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| Altimeter |
An altimeter is an instrument that shows a pilot just how high his
airplane is above the earth. Most
altimeters are aneroid (without air) barometers, made to read in feet of
altitude instead of inches of mercury.
Like all barometers, the aneroid altimeter shows changes in air
pressure, which lessens as the altitude increases.
Behind the dial is a small metal can called a diaphragm from which
most of the air has been removed. As the
airplane goes up, the air pressing against the diaphragm lessens, allowing the
diaphragm to expand.
The movement of the sides of the diaphragm move the hands on the
face of the altimeter.
Still another kind of altimeter is called a radio or radar
altimeter. It measures altitude by
sending radio signals to the ground.
It tells the altitude by measuring the time needed for the radio
signals to reach the ground and bounce back to the plane. – Dick Rogers
