Thermometer 1665 Christiaan Huygens (1629–1695) suggested using the melting and boiling points of water as standards, and in 1694 Carlo Renaldini (1615–1698) proposed using them as fixed points on a universal scale. In 1701, Isaac Newton (1642–1726/27) proposed a scale of 12 degrees between the melting point of ice and body temperature. In 1714 Dutch scientist and inventor Daniel Gabriel Fahrenheit invented the first reliable thermometer, using mercury instead of alcohol and water mixtures. In 1724 he proposed a temperature scale which now (slightly adjusted) bears his name. He could do this because he manufactured thermometers, using mercury (which has a
Thermodynamic temperature as SPRTs, PRTs or Platinum RTDs) and monochromatic radiation thermometers. For some types of thermometer the relationship between the property observed (e.g., length of a mercury column) and temperature, is close to linear, so for most purposes a linear scale is sufficient, without point-by-point calibration. For others a calibration curve or equation is required. The mercury thermometer, invented before the thermodynamic temperature was understood, originally "defined" the temperature scale; its linearity made readings correlate well with true temperature, i.e. the "mercury" temperature scale was a close fit to the true scale. The thermodynamic temperature is a measure of the average