# Charles, Jacque Alexandre César (1746–1823)

Jacque Charles was a French physicist, inventor, and mathematician who, with Nicholas Robert, made the first ascent in a hydrogen balloon (1783). He discovered the law relating the expansion of a gas to its temperature rise. Gay-Lussac published this work some 15 years after Charles' discovery and it is alternately known as Charles' law or Gay-Lussac's law. Charles is credited with inventing a thermometric hydrometer and improving Fahrenheit's aerometer and Gravesande's hydrometer.

## Charles' law

Charles' law states that for a gas held at constant pressure,
the volume (*V*) occupied by the gas
is directly proportional to its temperature (*T*). That is:

*V* ∝ *T*, or

*V*/*T* = *k*,

where *k* is a constant.

The relationship was discovered by Charles in 1787 and is a special case of the ideal
gas law. It is sometimes called **Gay-Lussac's law**, because
Joseph Gay-Lussac established it more
accurately in 1882.