Gay-Lussac's gas-expansivity experiments and the traditional mis-teaching of ‘Charles's Law’

Summary

Although gas thermometers have long been the standard against which all other thermometers are checked, English-language physics textbooks usually propose experiments for students to test the linearity of the relationship, at constant pressure, between gas volume and temperature indicated by a mercury thermometer. This absurd exercise receives support from many authoritative textbooks which wrongly associate with Gay-Lussac's classic 1802 paper in Annales de Chimie—in which he announced that all gases have the same mean expansivity over the range 0 to 100°C—a diagram of an apparatus he used, later, to compare mercury thermometers with air thermometers. This confusion is first found in Biot's Traité de Physique (1816) and has been perpetuated, with unfortunate consequences, by many subsequent authors.