Between 1792 and 1796, Volta devoted himself to studying the density and tension of saturated vapours and their dependence on temperature. In this context, he invented a specific device for experimentally analysing the expansion of air.
The instrument consists of a glass vessel formed by a bulb connected to a graduated cylinder. The container, partially filled with water, is turned upside down and immersed in a second vessel filled with water, so as to trap a known quantity of air in the bulb and cylinder.
The volume of trapped air can be determined by reading the markings on the graduated cylinder. This makes it possible to accurately observe changes in air volume as experimental conditions vary.
By slowly heating the water surrounding the container, the air inside also heats up and expands, pushing some of the water out of the cylinder. If the heating is sufficiently gradual, the air temperature can be considered equal to that of the water.
By measuring the temperature of water and the volume of air at different temperatures, Volta derived the law of uniform expansion of air. He establishes that the increase in volume is proportional to the increase in temperature and that, for each degree, the volume increases by 1/273 of the volume of the gas at 0 °C.
