These points are adequate for approximate calibration, but both the freezing and boiling points of water vary with atmospheric pressure. Later thermometers that used a liquid other than mercury also gave slightly different temperature readings. In practice, these variations were very slight and remained close to the thermodynamic temperature, once the latter was discovered. These issues were explored experimentally with the gas thermometer. Until the discovery of true thermodynamic temperature, the mercury thermometer usually defined the temperature.
These points are adequate for approximate calibration, but both the freezing and boiling points of water vary with atmospheric pressure. Later thermometers that used a liquid other than mercury also gave slightly different temperature readings In practice, these variations were very slight and remained close to the thermodynamic temperature, once the latter was discovered. These issues were explored experimentally with the gas thermometer. Until the discovery of true thermodynamic temperature, the mercury thermometer usually defined the temperature.