This law, known as the Gay-Lussac law, observes the relationship between the pressure and temperature of a gas. Contrary to its name, this relationship was actually discovered by the French inventor and physicist Guillaume Amontons, and is occasionally referred to as Amontons' Pressure-Temperature Law. While Guy-Lussac explored the temperature-pressure relationship, the Guy-Lussac law is usually used to refer to the law of combination of volumes. Amontons came across this report while he was building an “air thermometer”. Although not many have been able to identify his exact method of experimentation, later scientists developed an apparatus consisting of a pressure gauge and a metal sphere. These two pieces were then attached and immersed in solutions at varying temperatures. From the research and experimentation of Amontons and Guy-Lussac, they established that pressure and volume had a direct relationship; as one increased, the other increased. It was then found that the quotient of pressure and temperature equals a constant, where, just like Boyle's law, it could be used to find one of two variables at another pressure or temperature, given one of the variables and that the other conditions remain the same. Instead of using various solutions at different temperatures as in the experiment described above, many experiments today use one solution in which the temperature is increased or decreased, as in the following