Thermometers measure temperature, by using materials at change in some way when they are heated or cooled. In a mercury or alcohol thermometer, the liquid expands as it is heated and contracts when it is cooled, so the length of the liquid column is longer or shorter depending on the temperature. Modem thermometers are calibrated in standard temperature units such as Fahrenheit or Celsius. The first thermometers were called thermoscopes and while several inventors invented a version of the thermo scope at the same time, Italian inventor Santorio was the first inventor to put a numerical scale on the instrument. Galileo Galilei invented a rudimentary water thermometer in 1593 which, for the first time, allowed temperature variations to be measured. In 1714, Gabriel Fahrenheit invented the first mercury thermometer, the modern thermometer.