The action of the lift pump is explained in terms of what happens when the plunger is moving upwards (up stroke) and when moving downwards (down stroke).

Up stroke.

Valve A closes due to the weight of water above it.

The weight above valve B reduces. This causes the atmospheric pressure acting on the surfsce of water in the well, to push the water up through the pipe into the burrel.

Consquently, water above the plunger is lifted upwards and it flows out through the spout.

Down stroke.

Valve B closes due to the pressure on it, while valve A opens due to the pressure exerted by water in the burrel.

Water the passes upwards through valve A into the area above the plunger.

Limitations of the lift pump
It can only raise water to a maximum height of 10 metres. This is because the atmospheric pressure can only support a water column of 10 metres.

2.The Force Pump:
The force pump is designed to overcome the limitations of the lift pump. It can raise water to heights greater than 10metres.


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The action of the force pump is also explained in terms upstroke and down stroke.

Up stroke.

Valve B closes and the atmospheric pressure forces the water into the barrel through valve A.

Down stroke.

Valve A closes due to the weight of the water above it.

The water in the barrel is forced through valve B into the reservoir, C and out of the spout D.

The air trapped in the reservoir is compressed and as aresult, it keeps on pushing the water out of the reservoir through the spout even when in upstoke.

3.Other Applications of atmospheric Pressure:
(i) Drinking straw
(ii) Sunction pad
(iii) Siphon
(iv) Rubber suckers
(v) Bicycle pump
(vi) Water supply system

Rubber Sucker
This is circular hollow rubber cap before it is put to use it is moisturized to get a good air seal and firmly pressed against a small flat surface so that air inside in pushed out then atmospheric pressure will hold it firmly against surface as shown below

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Uses of rubber sucker;

-It is used printing machines for lifting papers to be fed into the printer.

The siphon;

This is used to take the liquid out of vessels (eg. Aquarium, petrol tank)

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How a siphon works
The pressure at A and D is atmospheric, therefore the pressure at E is atmospheric pressure plus pressure due to
The column of water DE. Hence, the water at E can push its way out against atmospheric pressure..
NB: To start the siphon it must be full of liquid and end A must be below the liquid level in the tank.
Applications of siphon principle

  1. Automatic flushing tank:
    This uses siphon principle.
    Water drips slowly from a tap into the tank. The water therefore rises up the tube until it reaches and fills the bend
    In the pipe siphon action starts and the tank empties (the water level falls to the end of the tube).The action Is then repeated again and again.
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  1. Flushing tank of water closet:
    This also uses the siphon principle.
    When the chain or handle is pulled, water is raised to fill the bend in the tube as shown below:
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The siphon action at once starts and the tank empties.
Comparison of densities of liquids

(i) Miscible liquids
Here, a third liquid usually mercury is used to separate the two miscible liquids.

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-Pour one liquid in one arm of a manometer and pour the second liquid in the other arm.
-Measure the height of the liquids in the two arms, h1 and h2.


H+h1ρ1g=H+h1ρ1g h1ρ1


(ii) Immiscible Liquids.

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-Pour one liquid in one arm of a manometer and pour the second liquid in the other arm.
-Measure the height of the liquids in the two arms, h1 and h2.



=H+h1ρ1g h1ρ1=h1ρ1