An altimeter is an instrument that measures altitude (the distance above sea level or some other chosen point). Aircraft use two different types: the aneroid (barometric) altimeter and the radio altimeter.

The aneroid altimeter derives a quantity for altitude by measuring the decreasing air pressure that occurs with increase in altitude, while a radio altimeter bounces a radio signal from an object aloft to the ground and determines the distance aloft as a function of the time it takes for the signal to return.

The history of the altimeter begins with the invention of the mercury barometer, the first device to measure air pressure. In 1643, Italian physicist Evangelista Torricelli, a pupil of Galileo, filled a tube with mercury. One end of the tube was closed; the other open end was turned upside down and inserted in a cup of mercury. Because the air exerted pressure on the mercury...