Guillaume Amontons

Barometer (Guillaume Amontons)

Toward the end of the 1600s, the French physicist Guillaume Amontons built a thermometer based on the fact that the pressure of a gas is directly proportional to its temperature. The relationship between the pressure and the temperature of a gas is therefore known as Amontons' law.

P a T

Amontons' law explains why car manufacturers recommend adjusting the pressure of your tires before you start on a trip. The flexing of the tire as you drive inevitably raises the temperature of the air in the tire. When this happens, the pressure of the gas inside the tires increases.

Amontons' law can be demonstrated with the apparatus shown in below. Data obtained with this apparatus at various temperatures are given in the table below.

The Dependence of the Pressure of a Gas on its Temperature

Temperature (ºC)	Pressure (lb/in2)
100	18.1
74	16.7
24	14.5
0	13.2
-47	10.8
	The apparatus for demonstrating Amontons' law consists of a pressure gauge connected to a metal sphere of constant volume, which is immersed in solutions that have different temperatures.