On yesterday's date in 1686 Gabriel Fahrenheit was born. There is considerable debate as to who invented the thermometer. What we do know is that the scale used, zero to 212 degrees would bear his name. Fahrenheit constructed two thermometers by 1714. He eventually replaced alcohol with mercury. The Fahrenheit scale is the English unit and the Celsius scale is the metric unit, used first by the scientific community. The Fahrenheit scale includes feet and miles, while the metric scale includes meters and kilometers.
Earlier thermometers were invented, however using water. Galileo is most commonly considered the inventor of the (water) thermometer. In 1613 he constructed an instrument that did not have a scale, but in a letter dated 1638 another scientist, Benedetto Castelli describes Galileo's experiment and implies that Galileo did indeed use a scale when examining "the degrees and heat and cold". Galileo's thermometer was constructed of unsealed glass. A thin hollow glass tube about 18 inches in length was vertically placed in a glass base about the size of a hen's egg. He warmed the flask with his hands then turned the mouth upside down in some water, and the water rose a little in the thin piece of glass. Galileo never published anything stating that he divided the thermometer into measurements of equal degree.
A better candidate for inventor may be Santorio, who published a reference to his thermometer in 1612. This instrument had medical as well as meteorological uses. His
"thermometer" quantified changes in the air temperature as well as changes in the body temperature. Santorio also used glass with water in it. The question of the use of a scale also surrounds Santorio's invention. He did mark the temperature with threads tied around the thin glass tube.